For Loops in Python

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While loops continue to run until some requirement is met, for loops run a block of code a specific number of times.  Each time the block runs is called an iteration and it can run over a range, string, sequence, anything that can be iterated.

Iterating over a range

In the code below:

# the variable "counter" increases by 1 with each iteration
# the iterations start at the first number in the range
# the iteration cease prior to the last number in the range

for counter in range(0, 11): # <-- (0 included, 11 excluded)
    print(counter) # <-- This block is executed 10 times

The code below does the same thing, we assume a starting point of 0 – so 11 iterations

for counter in range(11):
    print(counter) # <-- This block is executed 10 times

We can loop forwards or backwards:

for counter in reversed(range(1,11)):
    print(counter)

print("!!~~BLAST OFF~~!!")

The for loop by range uses (first, last, step) – first is inclusive, last is exclusive and step determines how much to increment the counter variable by on each iteration.

for counter in (range(0, 21, 2)):
    print(counter)

Outputs:

0
2
4
6
8
10
12
14
16
18
20

Iterating over a string

greeting = "GIDDAY MATE!"

for counter in greeting:
    print(counter)

Commands continue or break

We want to list all of the numbers between 1 and 20 but we must exclude 13 The continue command allows the loop to continue iterating.

for counter in range(1, 21):
    if counter == 13:
        continue
    else:
        print(counter)

The break commend ends the look and breaks out of it

for counter in range(1, 21):
    if counter == 13:
        break
    else:
        print(counter)

More Examples:

# Iterating over a list
fruits = ['apple', 'banana', 'cherry']
for fruit in fruits:
    print(fruit)

# Iterating over a string
word = 'Python'
for letter in word:
    print(letter)

# Iterating over a range
for number in range(1, 5):
    print(number)

Exercise:

Create a program that uses a “for” loop to iterate through a list of numbers and print each number multiplied by 2.

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Python For Loops – Programming for Beginners – (5m:18s)

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