2. Branching and Iteration
MIT OpenCourseWare2017-02-15
Python#string#branching#conditionals#indentation#iteration#for loop#if loop
550K views|7 years ago
💫 Short Summary
The video covers Python programming topics such as strings, concatenation, input functions, comparison operators, branching, conditionals, loops, and the range function. It emphasizes the importance of understanding syntax, logic, and proper coding practices in Python to manipulate strings, make decisions, and iterate through sequences effectively. The speaker uses practical examples and analogies to simplify complex concepts for beginners, highlighting the significance of structured coding and control flow in programming. The video aims to provide a foundational understanding of key Python concepts for learners to enhance their coding skills and problem-solving abilities.
✨ Highlights
📊 Transcript
✦
Introduction to strings in programming.
02:05Python recognizes strings with double or single quotes and assigns them to variables using the equals sign.
Concatenation of strings using the plus operator to combine their values.
Demonstration in Python's Spider tool showing concatenation of 'hi' and 'name' resulting in 'hello thereana' without spaces.
Emphasis on the fact that the plus operator does not add spaces automatically during concatenation.
✦
The differences between using commas and plus operators in Python's print function.
06:48Commas automatically add spaces between values in the print function.
The plus operator concatenates strings without spaces in the print function.
Commas allow for mixing string and non-string objects, but result in spaces.
Plus operator requires all objects to be strings for concatenation in the print function.
✦
Getting user input in Python using the 'input' function.
09:55Users can input text or numbers, which are automatically converted into strings.
To work with numbers as numbers, users must cast the input to the desired type, such as integer or float.
Casting allows for mathematical operations with the input.
Python defaults to treating all input as strings, so explicit casting is necessary for numerical operations.
✦
Comparison operators in Python and logical operators for Boolean variables are discussed in the segment.
13:32The video explains how to compare integers, floats, and strings using operators like equal, not equal, greater than, and less than.
The difference between assignment (=) and equality (==) operators in Python is emphasized.
Comparing strings lexicographically is also covered in the segment.
Branching in programming is introduced as a way to enhance code functionality.
✦
Navigation algorithm at MIT using right-hand rule.
17:30The algorithm involves turning right, moving forward, and making decisions based on walls.
Relates algorithm to programming concepts in Python.
Emphasizes how programmers create decisions for computers.
Highlights the control flow in programming.
✦
Explanation of if-else and elif constructs in Python for decision-making.
21:37The video shows how to execute different instructions based on true or false conditions.
elif is introduced as shorthand for else if, allowing multiple conditional checks.
Importance of using double equals signs for equality checks to prevent syntax errors.
Indentation in Python is emphasized for denoting code blocks, improving readability and structured coding practices.
✦
Highlights of Loops in Programming
26:17The video segment explains the concept of branching, conditionals, and introduces while loops.
A game example involving the Lost Woods from Legend of Zelda is used to illustrate the need for loops in coding.
While loops are described as a way to repeat a set of instructions as long as a condition is true.
The importance of loops in handling scenarios where multiple iterations are required is emphasized, showcasing practical application in a simple game scenario.
✦
Importance of Loop Counters in User Input and Efficiency.
31:30Initializing and incrementing loop counters is crucial to prevent infinite loops in programs.
The segment illustrates this concept through a program simulating decision-making while lost in a forest.
Introduces the more efficient alternative to while loops, the for loop, for streamlined counting and iterating.
Properly managing loop counters is emphasized to avoid program errors and inefficiencies.
✦
Explanation of the 'range' function in Python.
36:10'Range' can start at a specific value and increment by a different step size.
Importance of providing a 'stop' value to 'range' and default values for 'start' and 'step' discussed.
Demonstrates how 'range' can be used with different parameter combinations for specific iteration patterns.
Clear understanding of how 'range' can be customized for various programming needs.
✦
Explanation of loop variable i starting at 7 and incrementing by one each iteration.
38:21Calculation of sum by adding the current value of i to the previous sum.
Explanation of using the break statement to exit a loop prematurely.
Comparison between for and while loops, with for loops being ideal for a known number of iterations and while loops for unpredictable tasks like user input.
Both loops can be exited early using the break statement.
✦
Importance of Counters in Loops
42:53Initializing and incrementing the counter correctly is crucial to prevent infinite loops.
For loops can be converted to while loops, but the reverse is not always possible.
User input is used as an example to demonstrate loops with unknown iterations.
The segment ends with a brief summary and closure of the topic.
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00:31PROFESSOR: All right.
00:33Let's get started, everyone.
00:35So, good afternoon.
00:37Welcome to the second lecture
of 60001 and also of 600.
00:42So as always, if you'd like to
follow along with the lectures,
00:46please go ahead and download
the slides and the code
00:48that I'll provide at least an
hour before class every day.
00:53All right.
00:53So a quick recap of
what we did last time.
00:56So last time, we talked a little
bit about what a computer is.
