Module 5: Logic and Control Flow


Now that you’ve got basic data types conditional logic, and lists down, it’s
time to add another element to the equation. Control flow opens many doors for
crafting interesting, usefully repetitive programs. We’ll add a lot of new
syntax in this module, and it’s more important than ever to practice and
explore on your own

Programming is an Art Form

This is where creativity can really come in handy– with control flow and loops, the world is your oyster! Want to print each letter of a word, one at a time? You can! Want to ask the user for input until they type “quit”? You can! And so much more.



  • The Why of loops
  • While-loops
  • Boolean expressions
  • For-loops

Introduction to Control Flow

So far, we’ve been writing code that tells a computer what to do and in what order. For example, a calculator takes user input in the form of numbers and operations, and the precise sequence of operations it performs to arrive at the answer depends on the input. This conditional nature of the calculator code is very powerful.

Often, it’s also necessary to evaluate some conditional logic more than once. In the case of the calculator, we’d want to continue to evaluate the user’s input until they “quit” the calculator, or turn it off.

The code responsible for determining how many times to do something, when to stop doing something, or when to keep doing something is called control flow. Let’s look at some common ways to specify control flow operations and decisions in Python.


A Simple Example

How could you print ‘hello world’ 100 times? You could write print 'hello world' 100 times (copy/paste would be helpful), but that is absurdly inefficient. Imagine if you realized you made a typo after finishing those 100 lines of code.

It’s much better to use a loop.


Here’s how we could print ‘hello world’ 100 times using only 4 lines of Python code.

my_counter = 0

while my_counter < 100:
    print "hello world"
    my_counter = my_counter + 1

A while-loop is the simplest way to execute code repeatedly, as long as some condition is True - in the above example, that the my_counter variable is less than 100.

Just like in an if-statement, a while-loop needs a boolean expression to evaluate. Where’s the boolean expression in the code above?

It’s right after while

The boolean expression is my_counter < 100. Initially, this expression evaluates to True, because my_counter is 0. Definitely less than 100. That’s why the while-loop starts!

Also just like an if-statement, a while-loop has a body. What is the body of the above while-loop?

The body of a while loop is the indented block

The body of the while loop is composed of the highlighted lines below:

my_counter = 0

while my_counter < 100:
    print "hello world"
    my_counter = my_counter + 1

Anything indented underneath the first line of the while-loop is said to be the body of the while-loop.

The main difference between an if-statement and a while-loop is that once the body of the if-statement happens, it doesn’t happen again. The if-statement checks the boolean expression, and either executes the code in the if-statement body, or does not.

However, with a while-loop, the body is executed again and again, and again as long as while-loop’s boolean expression evaluates to True.

So, for the above example, the execution of the code goes something like this:

set the my_counter variable to 0

start while loop
print "hello world"
increase the my_counter variable by 1
check if my_counter still less than 100- it is!
print "hello world"
increase the my_counter variable by 1
check if my_counter still less than 100- it is!
print "hello world"
... 98 more times!
check if my_counter still less than 100- it's not!
stop while loop

Making While-Loops End: Natural Breaks

It’s important to have a plan for when a while-loop should finish. Otherwise, the while-loop will just keep going forever (or until you stop the program from running). This is called an infinite loop.

As stated above, the main way to make a while-loop end is for it’s boolean expression to evaluate to False. So, it’s common for part of a while-loop body to handle making progress towards the expression eventually evaluating to False.

Here are some examples:

Counting down until some_number is not greater than 5 any more

some_number = 10

while some_number > 5:
    some_number = some_number - 1

Take a look at this code:

keep_going = True

while keep_going:
    print "I'm in the while loop"
    keep_going = False

How many times does this while-loop’s body run?

Just one time!

As soon as the while-loop starts, on the second and last line of the body, the boolean condition (in this case, it’s just the variable keep_going) becomes False.

It’s important to note that a boolean type, as in True and False, are also valid conditions for a while-loop. We’ll use this in the next section on Explicit Breaks as well.

