“for” Keyword in Python: Looping Through Iterables

In Python, for is a looping keyword that helps iterate through sequences, such as lists, tuples, dictionaries, and strings. By using for, you can perform various tasks on each item of an iterable. It allows you to execute a block of code repeatedly for each item in an iterable until you’ve looped through all the elements.

The syntax for a for loop is:

for <variable> in <iterable>:
    <statement 1>
    <statement 2>
    ...
    <statement n>

Here, the <iterable> is an object that can return its elements one at a time, and the <variable> refers to each element of the iterable in turn.

In this tutorial, you’ll learn the various ways to use the for keyword to loop through iterables in Python.

Using “for” to Loop Through a List

A list is an ordered collection of elements, and it is iterable in Python. You can use a for loop to cycle through each item of a list and perform operations on them.

Suppose you have a list of languages:

languages = [‘Python’, ‘C++’, ‘Java’, ‘PHP’, ‘JavaScript’]

To loop through this list, you can use the following code:

languages = ['Python', 'C++', 'Java', 'PHP', 'JavaScript']
for language in languages:
    print(language)

This code will output:

Python
C++
Java
PHP
JavaScript

You can use the range() function and the len() function with the for loop to access indices and elements of a list:

languages = ['Python', 'C++', 'Java', 'PHP', 'JavaScript']
for index in range(len(languages)):
    print(languages[index], " is at index ", index)

You can also loop through a list of numbers and perform arithmetic operations on them:

numbers = [1, 2, 3, 4, 5]
sum = 0
for number in numbers:
    sum += number
print("Sum of numbers in the list is: ", sum)

This will output:

Sum of numbers in the list is: 15

Using “for” to Loop Through a Tuple

A tuple is an immutable collection of elements, and it is iterable. You can use for to loop through a tuple just as you would for a list:

fruits = ('apple', 'banana', 'orange')
for fruit in fruits:
    print(fruit)

This will output:

apple
banana
orange

Similarly, you can use the range() function and the len() function with the for loop to access indices and elements of a tuple:

fruits = ('apple', 'banana', 'orange')
for index in range(len(fruits)):
    print(fruits[index], " is at index ", index)

You can’t modify a tuple, but you can define a tuple with just one element by following it with a comma:

t = (5,)

Using “for” to Loop Through a Dictionary

A dictionary is a collection of key-value pairs, and it is iterable in Python. To loop through a dictionary, you can use a for loop that iterates over its keys, values, or items:

person = {'name': 'John', 'age': 24, 'country': 'USA'}
for key in person:
    print(key, ":", person[key])

This will output:

name : John
age : 24
country : USA

You can also use the values() method to loop through the values of a dictionary:

for value in person.values():
    print(value)

This will output:

John
24
USA

Or, you can use the items() method to loop through both key-value pairs:

for key, value in person.items():
    print(key, ":", value)

This will output:

name : John
age : 24
country : USA

Using “for” to Loop Through a String

A string is an iterable in Python, and you can use the for loop to cycle through each character of a string:

greeting = "Hello, World!"
for char in greeting:
    print(char)

This will output:

H
e
l
l
o
,
 
W
o
r
l
d
!

Similarly, you can use the range() function and the len() function with the for loop to access indices and characters of a string:

greeting = "Hello, World!"
for index in range(len(greeting)):
    print(greeting[index], " is at index ", index)

This will output:

H  is at index  0
e  is at index  1
l  is at index  2
l  is at index  3
o  is at index  4
,  is at index  5

W  is at index  7
o  is at index  8
r  is at index  9
l  is at index  10
d  is at index  11
!  is at index  12

Nested “for” Loops

You can use nested for loops to loop through multiple sequences simultaneously. For example, you can loop through a list of colors and a list of fruits:

colors = ['red', 'green', 'blue']
fruits = ['apple', 'banana', 'orange']

for color in colors:
    for fruit in fruits:
        print(color, fruit)

This will output:

red apple
red banana
red orange
green apple
green banana
green orange
blue apple
blue banana
blue orange

Conclusion

The for loop is a powerful tool for iterating through sequences in Python. It helps you automate repetitive tasks and perform operations on each element of an iterable. With the for keyword, you can loop through various types of iterables, such as lists, tuples, dictionaries, and strings. Understanding how to use the for loop correctly is essential for any Python programmer.