01:00And I think the main takeaway
from the last lecture
01:03is really that a computer only
does what it is told, right?
01:06So it's not going to
spontaneously make
01:08decisions on its own.
01:10You, as the programmer,
have to tell it
01:12what you want it to do
by writing programs.
01:15OK.
01:16So we talked about
simple objects.
01:18And these objects were
of different types.
01:22So we saw integers,
floats, and Booleans.
01:25And then we did a couple of
simple operations with them.
01:28Today, we're going to
look at a different--
01:30a new type of object
called a string.
01:33And then we're
going to introduce
01:35some more powerful things
in our programming toolbox.
01:42So we're going to look at how
to branch within a program,
01:44and how to make things-- how
to make the computer repeat
01:47certain tasks
within our program.
01:50All right.
01:51So let's begin by
looking at strings.
01:53So strings are a
new object type.
01:56We've seen so far
integers, which
01:58were whole numbers, floats,
which were decimal numbers,
02:02and we have seen Booleans,
which were true and false.
02:05So strings are going to be
sequences of characters.
02:09And these characters
can be anything.
02:11They can be letters,
digits, special characters,
02:14and also spaces.
02:17And you tell Python that you're
talking about a string object
02:20by enclosing it in
quotation marks.
02:23So in this case, I'm creating an
object whose value is h-e-l-l-o
02:29space t-h-e-r-e.
02:32And Python knows it's a
string object, because we're
02:34enclosing it in quotations.
02:36They can be either double
quotes or single quotes,
02:38but as long as you're
consistent, it doesn't matter.
02:41And this object, we're binding
it to this variable named hi.
02:46And we're using that using
the equals sign, which
02:49is the assignment operator.
02:51So from now on, whenever we
refer to this variable hi,
02:55Python is going to say, oh,
I know what the value is,
02:57and it's that string
of characters.
03:00So we're going to
learn about two things
03:02that you can do on strings
today, two operations.
03:05One is to concatenate them.
03:07And concatenation is
really just a fancy word
03:09for using this plus
operator, which
03:11means put the strings together.
03:14So I have this original
variable named hi,
03:18and I create a new
variable called name.
03:20And in it, I'm going
to assign the string
03:24a-n-a to the variable name.
03:27And when I use the plus
operator in between hi and name,
03:31those two variables,
Python is going
03:32to look at the
values of those two,
03:34and it's going to just
put them together.
03:38OK.
03:39I'm going to switch to Spider.
03:43And this is just that
example from the slides.
03:49So let's see what happens.
03:50So I have the variable
hi, the variable name,
03:54and I'm just concatenating
those two together.
03:57And then I'm going
to print that out.
03:59So if I run the code, notice
it prints out "hello thereana."
04:05There's no space.
04:06And there's no space because
the concatenation operator,
04:09the plus, doesn't add
any spaces implicitly.
04:13So again, another example
of just computer just
04:16doing what it's told.
04:17If we want to add a space,
we'd have to actually insert
04:20the space manually.
04:22So that's this
line here, line 8.
04:25And in this line,
we're concatenating
04:27the value of the
variable hi with a space.
04:31Notice we're putting
it in quotation marks.
04:33Just a space.
04:34And then with name.
04:37So if we'll go ahead
and print that value,
04:41notice this was that
garbage greeting there.
04:45And now we have a
proper greeting, right?
04:52So that's the concatenation
between strings.
04:56And then the other
thing we're going
04:57to look at related to
strings is the star operator.
05:04So that's this one
here on line 10.
05:07So Python allows you to use
the star operator, which
05:09stands for multiplication,
between a string and a number.
05:15And when you do that,
Python interprets it
05:19as repeat that string
that many number of times.
05:25So in this case, I'm
creating a silly greeting,
05:29and I'm concatenating the value
of hi, which is "hello there"
05:33with the space plus the name.
05:37So notice here, I'm
using parentheses
05:38to tell Python, do this
operation first, and then
05:42multiply whatever the
result of this is by 3.
05:48So if I print that
out, it's going
05:51to multiply the space
with my name three times,
05:55and it's going to concatenate
that with "hello there."
05:58So that's exactly what
it printed out there.
06:02Last lecture, we talked
a little bit about print.
06:05Today, I'm going to talk about
some nuances related to print.
06:09So you use print to
interact with the user.
06:12It's cool to write programs that
print things out to the user.
06:15So the key word
here being print.
06:19And then you put
parentheses after print.
06:22And in the parentheses,
you put in whatever
06:25you want to show the user.
06:27So in this little
program, I have--
06:30I created a variable named x.
06:31I assigned it the value
1, and then I print 1.
06:35Here, I'm casting.
06:38So I'm taking the number
one, the integer 1,
06:40and I'm casting it to a string.
06:43And you'll see why in a moment.
06:46So I want to bring
to your attention
06:47a couple of things here.
06:48So in the first print, I'm
using commas everywhere here.
