Python Loops

A loop statement allows us to execute a statement or group of statements multiple times.

Python has two primitive loop commands:

while loops

for loops

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:

for iterating_var in sequence:
    statements(s)

The first item in the sequence is assigned to the iterating variable iterating_var.

Next, the statements block is executed.

Each item in the list is assigned to iterating_var, and the statement(s) block is executed until the entire sequence is exhausted.

# Program to print the fruits available inside the list
fruits = ["Apple", "Banana", "Cherry","Mango","Pineapple","Jackfruit"]

for x in fruits:
    print(x)
    
Output:
Apple
Banana
Cherry
Mango
Pineapple
Jackfruit

# Program to print the numbers available inside the list
numbers = [0, 1, 5, 10, 15, 45, 50]

for i in numbers:
    print(i)
    
Output:
0
1
5
10
15
45
50

Using else Statement with Loops

If the else statement is used with a for loop, the else statement is executed when the loop has exhausted iterating the list.

# Program to print the numbers available inside the list using else statement
numbers = [0, 1, 5, 10, 15, 45, 50]

for i in numbers:
    print(i)
else:
    print("No items left")
    
Output:
0
1
5
10
15
45
50
No items left

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

# List of numbers
numbers = [0, 1, 5, 10, 15, 45, 50]

# variable to store the sum
sum = 0

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

print("The sum of numbers in the list is", sum)

Output: 
The sum of numbers in the list is 126

Looping Through a String

# Program to print each character of a string
for x in "banana":
    print(x)
    
Output:
b
a
n
a
n
a

The range() function

We can generate a sequence of numbers using range() function.

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.

# Program to generate 10 numbers starting from 0 to 9 using Range Function
print(range(10))

Output:
range(0, 10)

# Program to display the numbers using Range Function
for x in range(10):
    print(x)

Output:
0
1
2
3
4
5
6
7
8
9

# Program to display the numbers between 2 values using Range Function 
for x in range(2,6):
    print(x)

Output:
2
3
4
5

# Program to display the numbers between 2 values using step size
for x in range(1, 30, 2):
    print(x)

Output:
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29

Iterating by Sequence Index

An alternative way of iterating through each item is by index offset into the sequence itself.

# Program to iterate through a list using indexing
fruits = ["Apple", "Banana", "Cherry","Mango","Pineapple","Jackfruit"]

for index in range(len(fruits)):
    print ('Fruit name is :', fruits[index])

Output:
Fruit name is : Apple
Fruit name is : Banana
Fruit name is : Cherry
Fruit name is : Mango
Fruit name is : Pineapple
Fruit name is : Jackfruit

# A list of fruits
fruits = ["Apple", "Banana", "Cherry","Mango","Pineapple","Jackfruit"]

## Read fruits list and enumerate into index and value
# Enumerate() method adds a counter to an iterable and 

returns it in a form of enumerate object.
for index,value in enumerate(fruits):
    print(index,value)
#  print ('Fruit Index is',index,'and Fruit name is :',value)

Output:
0 Apple
1 Banana
2 Cherry
3 Mango
4 Pineapple
5 Jackfruit

Nested For Loop

Python programming language allows to use one for loop inside another for loop.

# Nested for loop example 
for x in range(1,4):  # 1 ,2, 3
    ### Inner for loop	 ###
    for y in range(1,4):
        print(x,y)

Output:
1 1
1 2
1 3
2 1
2 2
2 3
3 1
3 2
3 3

# Nested for loop example
adj = ["red", "big", "tasty"]
fruits = ["apple", "mango", "banana"]

for x in adj:
    for y in fruits:
        print(x, y) 

Output:
red apple
red mango
red banana
big apple
big mango
big banana
tasty apple
tasty mango
tasty banana