Counting up until `n` is equal to 100

n = 1

while n != 100:
    print n
    n = n + 1

Making While-Loops End: Explicit Breaks

Another way to end a loop is with the break statement. The following example does the same thing as the last example in the previous section.

n = 1

while True:
    print n
    n = n + 1
    if n == 100:

Read both while-loops carefully, comparing the syntax of each.

Here, the boolean expression to set up the while-loop is always True - in fact, it is just the value True itself. So, we need to explicitly end the loop by checking for when the n is equal to 100, and execute break to end the loop.


Just like while-loops, for-loops provide a vocabulary and framework for executing the same block of code over an over. However, instead of checking the some condition in order to decide whether to keep looping, as in a while-loop, for-loops iterate over something – most commonly, a list!

Here’s simple first example:

desserts = ["cupcake", "ice cream", "cookie"]
for dessert in desserts:
    print dessert

If we run this in our Python repl console, we see each dessert string get printed out:

>>> desserts = ["cupcake", "ice cream", "cookie"]
>>> for dessert in desserts:
...     print dessert
ice cream

Here’s another example.

Suppose we had a list of numbers 0 through 99. We can print out each item in this list from start to end. Consider this example:

nums = range(100)

for n in nums:
    print n

As you should recall, the range function returns a list of numbers, starting at 0 and up to but not including 100. The for-loop assigns the variable n to each number in nums, in order, and then prints out its value every time.

Try this out yourself.

Each Item, One-by-One

As in the 2 above examples, when you iterate over a list using a for-loop, you get each item in the list, one-by-one. This is a wonderful thing!

Here are a few more examples to try on your own.

colors = ['red', 'orange', 'green']

for color in colors:
    print color
my_cool_shapes = ['triange', 'rectangle', 'square']

for shape in my_cool_shapes:
    print shape

Lastly, check out this code. It’s similar to the above example, with one important difference.

my_cool_shapes = ['triange', 'rectangle', 'square']

for anything_i_want in my_cool_shapes:
    print anything_i_want

Syntax of For-loops

Let’s call out the specific syntax of a for-loop.

  1. Structure the first line of the for-loop like so:
for [could be any word here] in [some list]
  1. Indent the body of the for-loop, or the code to be executed again and again.
  2. The for-loop body can be multiple lines, or just one.

For-loops with Conditionals

We’ve already seen an example of using an if conditional in a while-loop. We can also use conditionals inside of a for-loop.

For example, we want to print out the numbers 1 through 100, but in addition whether the number is even or odd:

for n in range(1, 101):
    if n % 2 == 0:
        print n, 'is even'
        print n, 'is odd'

If we want to use a conditional to break out of a for-loop, we can use the break statement, as we’ve done with while-loops.



Login to your account and start a new repl here. Title it

Complete the practice problems below on

  1. Write a while loop that prints out the numbers from 1 to 7, including 7. Use a similar technique as the below while-loop. Type this out– don’t copy/paste the below while-loop.

    counter = 1
    while counter < 10:
        print counter
        counter = counter + 1
  2. Write a for-loop that prints out the numbers from 1 to 20. Use a similar technique as the below for-loop. Again- don’t copy/paste this.

    for num in range(4):
        print num
  3. Write a for-loop that prints every number between 1 and 10 except the numbers 3 and 5. Use a similar technique as below.

    for num in range(10):
        if num != 8:
            print num


Use a nested if-statement

If you nest two if statements, you can do something only if both conditions are True.

num = 4

if num != 3:
    if num != 5:
        print "It's not 3 or 5!"

num2 = 5

if num2 != 3:
    if num2 != 5:
        print "It's not 3 or 5!"
  1. Write a while loop that prints out the numbers 100 to 1 in descending order. First, it’s helpful to figure out the condition your while-loop will use to know whether to keep looping or not. What’s the condition?

When some counter variable is < 1

You’ll need to set a counter variable to be equal to 100 first. Then, the while-loop’s condition to check after each iteration of the loop will be whether that counter variable has dropped below 1.

