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Python: loops
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Loops repeat code so you do not have to type it again and again. Python has two kinds: for loops and while loops.
for loops
A for loop repeats a set number of times. It is a count-controlled loop.
for i in range(5):
print("Hi")
prints Hi 5 times.
range(5) gives 0, 1, 2, 3, 4. range(1, 5) gives 1, 2, 3, 4: it stops one before the end number.
range(2, 11, 2) counts up in 2s: 2, 4, 6, 8, 10.
for i in range(5):
print("Hi")
prints Hi 5 times.
range(5) gives 0, 1, 2, 3, 4. range(1, 5) gives 1, 2, 3, 4: it stops one before the end number.
range(2, 11, 2) counts up in 2s: 2, 4, 6, 8, 10.
while loops
A while loop repeats while a condition is True. It is a condition-controlled loop.
count = 10
while count > 0:
count = count - 3
print(count)
count goes 10, 7, 4, 1, -2. When count is no longer more than 0, the loop stops and -2 is printed.
Use while when you do not know in advance how many repeats you need, for example asking until the user types the right password.
count = 10
while count > 0:
count = count - 3
print(count)
count goes 10, 7, 4, 1, -2. When count is no longer more than 0, the loop stops and -2 is printed.
Use while when you do not know in advance how many repeats you need, for example asking until the user types the right password.
Running totals
total = 0
for n in range(1, 5):
total = total + n
print(total)
Each time round, n is added to total: 0 + 1 + 2 + 3 + 4 = 10.
If the condition of a while loop never becomes False, you get an infinite loop.
for n in range(1, 5):
total = total + n
print(total)
Each time round, n is added to total: 0 + 1 + 2 + 3 + 4 = 10.
If the condition of a while loop never becomes False, you get an infinite loop.
What does this print?
total = 0
for n in range(1, 5):
total = total + n
print(total)
- range(1, 5) gives 1, 2, 3, 4.
- total: 1, 3, 6, 10.
Answer: 10
for loops repeat a fixed number of times (range stops one before the end number). while loops repeat while a condition is True. A condition that never becomes False makes an infinite loop.
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