While loops

Introduction to Python for Developers

Jasmin Ludolf

Senior Data Science Content Developer

If statement

Flow of an if statement: start > condition met > perform action > exit

If statement versus while loop

If statement

Flow of an if statement: start > condition met > perform action > exit

While loop

Flow of a while loop: start > condition met > perform action > loop > repeat until condition is no longer met

While loop

while condition:
    action

Any continuous task
- Accelerate while a button is pressed

Games console with a racing game

¹ https://unsplash.com/@joaoscferrao

While loop

ingredients_to_add = 5
items_added = 0

# Keep adding while we have items left
while items_added < ingredients_to_add:

    items_added += 1
    remaining = ingredients_to_add - items_added
    print(remaining, "ingredients left to add")

Output

4 ingredients left to add
3 ingredients left to add
2 ingredients left to add
1 ingredients left to add
0 ingredients left to add

Loop exits when items_added equals ingredients_to_add

A word of caution

while runs continually while the condition is met

ingredients_to_add = 5
items_added = 0

while items_added < ingredients_to_add:
    remaining = ingredients_to_add - items_added
    print(remaining, "ingredients left")

Running forever

ingredients_to_add = 5
items_added = 0

# INFINITE LOOP - never exits!
while items_added < ingredients_to_add:
    remaining = ingredients_to_add - items_added
    print(remaining, "ingredients left")
    # Forgot to increment items_added!

Condition never becomes False
Loop runs forever, program freezes
Common developer mistake

Breaking a loop

while items_added < ingredients_to_add:
    remaining = ingredients_to_add - items_added
    print(remaining, "ingredients left")


    # Terminate the loop
    break

break can also be used in for loops
If the code is already running: Control + C / Command + C

Conditional statements within while loops

ingredients_to_add = 5
items_added = 0


while items_added < ingredients_to_add:
    items_added += 1
    remaining = ingredients_to_add - items_added

    if remaining > 3:
        print("Several ingredients remaining")

    elif remaining >= 1:
        print("Almost done!")

    else:
        print("Shopping list complete!")

Conditional statements output

Several ingredients remaining
Several ingredients remaining
Almost done!
Almost done!
Shopping list complete!

Let's practice!

Introduction to Python for Developers