Control Flow: Loops#

Goals of this lecture#

Control flow, revisited.
What is a loop? What is iteration?
for loops in action.

Control flow, revisited#

By default, Python commands are executed in a linear order, i.e., line by line.

Unless we tell Python otherwise, each line will be executed once.

But sometimes it’s useful to control:

Which lines get executed?
How many times do those lines get executed?

These control “parameters” correspond, roughly, to:

Conditional statements (if/elif/else).
Loops (e.g., for or while).

Loops, explained#

A loop is a way to repeat the same piece of code multiple times.

When should you use a loop?#

Rule of thumb: if you find yourself copying/pasting the same code many different times…you might think about using a loop!

More generally: in programming, we often want to execute the same action multiple times.

Apply the same instruction to every item on a list.
Continue running some code until a condition is met.

A loop is an example of iteration#

Iteration simply means: repeating some sequence of instructions until a specific end result is achieved.

That “end result” could be any number of things:

You reach the end of a list.
Some other condition is met.

In general, we’ll use the term iterate to mean “do over and over again”.

The expression “iterate over a list” means: Do X to every item of that list.

Two kinds of loops#

There are two main kinds of loops you’ll use in Python: for loops and while loops.

A for loop runs some code for every item of a list or sequence.
A while loop runs a piece of code until some condition is met (e.g., while condition == True).

Today, we’ll focus on for loops.

Side note: lists#

We haven’t discussed list objects in detail yet, but we will introduce them as part of the lecture today.
High-level:
- A list is an ordered collection of elements.
- Different elements can be accessed by indexing through the list.

## This is a list in Python
numbers = [1, 2, 3]
### This is how we "index" particular elements in that list
numbers[0]

`for` loops in action#

A for loop is used for iterating over a sequence.

A for loop uses the syntax: for elem in list_name: ...

## This is a list in Python
numbers = [1, 2, 3]
### This is a for loop
for number in numbers:  
    print(number)

1
2
3

Check-in#

What do you expect the following code block to do, if you executed it?

for l in "apple":
    print(l)

A for loop tells Python to iterate over each element in a sequence.

The content of that loop––the indented code underneath the for statement––tells Python what to do each time.

for l in "apple":
    print(l)

a
p
p
l
e

Check-in#

Approximately how many lines of code would we need if we wanted to print each element of a list with 100 items, without using any kind of loop? (I.e., copy/paste the same code?)

Compare and contrast#

### This code prints each number independently
numbers = [1, 2, 3, 4, 5]
print(numbers[0])
print(numbers[1])
print(numbers[2])
print(numbers[3])
print(numbers[4])

### This code iterates through the list
numbers = [1, 2, 3, 4, 5]
for number in numbers:
    print(number)

What can you use a `for` loop on?#

You can use a for loop on any sequence.

We’ll talk more about sequences next week, but they include:

Lists, e.g., [1, 2, 10].
Strings, e.g., "apple".
Ranges, e.g., range(10)

Introducing `range`#

range is an operator used to create a range of numbers, e.g., from 0 to 100. It’s very useful if your main goal is to execute some code $N$ times.

Note that range(10) will return an iterable object of 10 numbers from 0 to 9.

for i in range(10):
    print(i)

If you want to start at a different number (e.g., 3), you can enter that as an argument as well.

for i in range(3, 10):
    print(i)

Remember: range will produce numbers going all the way up to $N - 1$, not $N$.

for i in range(5):
    print(i)

Check-in#

Write a for loop that prints out every letter in the string "CSS".

## First, define a string using something like: word = "CSS"
### YOUR CODE HERE

Check-in#

Write a for loop that prints out every letter in the string "Computational Social Science", except for the spaces.

HINT: Think about how you could combine a for loop with an if statement.

## First, define a string using something like: word = "Computational Social Science"
### YOUR CODE HERE

Loops and conditionals, combined#

The real expressive power of a for loop comes into play when we use conditional statements.

Remember that an if statement allows us to run a piece of code only if some condition is met.

Check-in#

Why/how could an if statement be helpful when using a for loop?

`for` and `if`: a simple use-case#

Suppose you are instructed to write a program that prints out all the even numbers between 1 and 11.

Breaking the problem down:

First, we want to iterate through a range from (1, 22).
Then, we want to check if each element of that range is even.
if a given element is even, we print it out.

Check-in#

How might we determine if a number is even?

Hint: Think about the modulo operator (%).

`print`ing even numbers in `range(1, 22)`#

for num in range(1, 22): ## for loop
    if num % 2 == 0:  ## conditional statement
        print(num)  ## the code we want to execute

Check-in#

Suppose you’re going grocery shopping. Here are the costs of each item:

costs = [5, 8, 4, 10, 15]
costs

[5, 8, 4, 10, 15]

You want to keep your costs low, so you decide not to buy anything above $9. How would you write a for loop that:

Iterates through costs.
Tracks a final_bill variable.
Only adds items to final_bill if they’re below 9$?

Try to implement this code before looking at the solution below.

