Mastering For Loops in Python: A Comprehensive Guide

A  for  loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).

This is less like the  for  keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.

With the  for  loop we can execute a set of statements, once for each item in a list, tuple, set etc.

Python for loop Syntax

Python for loop can iterate over a sequence of items. The structure of a for loop in Python is different than that in C++ or Java. That is, for(int i=0; i < n; i++) won’t work here. In Python, we use the ‘in’ keyword.

The syntax for a For Loop in Python is as follows:

for variable in sequence:
    # loop code here

Here, "sequence" could be any iterable object, such as a list, tuple, or string. The "variable" takes on the value of each element in the sequence one by one, with each iteration of the loop.

You can change the name of variable like below:

  
for x in sequence:
    # loop body 

Python for loop flowchart

Here is a flowchart for Python For Loops:

Looping through a list

Let's look at some examples to get a better understanding of For Loops in Python.

fruits = ["Cabbage", "Spinach", "Cauliflower"]
for x in fruits:
  print(x) 

Output:

Cabbage
Spinach
Cauliflower

Looping Through a String

Strings are iterable objects, they contain a sequence of characters: The for statement is used to iterate over a sequence of elements, such as a string. The syntax for the for loop is for variable in sequence:. In this case, the sequence is "Hello", and the loop variable is x.

Loop through the letters in the word "Hello":

Code

for x in "Hello":
  print(x) 

Python For Loop that iterates over a string "Hello". The loop variable x takes on the value of each character in the string "Hello" one by one, with each iteration of the loop.

The output of the code will be:

H
e
l
l
o 

Looping through a range of numbers

for i in range(5):
    print(i)

0
1
2
3
4

Using the range() Function: In the second example, we used the range() function to loop through a range of numbers. The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and stops before a specified number. We can specify the start and step values as well.

for i in range(2, 10, 2):
    print(i)

# Output:
# 2
# 4
# 6
# 8

Python for loop that iterates over a dictionary

Here's an example of a Python for loop that iterates over a dictionary:

my_dict = {'name': 'John', 'age': 30, 'country': 'USA'}

for key in my_dict:
  print(key, ':', my_dict[key])

Explanation:

1. The my_dict dictionary is created, which has three key-value pairs.
2. The for loop starts by iterating over the keys in my_dict.
3. In each iteration, the loop variable key takes on the value of each key in the dictionary.
4. The print function is used to print each key-value pair by accessing the values in the dictionary using square bracket notation my_dict[key].

The output of the code will be:

name : John
age : 30
country : USA

Note: Another way to iterate over a dictionary is to use the items method, which returns a view of the dictionary's key-value pairs as a sequence of (key, value) tuples. Here's an example:

my_dict = {'name': 'Rumman', 'age': 22, 'country': 'India'}
for key, value in my_dict.items():
  print(key, ':', value)

The output of the code will be:

name : Rumman
age : 22
country : India