Loops in Python

Loops allow us to repeat a block of code multiple times without writing it again.
They are essential for automating repetitive tasks and iterating over collections.

Why use loops?

  • Loops save time and reduce code duplication.
  • They help process lists, strings, dictionaries, and other collections efficiently.
  • Used for repetitive tasks like calculations, data processing, and automation.

Examples of real-time usage:

  • Iterating over a list of users to send notifications
  • Processing each line in a file or CSV
  • Summing numbers in a dataset
  • Generating repeated patterns or reports
  • Running a program until a certain condition is met (e.g., game loop, waiting for input)

We will cover:

  1. for loops → iterate over sequences (lists, strings, ranges)
  2. while loops → repeat while a condition is True
  3. Nested loops → loops inside loops
  4. Loop control → break, continue, pass

Example: for loop

# ---- Example: for loop ----
# Use "for" to iterate over a sequence like a list or range.

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:    # 'fruit' takes each value in the list
    print("I like", fruit)

# Using range() to loop over numbers
for i in range(5):     # 0, 1, 2, 3, 4
    print("Iteration:", i)

I like apple
I like banana
I like cherry
Iteration: 0
Iteration: 1
Iteration: 2
Iteration: 3
Iteration: 4

Example: while loop

# ---- Example: while loop ----
# Use "while" to repeat code as long as a condition is True.

count = 0
while count < 5:   # loop continues while count is less than 5
    print("Count is:", count)
    count += 1     # increment count to eventually stop the loop

Count is: 0
Count is: 1
Count is: 2
Count is: 3
Count is: 4

Example: nested loops

# ---- Example: nested loops ----
# A loop inside another loop. Useful for 2D data or repeated patterns.

for i in range(1, 4):       # outer loop
    for j in range(1, 4):   # inner loop
        print(f"i={i}, j={j}")

i=1, j=1
i=1, j=2
i=1, j=3
i=2, j=1
i=2, j=2
i=2, j=3
i=3, j=1
i=3, j=2
i=3, j=3

Use of break

# ---- break ----
# Stops the loop immediately
for i in range(1, 6):
    if i == 3:
        print("Breaking loop at i =", i)
        break
    print(i)

# ---- continue ----
# Skips current iteration and moves to next
for i in range(1, 6):
    if i == 3:
        print("Skipping i =", i)
        continue
    print(i)

# ---- pass ----
# Does nothing, acts as a placeholder
for i in range(1, 4):
    pass   # can be used when loop syntax is required but no action is needed

1
2
Breaking loop at i = 3
1
2
Skipping i = 3
4
5

Summary

We learned:

  • for loops → iterate over sequences or ranges
  • while loops → repeat while a condition is True
  • Nested loops → one loop inside another
  • Loop control:
    • break → exit loop
    • continue → skip current iteration
    • pass → placeholder, does nothing

Next notebook: 06_functions_basics.ipynb