Python For loop

1. What is a Python For Loop?

A for loop in Python requires at least two variables to work. The first is the iterable object such as a list, tuple or a string. And second is the variable to store the successive values from the sequence in the loop.

1.1. Python For Loop Syntax

In Python, you can use the “for” loop in the following manner.

for iter in sequence:
    statements(iter)

The “iter” represents the iterating variable. It gets assigned with the successive values from the input sequence.

The “sequence” may refer to any of the following Python objects such as a list, a tuple or a string.

1.2. For Loop WorkFlow in Python

The for loop can include a single line or a block of code with multiple statements. Before executing the code inside the loop, the value from the sequence gets assigned to the iterating variable (“iter”).

Below is the flowchart representation of a Python For Loop.

Regular Python For Loop Flowchart

1.3.1. Python For Loop Example – Print Characters of a String

vowels="AEIOU"
for iter in vowels:
    print("char:", iter)

The above code is traversing the characters in the input string named as the vowels. Its output is as follows.

char: A
char: E
char: I
char: O
char: U

1.3.2. Python For Loop Example – Find the Average of N Numbers

We’ll use the following steps to calculate the sum of N numbers.

1. Create a list of integers and populate with N (=6) values.

2. Initialize a variable (sum) for storing the summation.

3. Loop N (=6) number of times to get the value of each integer from the list.

4. In the loop, add each value with the previous and assign to a variable named as the sum.

5. Divide the “sum” with N (=6). We used the len() function to determine the size of our list.

6. The output of the previous step is the average we wanted.

7. Finally, print both the “sum” and the average.

Below is the Python code for the above program.

int_list = [1, 2, 3, 4, 5, 6]
sum = 0
for iter in int_list:
    sum += iter
print("Sum =", sum)
print("Avg =", sum/len(int_list))

Here is the output after executing the above code.

Sum = 21
Avg = 3.5

2. Range() Function with For Loop

2.1. What is Range() Function?

The range() function can produce an integer sequence at runtime. For example, a statement like range(0, 10) will generate a series of ten integers starting from 0 to 9.

Below snippet is interpreting more about the functional aspect of the range() function.

>>> type(range(0, 10))
<class 'range'>
>>> range(0, 10)[0]
0
>>> range(0, 10)[1]
1
>>> range(0, 10)[9]
9
>>> len(range(0, 10))
10
>>>

2.2. Range() Function Example

Let’s now use the range() with a “for” loop.

for iter in range(0, 3):
    print("iter: %d" % (iter))

It will yield the following result.

iter: 0
iter: 1
iter: 2

By default, the “for” loop fetches elements from the sequence and assigns to the iterating variable. But you can also make the “for” loop returning the index by replacing the sequence with a range(len(seq)) expression.

books = ['C', 'C++', 'Java', 'Python']
for index in range(len(books)):
   print('Book (%d):' % index, books[index])

The following lines will get printed.

Book (0): C
Book (1): C++
Book (2): Java
Book (3): Python

3. Else Clause with Python For Loop

Interestingly, Python allows using an optional else statement along with the “for” loop.

The code under the else clause executes after the completion of the “for” loop. However, if the loop stops due to a “break” call, then it’ll skip the “else” clause.

3.1. Syntax

# Foe-Else Syntax

for item in seq:
    statement 1
    statement 2
    if <cond>:
        break
else:
    statements

Look at the below For Loop with Else flowchart.

3.2. For-Else Flowchart

3.3. For-Else Example

birds = ['Belle', 'Coco', 'Juniper', 'Lilly', 'Snow']
ignoreElse = False

for theBird in birds:
    print(theBird )
    if ignoreElse and theBird is 'Snow':
        break
else:
    print("No birds left.")

The above code will print the names of all birds plus the message in the “else” part.

Belle
Coco
Juniper
Lilly
Snow
No birds left.

Setting the “ignoreElse” variable to “True” will get the “else” part ignored. And only the names will get displayed.

Python For Loop Summary

In this tutorial, we covered “Python for Loop” and a couple of ways to use it in real Python programs. If you have any question about this topic, please do write to us.