Python for Loop

The for loop in Python is used to iterate over a sequence (list, tuple, string) or other iterable objects. Iterating over a sequence is called traversal.

Syntax of for Loop

for val in sequence:
    Body of for

Here, val is the variable that takes the value of the item inside the sequence on each iteration. Loop continues until we reach the last item in the sequence. The body of for loop is separated from the rest of the code using indentation.

Flowchart of for Loop

Example: Python for Loop

# Program to find
# the sum of all numbers
# stored in a list

# List of numbers
numbers = [6,5,3,8,4,2,5,4,11]

# variable to store the sum
sum = 0

# iterate over the list
for val in numbers:
    sum = sum+val

# print the sum
print("The sum is",sum)

Output

The sum is 48

The range() function

We can generate a sequence of numbers using range() function. range(10) will generate numbers from 0 to 9 (10 numbers). We can also define the start, stop and step size as range(start,stop,step size). step size defaults to 1 if not provided. This function does not store all the values in memory, it would be inefficient. So it remembers the start, stop, step size and generates the next number on the go. To force this function to output all the items, we can use the function list().
The following example will clarify this.

>>> range(10)
range(0, 10)
>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> list(range(2,8))
[2, 3, 4, 5, 6, 7]
>>> list(range(2,20,3))
[2, 5, 8, 11, 14, 17]

We can use the range() function in for loops to iterate through a sequence of numbers. It can be combined with the len() function to iterate though a sequence using indexing. Here is an example.

# Program to iterate
# through a list
# using indexing

# List of genre
genre = ['pop','rock','jazz']

# iterate over the list using index
for i in range(len(genre)):
    print("I like",genre[i])

Output

I like pop
I like rock
I like jazz

for loop with else

A for loop can have an optional else block as well. The else part is executed if the items in the sequence used in for loop exhausts. break statement can be used to stop a for loop. In such case, the else part is ignored. Hence, a for loop's else part runs if no break occurs.
Here is an example to illustrate this.

# Program to show
# the control flow
# when using else block
# in a for loop

# a list of digit
list_of_digits = [0,1,2,3,4,5,6]

# take input from user
input_digit = int(input("Enter a digit: "))

# search the input digit in our list
for i in list_of_digits:
    if input_digit == i:
        print("Digit is in the list")
        break
else:
    print("Digit not found in list")

Output 1

Enter a digit: 3
Digit is in the list

Output 2

Enter a digit: 9
Digit not found in list

Here, we have a list of digits from 0 to 6. We ask the user to enter a digit and check if the digit is in our list or not. If the digit is present, for loop breaks prematurely. So, the else part does not run. But if the items in our list exhausts (digit not found in our list), the program enters the else part.