What’s the body of your while loop?

Print and make progress

The main thing happening in the body of this while-loop is printing out a number.

Then, you’ll need to subtract 1 from the counter variable, so that eventually, the while loop will naturally break.

  1. Modify the loop so that only multiples of 11 are printed.

How can you find out if some number is a multiple of of another?

Use the modulus operator

In Python, there is a mathematical operator % that tells you the remainder if one number we to be divided by another number. For example:

>>> 4 % 2
>>> 3 % 2

When 4 is divided by 2, there is no remainder– they divide perfectly into each other. So, 4 is a multiple of 2.

When 3 is divided by 2, there is a reminder– 1. They do not divide perfectly into each other, and 3 is not a multiple of 2.

  1. Using a for-loop and the range function, print every number from 0 to 79.

  2. Type the following list into your iPython session. Then, print each item in the list using a for-loop.

    colors = ["black", "orange", "purple"]
  3. Using the same list as the previous problem, as well as list-indexing print the first letter in each word of the colors list.

    Recall that you can print the first letter of a string with the following syntax:

    name = "Balloonicorn"
    print name[0]
  4. Using anything from this lesson (be sure to use a loop!), find a way to produce the following output:



Use a While loop

Using a while-loop would be the most straightforward way to do this. Make a variable called counter, and set it to 7. Then, your while loop could look like this:

while counter < 13:
    print counter
    counter = counter + 1
  1. Using a similar technique as above, find a way to produce the following output:



Use a While loop

Notice that instead of adding 1 to the counter variable, we could be adding 2. If you simply modify the solution from the previous problem to change

counter = counter + 1


counter = counter + 2

you should be all set.



In the following problems, you’ll find code that is invalid or not allowed in some way. Read the code, and see if you can predict what is wrong. When you’re ready, hover over the solution area to reveal the error message that Python shows, along with an explanation of what is going wrong.

  1. What’s wrong with this code?
>>> for num in 5:
...     print num

Can’t iterate over integers

It’s not possible to loop over an integer.

So, you’d get the following error:

>>> for num in 5:
...     print num
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable

In order to have numbers in a for loop, you need to have a list of numbers. So, use range!

>>> for num in range(5):
...     print num
  1. What’s wrong with this code?
>>> colors = ["orange", "yellow", "red"]
>>> while color in colors:
...     print colors

While loops need a condition

While loops don’t use the thing in things syntax. To start a while-loop, you must construct some kind of boolean expression, like a comparison of equality (i == 5) or value (i < 6).

So, you’d get the following error:

>>> while color in colors:
...     print color
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'color' is not defined

External Resources


Final Assignment

Create a new repl session called

Part 1: Quiz Questions

  1. Define a list of quiz questions of your choosing. This list should be a list of strings.
  2. Loop through your list of quiz questions (use a for-loop!), and ask the user for the answer to each question (use raw_input).

Part 2: Todo List Program

  1. In the same repl, make a list called todo_list and add one item to it. “Do Laundry” is a safe choice.
  2. Ask the user if they’d like to A) add a todo list item or B) quit. Capture their input into a variable called command.
  3. Start a while-loop. It should keep looping until the command variable is equal to "quit".
  4. In the body of the while loop, check if the user’s command is "add". If so, ask them what they’d like to add using raw_input. Once you have the item they’d like to add, append it to the list called todo_list.
  5. Still in the body of the while-loop, in the last line, prompt the user again for what they’d like to do next: A) add a todo list item or B) quit. Be sure to capture their input in a variable called command. Otherwise, the while loop will never end.

Part 3: Print Todo List

  1. In the same repl (outside of the while-loop), print each item of the todo_list.

Your output should look something like this (it’s ok if you end up with a different order of ingredients):

What is 2 + 2?
What is 5 + 5?
What is the capital of the US?
Washington DC
What would you like to do A) add an item B) quit?
What item would you like to add?
Take dog for a walk
What would you like to do A) add an item B) quit?
Do Laundry
Take dog for a walk