Python’s for Loop

You’ve seen the while loop but, with the introduction of another iterable, i.e., the list (the string being the first), it’s time to learn the for loop, a means of repeating code blocks for the extent of an iterable’s length.

The while loop typically is designed to repeat a specific block of code an unknown number of times until a condition is met. In some cases, it is intended to repeat without end.

But if the number of loops can be known ahead of time, you typically will use the for loop.

When we say the number of loops “can be known ahead of time,” that doesn’t necessarily mean a specific number. Rather, it means the number of repetitions can be known with respect to something else. Thus, when you know you need to iterate over a string or list, you know that the repetitions will be no more than the size of the string or list or whatever other iterable you are moving through.

Syntax

The most basic and common for loop contains two keywords (for and in) and two variable names:

for var in some_iterable:
    # code block

In the for loop above, the var is simply the variable name you provide and some_iterable is just that – a string or list or some other iterable over which you loop. var could be any variable name upon which you decide – following all the same rules for variable naming – and upon each iteration through the loop, the var will be assigned to the next item in the iteratable.

Thus:

names = ['George', 'John', 'James', 'Benjamin']

for name in names:
    print(name)

# George
# John
# James
# Benjamin

Use of range in for Loop

Python’s range() function creates a series of integers on the fly …

for i in range(10):
    #
    print(n)

# 0
# 1
# 2
# 3
# 4
# 5
# 6
# 7
# 8
# 9

… and it works quite well with for loops because the integers generated can be used to index into sequences like lists and strings.

names = ['George', 'John', 'James', 'Benjamin']

for i in range(len(names)):
    print(i, names[i])

# 0 George
# 1 John
# 2 James
# 3 Benjamin