06:54And in the second
print, I'm using plus.
07:01So by definition, if
you-- you can use commas
07:04inside a print-- inside
the parentheses of print.
07:08And if you use a comma, Python
is going to automatically
07:13add a space in
between the two things
07:17that the comma is in
between, the values.
07:20So "my fav num is"
is the first thing.
07:23And the second thing is
whatever's after the comma.
07:27Let's take x.
07:29So if you use a comma, Python
is going to automatically insert
07:32a space for you.
07:34Sometimes, you might want
that, sometimes you might not.
07:36If you don't want that, you
can use the concatenation
07:39operation, the plus.
07:41And you can add all
of your little bits
07:44together to create
one big string.
07:48If you're using
commas, the items,
07:50the objects in
between the commas,
07:52do not all have to be strings.
07:54That's the plus side
of using commas.
07:56But the downside is you
get spaces everywhere.
08:00If you use plus
operator, the plus side
08:03is Python does exactly
what you tell it to do,
08:06but everything has to
be a string object.
08:09So "my fav num is"
is a string object.
08:12You have to convert all of
your numbers to string objects,
08:15and so on.
08:18So if we look at Spider--
This is the same-- almost
08:29the same code.
08:30So here, I don't
have spaces anywhere.
08:34So you can see that
the first line here
08:37has commas everywhere.
08:39So I'm going to have spaces in
between every one of the things
08:43that I'm printing out.
08:47This line here is sort of a
combination between commas
08:50and concatenation.
08:54So depending on where
I used the comma,
08:56I'm going to have
an extra space.
08:58And this line here just has
concatenation everywhere.
09:02So if I run this, notice this
very first line added spaces
09:08everywhere in between
all my objects.
09:10The second one added
spaces somewhere.
09:11And you can sort of trace
through and see exactly where
09:14the spaces were added.
09:16And the last line here
didn't add spaces anywhere.
09:33So printing things out
to the console is nice,
09:37but the second part of sort of
writing an interactive program
09:40is getting input from the user.
09:43And that's the more
interesting part.
09:45So if you've done problem set 0,
you might have sort of already
09:49tried to understand
this on your own.
09:50But here we are.
09:52So the way you get
input from the user
09:55is using this command
function called input.
10:00And inside the parentheses,
you type in whatever you'd
10:03like to prompt the user with.
10:07So in this case, in my
example here, I have input,
10:11and then here I said
"type anything."
10:14So the user is going
to see this text here,
10:16and then the program
is just going to stop.
10:19And it's going to
wait for the user
10:20to type in something
and hit Enter.
10:23As soon as the user
types in Enter,
10:27whatever the user types
in becomes a string.
10:31If a user types in a
number, for example,
10:33that becomes the
string of that number.
10:36So everything the
user types in is
10:38going to be made as a string.
10:43In this line right here,
whatever these the user types
10:46in becomes a string.
10:47And we're going to
bind that string object
10:50to this variable named text.
10:54So now, further in my
program, I could do whatever
10:56I want with this variable text.
10:58In this case, I'm
going to print 5*text.
11:02OK.
11:03So if the user, for
example, gave me "ha,"
11:07I'm going to print "ha" 5 times.
11:10If the user gave
me 5, what do you
11:13think the user is--
what do you think
11:15is going to be printed out?
11:1825 or 5 five times?
11:22Great.
11:23Yes.
11:23Exactly.
11:235 five times.
11:28Oftentimes, you don't want to
work with numbers as strings,
11:31right?
11:32You want to work with
numbers as numbers, right?
11:34So you have to cast.
11:36And we learned
that last lecture.
11:38You cast by just putting
in this little bit
11:41right in front of the input.
11:43And you can cast it to
whatever type you want.
11:45Here I cast it to an int, but
you can also cast to a float
11:48if you want to work with floats.
11:50And that converts whatever
the user typed in,
11:53as long as it's some number that
Python knows how to convert,
11:57into the number itself.
11:59So in this case, if
the user gives me 5,
12:01I'm going to print out 5 times
5 instead of 5 five times.
12:07So that's the code here.
12:14So the first bit
is I'm going to get
12:16the user to type in anything,
and I'm going to put 555.
12:23And then when I type in the
number, since I'm casting it,
12:25I'm going to do operations
with the number.
12:27Yeah, question.
12:28AUDIENCE: [INAUDIBLE]
12:32PROFESSOR: Why do you
want to cast to-- oh.
12:37The question is why do you
want to cast to a string?
12:41Why do you want to cast
a string to a number?
12:42AUDIENCE: [INAUDIBLE]
12:46PROFESSOR: Oh, so
Python always--
12:50whatever you type
in, just by default,
12:53by definition of
the input command,
12:55Python always makes it a string.
12:58So if you want to
work with numbers,
12:59you have to explicitly
tell it, I'm
13:00going to work with a number.
13:03So even if you give
it the number 5,
13:04it's going to think
it's the string 5.
