For Loop

​
What is for loop in Python?
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
1
for val in sequence:
2
	# Body of for loop
Copied!
Here, val is the variable that takes the value of the element 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
Flowchart of for Loop in Python
Flowchart of for Loop in Python programming
Example: Python for Loop
1
# Program to find the sum of all numbers stored in a list
2
​
3
# List of numbers
4
numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11]
5
​
6
# variable to store the sum
7
sum = 0
8
​
9
# iterate over the list
10
for number in numbers:
11
	sum = sum + number
12
​
13
print("The sum is", sum)
Copied!
When you run the program, the output will be:
1
The sum is 48
Copied!
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.
The range object is "lazy" in a sense because it doesn't generate every number that it "contains" when we create it. However, it is not an iterator since it supports in, len and __getitem__ operations.
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.
1
print(range(10))
2
​
3
print(list(range(10)))
4
​
5
print(list(range(2, 8)))
6
​
7
print(list(range(2, 20, 3)))
Copied!
Output
1
range(0, 10)
2
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
3
[2, 3, 4, 5, 6, 7]
4
[2, 5, 8, 11, 14, 17]
Copied!
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 through a sequence using indexing. Here is an example.
1
# Program to iterate through a list using indexing
2
​
3
genre = ['pop', 'rock', 'jazz']
4
​
5
for g in genre:
6
	print("I like", g)
7
​
8
# iterate over the list using index
9
for i in range(len(genre)):
10
	print("I like", genre[i])
Copied!
Output
1
I like pop
2
I like rock
3
​I like jazz
Copied!
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.
The break keyword can be used to stop a for loop. In such cases, the else part is ignored.
Hence, a for loop's else part runs if no break occurs.
Here is an example to illustrate this.
1
digits = [0, 1, 5]
2
​
3
for i in digits:
4
    print(i)
5
else:
6
    print("No elements left.")
Copied!
When you run the program, the output will be:
1
0
2
1
3
5
4
No elements left.
Copied!
Here, the for loop prints items of the list until the loop exhausts. When the for loop exhausts, it executes the block of code in the else and prints No items left.
This for...else statement can be used with the break keyword to run the else block only when the break keyword was not executed. Let's take an example:
1
# program to display people's ages from record
2
person_name = 'Daniel'
3
​
4
people = {'John': 45, 'Diana': 32, 'Artur': 90}
5
​
6
for person in people:
7
    if person == person_name:
8
        print(people[person])
9
        break
10
else:
11
    print('No entry with that name found.')
Copied!
Output
1
No entry with that name found.
Copied!

if...else Statement

Functions

Last modified 1yr ago

Copy link

Contents

What is for loop in Python?

The range() function

for loop with else