Loops with for and while

Loops reduce repetition, but their stopping rule must be clear.

What you will learn

Loop over a range
Choose between for and while
Avoid an unintended infinite loop

Repeat over known items

A for loop takes each item from an iterable in turn and runs an indented body for it. range(1, 4) produces 1, 2, and 3; the ending number is excluded. This predictable behavior is useful when the number of repetitions is known or when a collection already contains the items to process.

Repeat while a condition holds

A while loop checks its condition before each pass. Something in the loop usually needs to move the program toward making that condition false. If nothing changes, the loop may run forever. Start with a small test value and confirm the update and stopping condition before using larger input.

Use the loop variable clearly

The name after for refers to the current item. It changes each time the loop moves to the next item, so avoid modifying a collection while iterating over it unless you understand the consequences. A descriptive variable name makes the body easier to read than a single letter when the item has meaning.

Choose the simpler loop

Prefer for when processing each item or repeating a known count. Prefer while when the decision to continue depends on a condition that changes during execution, such as waiting for a valid choice. The example uses for because its three values are known in advance.

Trace one pass at a time

For a loop that updates a total, make a small table with the current item and total before and after the pass. This exposes whether each item is counted once and whether the initial value is appropriate. For a while loop, add the condition to the table too; its change should make the stopping point apparent.

total = 0
for number in range(1, 4):
    total += number
print(total)

Expected output

Practice exercise

Use a for loop to print the even numbers 2, 4, and 6.

Show a hint

A range can use a step value.

Show one possible solution

for number in range(2, 7, 2): print(number) visits 2, 4, and 6.

Quick check

What values does range(1, 4) produce? 1, 2, 3 1, 2, 3, 4

Frequently asked questions

When should I use while instead of for?

<p>Use while when repetition should continue until a condition changes, such as retrying input. Use for when iterating over known items or a known range; it makes the number of passes easier to see.</p>