13:08Yeah.
13:09That's just how input works.
13:13The next thing we're
going to look at
13:18is ways that you can start
adding tests in your code.
13:25And before you can start
adding tests in your code,
13:29you need to be able to
do the actual tests.
13:32So this is where comparison
operators come in.
13:39So here, let's assume that
i and j are variables.
13:44The following comparisons are
going to give you a Boolean.
13:48So it's either going to say,
this is true or this is false.
13:51So that's going to be your test.
13:54So if i and j are
variables, you're
13:56allowed to compare
ints with ints,
13:58floats with floats,
strings with strings.
14:01And you're allowed
to compare ints
14:03and floats between
themselves, but you're not
14:05allowed to compare a
string with a number.
14:09In fact, if you even try to
do that in Python-- in Spider
14:13here, if I try to say, is
the letter a greater than 5?
14:18I get some angry
text right here.
14:22And this just tells
me Python doesn't
14:24understand the
meaning of-- how do I
14:26compare a string with a number?
14:30OK.
14:31So just like in math, we can
do these usual comparisons.
14:36We can say if something
is greater than something,
14:38greater or equal to, less
than, less than or equal to.
14:41I'd like to bring to your
attention the equality.
14:44So the single equals
sign is an assignment.
14:46So you're taking a
value, and you're
14:48assigning it to a variable.
14:49But when you're doing
the double equals sign,
14:51this is the test for equality.
14:53Is the value of
variable i the same
14:55as the value of the variable j?
14:58And that's, again,
also going to give you
14:59a Boolean either true or false.
15:02And you can also test for
inequality with the exclamation
15:05equal.
15:06So that means, is the
value of the variable i
15:09not equal to the value
of the variable j?
15:12True if yes, false if no.
15:16OK.
15:17So those are comparison
operators on integer,
15:19floats, and strings.
15:21On Booleans, you can do
some logic operators.
15:25And the simplest
is just inverting.
15:30So if a is a variable
that has a Boolean value,
15:35not a is just
going to invert it.
15:37So if a is true, then not
a is false, and vice versa.
15:42This is a table that sort of
represents what I've said here.
15:45So you can do-- you
can use and and or.
15:49These are key words in Python.
15:52You can use those two
key words on variables,
15:54on Boolean variables.
15:57And you get the result
a and b is only true
16:01if both a and b are true.
16:04And a or b is only false
if a and b are false.
16:11And this is the complete
table just in case
16:13you need to reference it.
16:17All right.
16:17So now that we have ways to do
logical-- question right there.
16:21AUDIENCE: [INAUDIBLE]
16:26PROFESSOR: Yeah, great question.
16:27So what does it mean to
compare a string with a string
16:29with the greater than?
16:30So that's just going to compare
them, lexicographically.
16:34So does it come first
in the alphabet?
16:37So we can even test that out.
16:39We can say, is a greater than b?
16:44And it's false.
16:48So b comes later in
the alphabet than a.
16:53OK.
16:54So now we have ways
to do the tests.
16:56So we can add some branching
to our programming toolbox
17:02now that we have
ways to do tests.
17:05This is a map of MIT.
17:06I'm going to go through
sort of a little example
17:10to motivate why we would want
to do branching in our code.
17:15And I think after this lecture,
you'll be able to sort of code
17:17up this algorithm that
I'm going to explain.
17:20So most of us see MIT as a maze.
17:21I first did when I came here.
17:26When I first came
here, obviously, I
17:28signed up for the free
food mailing list.
17:30And MIT, being a maze, I
had no idea where to go,
17:34what the shortest
path was to free food.
17:37So one way to think about
it is all I wanted to do
17:40was get to the free food.
17:44A very simple algorithm to
get there would be to say,
17:47OK, I'm going take
my right hand,
17:49and I'm going to make sure
that my right hand is always
17:51on a wall.
17:53And I'm going to go around
campus with my right hand
17:55always being at a wall.
17:56And eventually, I'll get
to where the free food is.
17:59There might not be
any left, right?
18:00But I'll be there.
18:03So the algorithm is as follows.
18:05If my right hand always
has to be on a wall,
18:07then I'm going to
say, if there's
18:10no wall to my right
side, then I'm
18:12going to go right
until I get to a wall.
18:17Then if there's a wall to my
right, and I can go forward,
18:22I'm just going to
keep going forward.
18:26If I keep going forward, and
there's a wall to my right
18:28and in front of me, I'm going
to turn around and go left.
18:31And then if there's a wall
to my right, in front of me,
18:34and to the left, then I'm going
to turn around and go back.
18:37So with this fairly
simple algorithm,
18:40I just follow the path always
keeping the wall to my right.
18:46And eventually, I would
end up where I need to be.
18:50So notice, I used, just in
plain English, a few key words.
18:54If, otherwise, things like that.
18:57So in programming, we have
those same constructs.
