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- Repetition and iteration: for and while
Repetition and iteration: for and while
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Loops save you from writing the same line again and again. Choosing the right loop is a key exam skill.
Count-controlled: for
Use for when you know how many times to repeat.
range(5) gives 0, 1, 2, 3, 4. range(1, 6) gives 1 to 5. range(0, 10, 2) counts in 2s: 0, 2, 4, 6, 8.
The end value is never included.
range(5) gives 0, 1, 2, 3, 4. range(1, 6) gives 1 to 5. range(0, 10, 2) counts in 2s: 0, 2, 4, 6, 8.
The end value is never included.
Condition-controlled: while
Use while when you repeat until something happens, such as a correct password.
The condition is checked before each pass. If it is never False, the loop runs for ever: an infinite loop.
n = 1
while n < 100:
n = n * 3
print(n)
The condition is checked before each pass. If it is never False, the loop runs for ever: an infinite loop.
n = 1
while n < 100:
n = n * 3
print(n)
Iteration over a data structure
for can visit every item in a list or string in turn.
prices = [4, 10, 3, 8]
total = 0
for p in prices:
total = total + p
print(total)
Output: 25
prices = [4, 10, 3, 8]
total = 0
for p in prices:
total = total + p
print(total)
Output: 25
What does this print?
n = 1
while n < 100:
n = n * 3
print(n)
- n: 1 → 3 → 9 → 27 → 81 → 243
- 243 < 100 is False, so the loop stops.
Answer: 243
for repeats a known number of times (range stops before the end value) or visits every item. while repeats while a condition is True and may run zero or many times.
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