Lesson 5: Control Flow

In this lesson, we will learn about...

• If...Else statements
• While Loops
• For Loops

If...Else Statements

These allow us to selectively run parts of a program depending on if a condition is met. Let's look at the anatomy of an if statement.

if CONDITION:
    do something
elif CONDITION:
    do another thing
else:
    do a third thing

This corresponds to the following actions in plain English:

1. If the first condition is true, "do something", then skip the rest of the if...else
2. Otherwise, if the second condition is true, "do another thing", then skip the rest of the if...else
3. If none of the above conditions are true, "do a third thing".

NOTE: Only the if is required. You can leave out elif and else entirely, and have as many elifs as you want. You can only have one if and one else.

x = 5
if x % 2 == 0:
    print(x, "is even!")
else:
    print(x, "is odd!")

5 is odd!

Conditions

Conditions are anything that resolve to either True or False. They can be...

• Boolean variables
• Boolean Expressions / Comparisons
• Function Calls

Falsy Values

Some values aren't the literal False but still evaluate to be False. These are called falsy. They are...

• Empty collections (lists, tuples, dictionaries, strings, ...)
• The number 0 in any form
• None
• False

Any other values are considered truthy and evaluate to True.

Comparisons and Logical Operators

Recall from Lesson 3 that there are several operators used for comparisons. There are also operators to combine expressions, such as and and or called logical operators.

Loops

Loops let us run a section of code over and over again. There are two main types of loops in python...

• While Loops
• For Loops

While Loops

These loops run while a condition is true. These conditions are the same as with if statements.

i = 1
while i < 6:
    print(i)
    i += 1

1
2
3
4
5

break and continue

You can use these keywords in a loop to stop the loop (break) or skip to the next iteration (continue).

i = 1
while i < 6:
    if i == 3:
        i += 1
        continue
    elif i == 5:
        break
    print(i)
    i += 1

1
2
4

For Loops

For loops are used to iterate over a sequence or finite range (list, tuple, set, string, ...)

fruits = ["banana", "orange", "strawberry"]

for fruit in fruits:
    print(fruit)

banana
orange
strawberry

range()

The range() function generates a sequence of numbers from 0 (by default) to a number with a step of 1 (by default).

• 1 parameter: 0 to $x-1$
• 2 parameters: $x$ to $y-1$
• 3 parameters: $x$ to $y-1$ in increments of $z$

for x in range(3):
    print(x)

0
1
2

for y in range (1,4):
    print(y)

1
2
3

for z in range (2,10,2):
    print(z)

2
4
6
8

NOTE: break and continue also work here in the same way as for loops.