19:01And those same sort
of intuitive words
19:03can be used to tell
Python to do something
19:07or to do something else or to
choose from a different set
19:11of possibilities.
19:14And this way, we
can get the computer
19:16to make decisions for us.
19:18And you might be
thinking, well, you
19:20said that computers can't
make decisions on their own.
19:23It's not.
19:24You, as programmers, are
going to build these decisions
19:26into the program,
and all the computer
19:28is going to do is going to reach
the decision point and say,
19:31OK, this is a decision
point, should I go left
19:34or should I go right?
19:35Or which one do I pick?
19:36And these sort of decisions are
created by you as a programmer.
19:40And the computer just
has to make the decision
19:42and choose a path.
19:43OK.
19:45So in programming, there's
three sort of simple ways
19:47that you can add control
flow to your programs.
19:50And that's making one
decision and choosing
19:53whether to execute something
or execute something else.
19:57The first is a simple if.
20:01And given a program
that just linearly
20:04has statements
that get executed,
20:07whenever I reach
an if statement,
20:11you're going to
check the condition.
20:13The condition is
going to be something
20:15that's going to get evaluated
to either true or false.
20:21So I've reached
the condition here.
20:25And if the condition
is true, then I'm
20:26going to additionally execute
this extra set of expressions.
20:31But if the condition
is false, then I'm
20:33just going to keep going
through the program
20:35and not execute that
extra set of instructions.
20:41How does Python know which
instructions to execute?
20:44They're going to be inside
this what we call code block.
20:48And the code block is
denoted by indentation.
20:51So it's going to be
everything that's
20:53indented is part of
that if code block.
20:58Typically, four
spaces is indentation.
21:01OK.
21:02So that's how you
write code that
21:06decides whether to execute
this extra thing or not.
21:10Now let's say I don't just
want to execute an extra thing,
21:14I want to reach a
point where I say,
21:17I'll either go down this path
or I'll do something else.
21:22That's this right here.
21:27So this if else construct
says this is my code,
21:34I've reached my
decision point here,
21:37if the condition
inside the if is true,
21:42then I'm going to execute maybe
this set of statements here.
21:48But if the condition
is not true,
21:50then I'm not going to execute
that set of statements,
21:53and instead I'm going to
execute under whatever else is.
22:00So using this
construct, I'm either
22:02going to do one set of
expressions or the other,
22:04but never both.
22:06And after I've executed
one or the other,
22:08I'm going to continue on with
just the regular execution
22:11of the program.
22:20OK.
22:20So we're able to either
choose one thing,
22:22choose one thing or
another, but what if we want
22:24to have more than one choice?
22:27So if some number is equal
to zero, I want to do this.
22:31If it's equal to 1,
I want to do this.
22:33If it's equal to 2, I want
to do this, and so on.
22:36That's where this
last one comes in.
22:39And we introduced this other
key word here called elif.
22:45So that stands for
short form for else if.
22:49So first we check if
this condition is true.
22:53So we're going
through our program,
22:54we've reached our
decision point,
22:56if the condition is true,
we're going to execute maybe
22:59this set of instructions.
23:04If the condition is
not true, maybe we'll
23:06check-- if the
condition is not true,
23:09we will check this
next condition.
23:11That's part of the
elif right here.
23:14And if that one's
true, we're going
23:16to execute a different
set of instructions.
23:18You can have more than one elif.
23:21And depending on
which one's true,
23:22you're going to execute a
different set of instructions.
23:25And then this last else
is sort of a catch all
23:28where if none of the previous
conditions were true,
23:31then just do this last
set of expressions.
23:35So in this case, you're
going to choose between one
23:38of these three-- one
of these four roots,
23:40or however many you have.
23:43And then when you're
done making your choice,
23:45you're going to execute the
remaining set of instructions.
23:51So the way this works is if
more than one condition is true,
23:54you're actually just going
to enter one of them.
23:57And you're going to enter the
very first one that's true.
24:01So you're never going
to enter more than one
24:02of these code blocks.
24:05You always enter one, and
you enter the first one
24:08that evaluates to true.
24:15So notice that we denoted
code blocks using indentation.
24:19And that's actually
one of the things
24:21that I really like about Python.
24:22It sort of forces you to
write pretty code and nice
24:26looking code and just
code that's very readable.
24:31And that forces you to indent
everything that's a code block.
24:36So you can easily see sort of
where the flow of control is
24:39and where decision making
points are and things like that.
24:44So in this particular example,
we have one if statement here,
24:49and it checks if two
variables are equal.
24:55And we have an if, elif, else.
24:58And in this example, we're
going to enter either this code
25:01block or this one or
this one, depending
25:04on the variables of x and y.
25:06And we're only going
into one code block.
25:08And we'll enter the
first one that's true.
25:13Notice you can have
nested conditionals.
25:16So inside this first if,
we have another if here.
25:22And this inner if is only going
to be checked when we enter
25:28the first-- this outter if.
