Course 2 · Python BasicsLesson 10 of 14Free to read

while loops · Python from Zero

How do you loop until something happens, when you can't know the count in advance?

Why you need it

"Keep asking for numbers until the user types 0." "Double the money until it passes 1000." You don't know how many rounds that takes. The robot used while not at_goal(); now you'll write conditions with variables.

The intuition

A while loop has three parts. If any is missing you get a bug:

Set up the variable the condition uses.
Check the condition at the top of each round.
Change the variable inside the body, so the condition can eventually become False.

money = 100            # 1. set up
years = 0
while money < 1000:    # 2. check
    money = money * 2  # 3. change
    years = years + 1
print(years)

Reading input until a stop value is a classic pattern: read once before the loop, then read again at the end of every round.

Definition

Loop condition

The loop condition of a while loop is evaluated before each iteration; the loop stops when it becomes False.

Sentinel value

A sentinel value is a special input, such as 0, that signals the end of the data.

Examples

Doubling

Try it yourself

How many doublings until 100 passes 1000?

money = 100
years = 0
while money < 1000:
    money = money * 2
    years = years + 1
print(years, money)

Step by step

L3100 < 1000: True, enter the loop.money = 100
L4Double: 200. years = 1.money = 200, years = 1
L4400 (2), 800 (3), 1600 (4)...money = 1600, years = 4
L31600 < 1000: False, the loop ends.
L6Print once after the loop.4 1600

Sum until 0

Try it yourself

Edit the Input box: any numbers, ending with 0.

total = 0
n = int(input())
while n != 0:
    total = total + n
    n = int(input())
print(total)

Input (one line per input() call)

Step by step

L2Read the first number before the loop.n = 5
L35 != 0, so add it.total = 5
L5Read the next number at the end of the round.n = 8
L3...when 0 is read the condition is False and the loop stops.total = 15

Common mistakes

Forgetting to change the variable

Without n = int(input()) at the end of the body, n never changes and the loop never ends. This site stops it after a few seconds.

for or while?

Know the number of rounds (or have a sequence)? Use for. Waiting for a condition? Use while.

Quick quiz

1. What does this print?

n = 1
while n < 20:
    n = n * 3
print(n)

A
```
27
```
B
```
9
```
C
```
20
```

Show the answer

Answer: A

1 → 3 → 9 → 27. 27 < 20 is False, so the loop stops at 27.

2. What does this print?

i = 5
while i > 0:
    i = i - 2
print(i)

A
```
-1
```
B
```
1
```
C
```
0
```

Show the answer

Answer: A

5 → 3 → 1 → -1. The check happens only at the top of each round.

Practice

3 exercises in three steps: predict what a program prints, change a program to meet a new requirement, then write one from scratch. Your code runs in the browser and is checked the moment you submit.

PredictHalving
Type exactly what this program prints.
ModifyAlso count them
The program adds numbers until 0. Also print, on a second line, how many numbers were added (not counting the 0).
WriteCollatz steps
Read a positive whole number n. Repeat until n is 1: if n is even, n becomes n // 2; otherwise n becomes 3 × n + 1. Print how many steps it took. (For 6: 6 → 3 → 10 → 5 → 16 → 8 → 4 → 2 → 1 is 8 steps.)

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Terms in this lesson

Term	Chinese	Definition
Loop condition	循环条件	The expression a while loop checks before each round.
Sentinel value	结束标记	A special input value that marks the end of the data.
Counter	计数器	A variable that is increased by 1 each time something happens.