4.2 Loop Control: while loop, for loop (BT101CO)

The while loop is condition-based. It repeats as long as a certain boolean condition remains True. It is best used when you don't know exactly how many times you need to repeat the task.

# Syntax:
while condition:
    # code to repeat
    # update condition

Solved Example (Counting):

count = 1
while count <= 5:
    print("Iteration:", count)
    count += 1  # Incrementing the counter

graph TD
    A[Start] --> B{Condition True?}
    B -- Yes --> C[Execute Code Block]
    C --> D[Update Condition]
    D --> B
    B -- No --> E[End]
      

The for loop is collection-based. It is used to iterate over a sequence (like a list, string, or range). It is best used when you know the number of iterations in advance.

# Syntax:
for variable in sequence:
    # code to repeat

Solved Example (String Iteration):

for char in "PYTHON":
    print(char, end="-")
# Output: P-Y-T-H-O-N-

The for loop is frequently used with the range() function to generate a sequence of numbers.

• range(5) → 0, 1, 2, 3, 4
• range(1, 6) → 1, 2, 3, 4, 5
• range(1, 10, 2) → 1, 3, 5, 7, 9 (starts at 1, ends before 10, steps by 2)

Solved Example (Sum of first N numbers):

n = 5
total = 0
for i in range(1, n + 1):
    total += i
print(f"The sum of first {n} numbers is {total}")
# Output: The sum of first 5 numbers is 15

Sometimes you need to change the behavior of a loop while it is running. Python provides three keywords for this:

• break: Exits the loop immediately, regardless of the condition.
• continue: Skips the rest of the current iteration and jumps to the next one.
• pass: A "do nothing" placeholder used when a statement is syntactically required but you don't want any command to execute.

Example of break and continue:

for num in range(1, 10):
    if num == 5:
        break  # Stops the loop entirely when num is 5
    if num % 2 == 0:
        continue  # Skips printing even numbers
    print(num, end=" ")
# Output: 1 3

You can place a loop inside another loop. This is commonly used for working with multi-dimensional data or creating patterns.

Example A: Multiplication Table

for i in range(1, 3):       # Outer loop (rows)
    for j in range(1, 4):   # Inner loop (columns)
        print(f"{i}*{j}={i*j}", end="\t")
    print() # New line after inner loop finishes

Example B: Star Patterns (Ladders)

for i in range(1, 6):
    for j in range(1, i + 1):
        print("*", end=" ")
    print()

* 
* * 
* * * 
* * * * 
* * * * *

for i in range(5, 0, -1):
    for j in range(1, i + 1):
        print("*", end=" ")
    print()

* * * * * 
* * * * 
* * * 
* * 
* 

for i in range(1, 6):
    for j in range(1, i + 1):
        print(j, end=" ")
    print()

1 
1 2 
1 2 3 
1 2 3 4 
1 2 3 4 5

n = 5
for i in range(1, n + 1):
    # Print leading spaces
    for j in range(n - i):
        print(" ", end=" ")
    # Print stars
    for k in range(i):
        print("*", end=" ")
    print()

        * 
      * * 
    * * * 
  * * * * 
* * * * *

n = 5
for i in range(1, n + 1):
    print(" " * (n - i) + "*" * (2 * i - 1))

    *
   ***
  *****
 *******
*********

n = 5
for i in range(n):
    for j in range(n):
        if i == 0 or i == n-1 or j == 0 or j == n-1:
            print("*", end=" ")
        else:
            print(" ", end=" ")
    print()

* * * * * 
*       * 
*       * 
*       * 
* * * * *

num = 1
for i in range(1, 6):
    for j in range(1, i + 1):
        print(num, end=" ")
        num += 1
    print()

1 
2 3 
4 5 6 
7 8 9 10 
11 12 13 14 15