25:36I do want to make
one point, though.
25:39So sometimes, you might forget
to do the double equals sign
25:41when you are checking for
equality, and that's OK.
25:46If you just use one
equals sign, Python's
25:48going to give you an error.
25:50And it's going to
say syntax error,
25:53and it's going to
highlight this line.
25:55And then you're going to know
that there's a mistake there.
25:58And you should be
using equality,
26:00because it doesn't
make sense to be
26:01using-- to assign-- to be making
an assignment inside the if.
26:12So we've learned
about branching.
26:13And we know about conditionals.
26:17Let's try to apply
this to a little game.
26:22And spoiler, we
won't be able to.
26:24We'll have to learn
about a new thing.
26:27But back in the 1980s,
there was the Legend
26:29of Zelda-- cool
graphics-- where there was
26:33a scene with the lost woods.
26:36Oversimplification if
anyone's a Zelda die hard fan.
26:40But the basic idea was
if you entered the woods,
26:45you entered from the
left to the right.
26:47And then as long as
you kept going right,
26:49it would show you the same
screen over and over again.
26:53And the trick was you
just had to go backward,
26:56and then you'd exit the woods.
26:58So very simple.
27:00Using what we know so far, we
could sort of code this up.
27:04And we'd say
something like this.
27:06If the user exits right,
then set the background
27:08to the woods background.
27:11Otherwise, set the background
to the exit background.
27:15Now let's say the user-- and
then in the else, we're done.
27:18Let's say the user went right.
27:20Well, you'd show them
the woods background,
27:22and now ask them again,
where do they want to go?
27:25If they exit right,
set the background
27:26to the woods background.
27:27Otherwise, set the background to
the exit background, and so on.
27:31So you notice that there's
sort of no end to this, right?
27:35How many times-- do you
know how many times the user
27:38might keep going right?
27:39They might be really
persistent, right?
27:41And they'll be like maybe
if I go 1,000 times,
27:44I'll get out of the woods.
27:45Maybe 1,001?
27:47Maybe.
27:48So this would probably
be-- who knows how deep?
27:56These nested ifs.
27:57So we don't know.
28:00So with what we know
so far, we can't really
28:02code this cute little game.
28:04But enter loops.
28:07And specifically, a while loop.
28:11So this code here that could
be infinitely number of nested
28:16if statements deep
can be rewritten
28:18using these three lines.
28:21So we say while the
user exits right,
28:24set the background to
the woods background.
28:26And with a while
loop, it's going
28:28to do what we tell it
to do inside the loop,
28:30and then it's going to
check the condition again,
28:32and then it's
going to do what we
28:34say it should do
inside the code block,
28:36and it's going to check
the condition again.
28:39And then when the condition--
as long as a condition is true,
28:42it's going to keep doing
that little loop there.
28:45And as soon as the
condition becomes false,
28:47it's going to stop
doing the loop
28:48and do whatever's
right after the while.
28:52OK.
28:53So that's basically
how a while loop works.
28:57We have while.
28:58That's the key word.
29:00The condition is
something that gets
29:01evaluated to true or false.
29:03And once again, we have a
code block that's indented,
29:07and it tells Python,
these are the expressions
29:08I want to do as long as
the condition is true.
29:16So the condition is true,
you evaluate every expression
29:18in the code block.
29:19When you reach the end of the
expression-- end of the code
29:22block, you check
the condition again.
29:24If it's true still, you
keep doing the expressions.
29:27Check it again, and so on.
29:32So here's a little game.
29:35And with these lines
of code, we were
29:38able-- we can code up
the lost woods of Zelda.
29:43Even worse graphics, by the
way than the original Zelda
29:46is this one that
I coded up here.
29:48So I print out the
following things.
29:50"You're in the Lost Forest.
29:51Go left or right."
29:54And my program's going to say,
"You're in the Lost Forest.
29:57Go left or right."
29:58It's going to get user input.
29:59It's going to say while the
user keeps typing in right,
30:03show them this text,
and ask them again.
30:07So I'm asking them again by
just saying input here again.
30:11And that's it.
30:11That's going to just keep
getting input from the user.
30:15And if the user doesn't type in
right, and maybe types in left,
30:18you're going to exit out of
this loop, and print out,
30:21"You've got out of
the Lost Forest."
30:24So I have to show you this,
because I spent too much time
30:28on it.
30:30But I decided to improve on
the code that's in the slides.
30:37And I've written here ways that
you guys can also improve it.
30:41So if I run my code--
"You're in the Lost Forest.
30:45Go left or right."
30:46So if I say left, then yay,
I got out of the Lost Forest.
30:51But if I go right,
then I'm stuck, right?
30:56I took down some trees.
30:57You can see there's
no more trees here.
30:59I made a table, and
then I flipped it over.
31:04So the expansion to this
if you want to try it out--
31:07I put this in the comments
here-- is try to use a counter.
