Loops - Cycles of Nature

Welcome! Nature loves repetition: the sun rises every morning, waves crash on the shore, the herd migrates every year. In programming we also often need to repeat something - check every animal on a list, count expedition days, generate a report every hour. Instead of rewriting the same code many times, we use loops.

The for Loop - Go Through Every Element

The for loop goes through the elements of a collection one by one - like an explorer who observes each animal in the herd one after another.

animals = ["Lion", "Elephant", "Giraffe", "Python"]

for animal in animals:
    print(f"Observed: {animal}")

Python takes the first element of the list, assigns it to the variable animal, and runs the indented block. Then it takes the second element, the third - until the end of the list. Just like with if, there is a colon after the header and an indented block.

range() - Repeat a Specific Number of Times

When we want to do something a specific number of times, we use range(). This function generates a sequence of numbers.

# Print expedition days from 1 to 5
for day in range(1, 6):
    print(f"Day {day} of the expedition")

# range(5) generates 0, 1, 2, 3, 4
for i in range(5):
    print(f"Step {i}")

Remember: range(1, 6) gives numbers from 1 to 5 - the upper bound (6) is not included. And range(5) starts at 0 and gives 0, 1, 2, 3, 4.

The while Loop - Repeat As Long As the Condition Is Met

The while loop repeats code as long as a condition is True. We use it when we don't know in advance how many repetitions we'll need.

water_supply = 5

while water_supply > 0:
    print(f"Water remaining: {water_supply} liters")
    water_supply = water_supply - 1   # use up a liter

print("Out of water!")

Python checks the condition before each repetition. As long as water_supply > 0 is True, it runs the block. Very important: inside the loop we must change the condition (here: decrease the water supply), otherwise the loop will never end - this is called an infinite loop.

break and continue - Controlling the Loop

Sometimes we want to stop a loop early (break) or skip one repetition and move to the next (continue).

animals = ["Gazelle", "Lion", "Antelope", "Zebra"]

for animal in animals:
    if animal == "Lion":
        print("Predator! Stopping observation.")
        break   # exit the loop
    print(f"Safe: {animal}")

numbers = [1, 2, 3, 4, 5, 6]

for number in numbers:
    if number % 2 != 0:
        continue   # skip odd numbers
    print(f"Even number: {number}")

Practice

Write a loop that counts all animals observed during the expedition. Run the loops in the editor and add another animal to the list:

# Loops - counting animals during the expedition

animals = ["Lion", "Elephant", "Giraffe", "Zebra", "Python"]

# TODO: use a for loop to display each animal
for animal in animals:
    print(f"Observed: {animal}")

# TODO: count how many animals there are (count in the loop)
count = 0
for animal in animals:
    count = count + 1

print(f"Observed {count} animals in total")

# TODO: use range() to print 3 expedition days
for day in range(1, 4):
    print(f"Day {day} of the expedition")

Summary

In this lesson you mastered repeating code:

• for - goes through elements of a collection (e.g., a list)
• range() - generates a sequence of numbers for repetitions
• while - repeats as long as a condition is True
• break - stops the loop, continue - skips one repetition
• The danger of an infinite loop

In the last lesson of this block Darwin will show you functions - how to wrap code into a reusable tool!