Loops in Python

Loops are used to execute a block of code repeatedly until a condition is met or all items in a sequence are processed. The main types are For loops (iterating over sequences) and While loops (executing code based on a condition).

For Loop

For loops is used to iterate over a sequence such as a list, tuple, string or range. It executes a block of code once for each item in the sequence.

0
1
2
3

Explanation:

• n = 4 stores the ending value for the loop.
• range(0, n) generates numbers from 0 to 3.
• for i in range(0, n): iterates through each number in the range.

Below is the flowchart of For loop:

Iterating by Index of Sequences

A for loop can also iterate through sequence elements using their index values with the help of range() and len().

geeks
for
geeks

Explanation:

• len(a) returns the total number of elements in the list.
• range(len(a)) generates index values from 0 to 2.
• a[idx] accesses list elements using their index.

While Loop

While loop repeatedly executes a block of code as long as the given condition remains true. When the condition becomes false, the line immediately after the loop in the program is executed.

Hello Geek
Hello Geek
Hello Geek

Explanation:

• cnt = 0 initializes the counter variable.
• while (cnt < 3): runs the loop while the condition is true.
• cnt = cnt + 1 increases the counter value by 1.

Below is the flowchart of While loop:

Infinite While Loop

An infinite while loop runs continuously because its condition always remains true.

Explanation:

• while(True): creates a loop with a condition that is always true.
• print("Hello Geek") keeps executing repeatedly inside the loop.

Note: It is suggested not to use this type of loop as it is a never-ending infinite loop where the condition is always true and we have to forcefully terminate the program (or loop execution).

Nested Loops

A nested loop is a loop inside another loop. The inner loop executes completely for every iteration of the outer loop.

1 
2 2 
3 3 3 
4 4 4 4 

Explanation:

• Outer loop for i in range(1, 5) controls the number of rows and Inner loop for j in range(i) runs i times for each row.
• print(i, end=' ') prints the value of i on the same line and print() moves output to the next line after each row.

Related Articles:

• For loop vs while loop
• Eliminating Loop
• Control statements