31:12If the user types in
right the first two times,
31:14just make that a sad face.
31:17But if the user types
in more than two times,
31:19make them cut down some trees
and build a table and flip it.
31:24That's a cute little
expansion if you
31:25want to test yourself to make
sure you are getting loops.
31:29OK.
31:30So so far, we've used while
loops to ask for user input.
31:34And that's actually somewhere
where it makes sense
31:37to use while loops,
because you don't actually
31:39know how many times the user
is going to type in something.
31:43You can use while loops
to keep sort of a counter
31:47and to write code
that counts something.
31:52If you do that, though,
there's two things
31:55you need to take care of.
31:56The first is the
first line here,
32:00which is sort of an
initialization of this loop
32:03counter.
32:06And the second is
this line here,
32:09which is incrementing
your loop counter.
32:15The reason why the
second one is important
32:17is because-- let's look
at our condition here.
32:20So while n is less than five.
32:24If you didn't have
this line here,
32:26you would never increment n.
32:29So every time through the loop,
you just keep printing zeros.
32:33And you would have
an infinite loop.
32:34I do want to show,
though, what--
32:37if you do have an infinite loop,
it's not the end of the world.
32:40So I can say something like--
so while true, print zero.
32:53So this is going to give me an
infinite loop in my program.
32:57And-- whoop.
33:06OK.
33:08So notice it's just printing the
letter p over and over again.
33:12And if I let it go
any longer, it's
33:13going to slow down the computer.
33:15So I'm going to hit
Control-C or Command-C maybe.
33:18And it's going to stop
the program from printing.
33:22So just in case you ever
enter infinite loops
33:24in your programs, just go to
the console and hit Control-C,
33:28and that's going to
stop it from sort
33:31of slowing down the computer.
33:34OK.
33:35So going back to
this example, I was
33:36saying that if you're using
counters-- variables in order
33:40to sort of count up
inside the while loop,
33:42you have to take
care to initialize
33:44a counter variable first.
33:46And then to increment
it, otherwise you'll
33:49enter an infinite loop.
33:51That feels a little bit tedious.
33:53And so there's a shortcut for
doing that exact same thing.
33:57So these four lines,
you can rewrite those
34:00into these two lines right here
using this new type of loop
34:04called a for loop.
34:07So the for loop says, for some
loop variable-- in this case,
34:10I named it n.
34:11You can name it
whatever you want.
34:13In range 5-- we're
going to come back
34:15to what range means in
a little bit-- print n.
34:22So every time through
the loop, you're
34:23going to print out
what the value of n is.
34:26Range 5 actually
creates internally
34:31a sequence of numbers
starting from 0
34:33and going to that
number 5 minus 1.
34:36So the sequence is going
to be 0, 1, 2, 3, and 4.
34:41The first time through the
loop, you're going to say n
34:44is equal to 0.
34:45Or internally, this
is what happens.
34:47N gets the value of 0.
34:48You're going to print n.
34:51Then you're going to
go back to the top.
34:53N gets the value 1.
34:55Then you're going to go
execute whatever is inside.
34:58So you're going to print 1.
35:00Then you're going
to increment that
35:01to the next value
in the sequence.
35:03You're going to print
out 2, and so on.
35:07So this is the general
look of a for loop.
35:12So we have for some
loop variable-- again,
35:16can be named whatever you
want-- in range some number.
35:21Do a bunch of stuff.
35:23And again, these are part
of this for loop code block.
35:26So you should indent
them to tell Python
35:29that these are the things
that you should do.
35:32So when you're using
range some number,
35:34you start out with variable
getting the value 0.
35:41With variable having
value 0, you're
35:44going to execute all
of these expressions.
35:47After all the expressions
in the code block are done,
35:50you're going to go
on to the next value.
35:53So 1.
35:55You're going to execute
all these expressions
35:57with the variable being
value 1, and then so on
36:01and so on until you go
to some num minus 1.
36:10That-- so using
range in that way
36:13is a little bit constraining,
because you're always
36:16going to get values
starting from 0
36:18and ending at some
num minus 1, whatever
36:21is in the parentheses in range.
36:23Sometimes you might want
to write programs that
36:25maybe start at a custom value.
36:27Don't start at 0.
36:28Maybe they start at 5.
36:29Maybe they start at minus 10.
36:32And sometimes you might
want to write programs
36:34that don't go with-- don't
expect the numbers by 1,
36:37but maybe skip
every other number,
36:39go every two numbers, or every
three numbers, and so on.
36:42So you can customize
range to your needs.
36:47The one thing you do need
to give it is the stop.
36:50So if you give it only one
value in the parentheses
36:52that stands for stop.
36:55And by default, start is
going to have the value 0,
36:57and step is going
to have the value 1.
37:01If you give it two things
in the parentheses,
37:04you're giving it start and stop.
37:06So the first being start,
the second being stop.
37:08And step gets this
value of 1 by default.
