Loops - Cycles of Nature

Welcome, @name! 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.

1animals = ["Lion", "Elephant", "Giraffe", "Python"]
2
3for animal in animals:
4    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.

1# Print expedition days from 1 to 5
2for day in range(1, 6):
3    print(f"Day {day} of the expedition")
4
5# range(5) generates 0, 1, 2, 3, 4
6for i in range(5):
7    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.

1water_supply = 5
2
3while water_supply > 0:
4    print(f"Water remaining: {water_supply} liters")
5    water_supply = water_supply - 1   # use up a liter
6
7print("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

).

1animals = ["Gazelle", "Lion", "Antelope", "Zebra"]
2
3for animal in animals:
4    if animal == "Lion":
5        print("Predator! Stopping observation.")
6        break   # exit the loop
7    print(f"Safe: {animal}")

1numbers = [1, 2, 3, 4, 5, 6]
2
3for number in numbers:
4    if number % 2 != 0:
5        continue   # skip odd numbers
6    print(f"Even number: {number}")

Practice

Write a loop that counts all animals observed during the expedition. Complete the code in the editor:

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!