Flow of Control in Python

Indentation and blocks

Python uses indentation (spaces at the start of a line) to group statements into a block. A block starts after a line ending with a colon (if, else, for, while) and all its lines must be indented equally (4 spaces is the standard). Wrong indentation gives an IndentationError.

if marks >= 33:
    print('Pass')      # inside if
print('Done')          # outside, always runs

Selection: if, if-else, if-elif-else

if n > 0:
    print('positive')

if n % 2 == 0:
    print('even')
else:
    print('odd')

if m >= 90:
    g = 'A'
elif m >= 75:
    g = 'B'
elif m >= 50:
    g = 'C'
else:
    g = 'D'

In if-elif-else, only the first true branch runs; the rest are skipped. else is optional and has no condition.

The for loop and range()

for repeats a block once for each item in a sequence.

• range(5) → 0, 1, 2, 3, 4
• range(2, 7) → 2, 3, 4, 5, 6
• range(1, 10, 3) → 1, 4, 7
• range(10, 0, -2) → 10, 8, 6, 4, 2

The stop value is never included. for ch in 'cat': loops over each character.

The while loop

while repeats as long as its condition is true. Something inside must change the condition, or it loops forever (infinite loop).

n = 1
while n <= 5:
    print(n)
    n += 1

Use for when you know how many times to repeat; use while when you repeat until something happens.

break and continue

break ends the loop immediately; the program goes to the first line after the loop.

continue skips the rest of the current turn and goes to the next turn.

for n in range(1, 11):
    if n == 8:
        break
    if n % 3 == 0:
        continue
    print(n)      # 1 2 4 5 7

A loop may have an else part that runs only if the loop ended without break.

Nested loops and pattern programs

A loop inside another loop is a nested loop. For each turn of the outer loop, the inner loop runs completely.

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

Output: * / * * / * * * / * * * * (one row per line). Change what is printed to get number (j) or letter (chr(64 + j)) patterns.

Series programs

A series sum uses an accumulator that starts at 0 (or 1 for products) and adds a term each turn.

# 1 + 1/2 + 1/3 + ... + 1/n
s = 0
for i in range(1, n + 1):
    s += 1 / i

# 1 - x + x**2 - x**3 ... (n terms)
s = 0
for i in range(n):
    s += (-1) ** i * x ** i

Other practical programs: factorial, Fibonacci, prime check, Armstrong, perfect and palindrome numbers, GCD/LCM.

Key formulas and definitions

• range(start, stop, step): stop not included
• if … elif … else: only the first true branch runs
• break: leave loop · continue: next turn
• Accumulator: s = 0; s += term
• Nested loop runs inner loop fully for each outer turn

Worked examples

Common mistakes

• Forgetting the colon after if/for/while, or mixing tabs and spaces.
• Expecting range(1, 5) to include 5.
• Forgetting to change the variable inside while, causing an infinite loop.
• Using several separate ifs when only one branch should run; use elif.