37:12And if you give it three
things in the parentheses,
37:15you're giving it start,
stop, and step in that order.
37:22And you're always going to
start at the start value
37:26and stop at-- or so you're going
to start at the start value,
37:30and you're going to
go until stop minus 1.
37:32So those are the
sequences of numbers.
37:36So in this first
code right here,
37:39my sum is going to
get the value 0.
37:40And you're going
to have a for loop.
37:44We're going to start
from 7, because we're
37:46giving it two numbers.
37:47And when you give
it two numbers,
37:49it represents start and
stop with step being 1.
37:53So we're starting at 7.
37:55If step is 1, the
next value is 8.
38:00What's the value after that?
38:05If we're incrementing by 1?
38:099.
38:12And since we're going
until stop minus 1,
38:17we're not actually
going to pick up on 10.
38:21So this loop variable,
i, the very first time
38:23through the loop is going
to have the value 7.
38:28So my sum is going
to be 0 plus 7.
38:37That's everything that's
inside the code block.
38:40The next time through the
loop, i gets the value 8.
38:45So inside the for
loop, my sum gets
38:52whatever the previous value
was, which was 7, plus 8.
38:58OK.
39:00The next time through
the loop, my sum
39:04get the value 7 plus 8 plus 9.
39:08Obviously, replacing that
with the previous value.
39:10So 15.
39:13Since we're not going through
10, that's where we stop.
39:15And we're going to
print out my sum, which
39:17is going to be the value
of 7 plus 8 plus 9.
39:22Yeah?
39:24OK.
39:25Yeah.
39:26AUDIENCE: [INAUDIBLE]
39:27PROFESSOR: Do they
have to be integers?
39:32That's a great question.
39:33We can try that out.
39:34I'm not actually sure right
off the top of my head.
39:38So you can go on Spider and
say-- let's say in this example
39:46here.
39:52So we can say 7.1, 10.3-- yeah.
39:58So they have to be integers.
40:08OK.
40:09So that's that example.
40:10And let's erase that.
40:13In this particular example,
we have start, stop, and step.
40:16And here, we're going
every other value.
40:20So we're starting at 5.
40:22Tell me what the next
value is supposed to be.
40:25If we're taking every other one.
40:277, and then 9, and then--
are we doing 11 or not?
40:35Excellent.
40:35Nice.
40:36Yeah.
40:37So we're going to
the end minus 1.
40:41OK.
40:41So it's possible
that sometimes you
40:43write code where you might want
to exit out of the loop early.
40:47You don't want to go
through all of the sequences
40:49of your numbers.
40:51Maybe there's a condition
inside there where you just
40:53want to exit the loop early.
40:55Inside the while
loop, maybe you want
40:56to exit the loop before the
condition becomes false.
41:00So that's where the
break statement comes in.
41:02So the break works like this.
41:06It's going to-- as soon
as Python sees this break
41:09statement, it's
going to say, OK,
41:13I'm going to look at whatever
loop I'm currently in.
41:18I'm not evaluating
any expression
41:20after it that comes
within my loop.
41:23And I'm going to
immediately exit the loop.
41:26So I'm going inside
this while, this while,
41:28I'm evaluating this
one expression,
41:30and I suddenly see a break.
41:33Expression b does
not get evaluated.
41:37And break is going
to immediately
41:39exit out of the innermost
loop that it's in.
41:43So this while loop that
has condition 2, that's
41:46the innermost loop that
the break is found in.
41:50So we're going to exit out
of this inner most loop here.
41:54And we're evaluating
expression c.
41:57And notice, we're
evaluating expression c,
41:58because it's-- expression c is
part of the outer while loop.
42:05It's at the same
level as this one.
42:08And these ones are part
of the inner while loop.
42:13OK.
42:14Last thing I want to
say is just a little bit
42:16of a comparison between
for and while loops.
42:18So when would you
use one or the other.
42:21This might be useful
in your problem sets.
42:23So for loops you
usually use when you
42:24know the number of iterations.
42:27While loops are very useful
when, for example, you're
42:29getting user input, and
user input is unpredictable.
42:32You don't know how
many times they're
42:33going to do a certain task.
42:36For both for and
while loops, you
42:38can end out of the loop
early using the break.
42:40The for loop uses this counter.
42:42It's inherent
inside the for loop.
42:45A while loop you can use a
counter in order-- you can use
42:48a while loop to count things.
42:50But you must initialize the
counter before the while loop.
42:53And you have to remember to
increment it within the loop.
42:56Otherwise, you maybe
lead to an infinite loop.
43:00We've seen as the very
first example of a for loop
43:04that the while--
the for loop could
43:06be rewritten as a while
loop, but the vice versa
43:08is not necessarily true.
43:11And the counterexample to
that is just user input.
43:14So you might not
know how many times
43:16you might do a certain task.
43:18All right.
43:19Great.
43:20That's all for today.
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