#### YOUR CODE HERE

Grocery shopping: a solution#

costs = [5, 8, 4, 10, 15]
final_bill = 0
for item in costs:
    if item <= 9:
        final_bill += item
print(final_bill)

Controlling `for` loops#

Sometimes, you may want an even finer degree of control over for loops. There are two commands that give you this control:

continue: tells the for loop to continue onto the next item in the list (i.e., without necessarily doing anything with the current item).
break: cancels/stops the for loop.

`break` in action#

The following code iterates through a range, and breaks once it gets to 5.

for num in range(1, 10):
    if num == 5:
        break
    print(num)

`continue` in action#

The following code iterates through a range, and continues once it gets to 5 (i.e., “skips”).

for num in range(1, 10):
    if num == 5:
        continue
    print(num)

Check-in#

How are break and continue different?

Nested `for` loops#

Just as we can nest conditional statements, we can also nest loops.

A nested loop is a for or while loop contained within another for or while loop.

As with nested if statements, it’s very important to be careful about your indentation.

Nested loops in action (pt. 1)#

professors = ['Trott', 'Ellis', 'Fleischer']
classes = ['COGS 18', 'CSS 1']
for cl in classes:
    for prof in professors:
        print("Is {cl} taught by {prof}?".format(cl = cl, prof = prof))

Is COGS 18 taught by Trott?
Is COGS 18 taught by Ellis?
Is COGS 18 taught by Fleischer?
Is CSS 1 taught by Trott?
Is CSS 1 taught by Ellis?
Is CSS 1 taught by Fleischer?

Nested loops in action (pt. 2)#

Note: The end = " " parameter in the print function just tells Python not to print the str on a new line.

for i in range(1, 6):
    for j in range(1, i + 1):
        print("*", end=" ")
    print(" ")

*  
* *  
* * *  
* * * *  
* * * * *  

A note of caution#

Nested for loops can take a very long to execute if:
- Your lists are very long.
- You have many, many levels of nesting.
Technically, the code in a nested for loop will run $N * M$ times, where $N$ is the length of the outer loop, and $M$ is the length of the inner loop.
- This is beyond this course, but making programs more efficient is an important part of Computer Science.

Some final (challenging) practice#

Practice 1#

Write a for loop to count the number of vowels in a string. The code block below starts with a list of vowels alreay, which you can use to cross-reference when iterating through a string.

Hint: If you’re feeling extra ambitious, you might think about how to handle upper-case vowels.

vowels = ['a', 'e', 'i', 'o', 'u']
example_string = "CSS is great"
### Your code here

Practice 2#

Write a for loop that:

Iterates through a list of numbers.
if the number (e.g., i) is even, iterates through a nested for loop of all those same numbers, and…
- if a given number is odd and larger than i, prints out the sum of those numbers.

Hint: Remember that % can be used to figure out whether a given number is divisible by 2 (e.g., 4 % 2 == 0).

### Your code here

Answers to practice problems#

### Practice 1
vowels = ['a', 'e', 'i', 'o', 'u']
example_string = "CSS is great"
num_vowels = 0
for l in example_string:
    if l.lower() in vowels:
        num_vowels += 1
print(str(num_vowels) + " vowels in '{x}'".format(x = example_string))

3 vowels in 'CSS is great'

### Practice 2
numbers = range(1, 10)
for num in numbers:
    if num % 2 == 0:
        for num2 in numbers:
            if num2 % 2 == 1 and num2 > num:
                print("{x} + {y} = {z}".format(x = num,
                                               y = num2,
                                               z = num + num2))

+ 3 = 5
+ 5 = 7
+ 7 = 9
+ 9 = 11
+ 5 = 9
+ 7 = 11
+ 9 = 13
+ 7 = 13
+ 9 = 15
+ 9 = 17

Conclusion#

That was a (hopefully somewhat gentle) introduction to for loops. If you’re feeling like you want more practice:

W3 schools, as always, has some good practice problems.

Next lecture we’ll cover a new kind of loop: a while loop.

CSS 1

Control Flow: Loops

Contents

Control Flow: Loops#

Goals of this lecture#

Control flow, revisited#

Loops, explained#

When should you use a loop?#

A loop is an example of iteration#

Two kinds of loops#

Side note: lists#

for loops in action#

Check-in#

Check-in#

Compare and contrast#

What can you use a for loop on?#

Introducing range#

Check-in#

Check-in#

Loops and conditionals, combined#

Check-in#

for and if: a simple use-case#

Check-in#

printing even numbers in range(1, 22)#

Check-in#

Grocery shopping: a solution#

Controlling for loops#

break in action#

continue in action#

Check-in#

Nested for loops#

Nested loops in action (pt. 1)#

Nested loops in action (pt. 2)#

A note of caution#

Some final (challenging) practice#

Practice 1#

Practice 2#

Answers to practice problems#

Conclusion#

`for` loops in action#

What can you use a `for` loop on?#

Introducing `range`#

`for` and `if`: a simple use-case#

`print`ing even numbers in `range(1, 22)`#

Controlling `for` loops#

`break` in action#

`continue` in action#

Nested `for` loops#