07 Apr 2026

Control Flow: Conditionals and Loops

If-else decisions, while loops, for loops, ranges, and repeated calculations in Python.

msc semester-i computational-techniques conditionals loops python

A program is not always a straight list of instructions. Sometimes it must choose between alternatives, and sometimes it must repeat a calculation many times. Control flow decides which statements run and how many times they run.

The two basic ideas are decision and repetition. Conditionals handle decisions. Loops handle repetition.

Conditionals

A conditional executes code only when a condition is true.

energy = -13.6

if energy < 0:
    print("bound state")
else:
    print("unbound state")

Use elif when several cases are possible.

temperature = 300

if temperature < 273:
    print("below freezing")
elif temperature == 273:
    print("freezing point")
else:
    print("above freezing")

While loops

A while loop repeats while a condition remains true.

n = 1

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

The update step is important. Without it, the loop may never stop.

A while loop is natural when the stopping condition is based on convergence, such as repeating an iteration until the change becomes smaller than a chosen tolerance.

For loops

A for loop is useful when the number of repetitions is known.

for n in range(1, 6):
    print(n)

The command range(1, 6) gives 1, 2, 3, 4, 5.

A for loop is also useful for processing each value in a list of readings.

Looping over data

measurements = [1.2, 1.4, 1.3, 1.5]

total = 0.0
for value in measurements:
    total = total + value

average = total / len(measurements)
print(average)

This is a typical pattern: initialize, loop, update, and then use the final result.

Nested control flow

Conditionals can be placed inside loops.

for n in range(1, 11):
    if n % 2 == 0:
        print(n, "even")
    else:
        print(n, "odd")

Table of values

Print a table of $x$ and $x^2$ from $x=1$ to $x=5$.

for x in range(1, 6):
    print(x, x**2)

The output is

1 1
2 4
3 9
4 16
5 25

This is a standard use of a for loop: the number of repetitions is known in advance.

Conditional example

Classify a number as positive, negative, or zero.

x = -4

if x > 0:
    print("positive")
elif x < 0:
    print("negative")
else:
    print("zero")

The order of the tests matters. Python checks the conditions from top to bottom and runs the first matching block.

Key points

• Use if, elif, and else for decisions.
• Use while when repetition depends on a condition.
• Use for when processing a known sequence.
• Keep loop bodies small and readable.

Practice questions

1. Write an if-else program to test whether a number is positive or negative.
2. Distinguish between while and for loops.
3. Write a loop to print $x$ and $x^2$ for $x=1$ to $5$.
4. Find the average of values stored in a list using a loop.
5. Why must a while loop contain a proper update step?