Loops with for

Loops with for#

The for loop allows you to repeat one or more instructions. To write a for loop you need two things: First a **sequence-like object **(e.g. a list, a string, a dictionary or the output of a function producing sequences). Second, a variable that takes different values as the loop iterates over the sequence. The variable is easier, because it doesn’t matter what name you give it.

Things you can iterate over with for loops:

strings (character by character)
lists
dictionaries
files (line by line)
iterable functions (range, sorted, reversed)

Loops with for are useful whenever you want to repeat instructions a known number of times, or when you wnat to do something for all elements of a sequence. Below you find some examples.

Loops executing a given number of times#

With the range() function, you can set the number of iterations easily:

    
    for i in range(7):
        print(i)

or using an interval:

 for i in range(10, 17):
    print(i)

or backwards:

    
    for i in range(17, 10, -1):
        print(i)

Loops over a string#

With a string as the sequence, you obtain single characters in each iteration.

    
    for char in 'ABCD':
        print(char)

A
B
C
D

Loops over a list#

A list iterates simply once through each element:

    
    for elem in [1, 22, 333, 4444, 55555]:
        print(elem)

Loops over a dictionary#

With a dictionary, the for loop iterates over the keys. Note that the dictionary is inherently unordered. Theoretically, you could get the keys in a different order each time.

    pairs = {'Alice': 'Bob', 'Ada': 'Charlie', 'Visual': 'Basic'}
    for key in pairs:
        print(key)
        print(pairs[key])

Alice
Bob
Ada
Charlie
Visual
Basic

Looping over two lists simultaneously#

Sometimes, you want to look up corresponding items from two lists. A straightforward solution is to loop over an index:

    
    names = ['Alice', 'Bob', 'Charlie', 'Delia']
    jobs = ['admin', 'builder', 'cook', 'developer']

    for i in range(4):
        print(names[i] + ' works as a ' + jobs[i])

Alice works as a admin
Bob works as a builder
Charlie works as a cook
Delia works as a developer

However, the pythonic solution would be to use zip:

    for name, job in zip(names, jobs):
        print(name + ' works as a ' + job)

Alice works as a admin
Bob works as a builder
Charlie works as a cook
Delia works as a developer

Indented block#

All indented commands after the colon are executed within a for loop. The first unindented command is executed after the loop finishes.

    for i in range(5):
        print('inside')
        print('also inside')
    print('outside')

inside
also inside
inside
also inside
inside
also inside
inside
also inside
inside
also inside
outside