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Python for Loops and while Loops Explained with Examples (range, break, continue)

P6.5 Python while Loop with Examples

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Why. Sometimes you do not know how many times to repeat. You repeat until something happens: until the stock runs out, until the target is reached.

What. while condition: repeats as long as the condition is True. Something inside must change, or the loop never ends.

How.

Write a safe while loop

  1. Set the starting value.
  2. Write the condition.
  3. Change the value inside the loop.
  4. Check that the condition will become False.

Worked example A — Months to reach a savings goal.

Step 1 — What we do. A nurse in Dublin saves €300 a month to reach €2,000.

Step 2 — Type and run this code.

saved = 0
months = 0
while saved < 2000:
    saved += 300
    months += 1
print(months, "months, saved €", saved)

Step 3 — Output you should see.

7 months, saved € 2100

Step 4 — What it means. The loop repeats until saved reaches 2000 or more: 7 months.

Worked example B — Stock runs down.

Step 1 — What we do. A shop sells 40 units a day from 150 units.

Step 2 — Type and run this code.

stock = 150
day = 0
while stock > 0:
    day += 1
    stock -= 40
print("Out of stock on day", day)

Step 3 — Output you should see.

Out of stock on day 4

Step 4 — What it means. Day 4 the stock goes to zero or below. -= is short for "subtract".

Ravindra Bagale's Tip

If Jupyter keeps running forever, your while condition never becomes False. Press the ■ stop button (Interrupt the kernel) and fix the change inside the loop.