Selection, Repetition and Iteration in Python

The Three Programming Constructs

Every program, regardless of language, is built from three fundamental constructs: sequence, selection, and iteration. Python implements all three. Any problem that can be solved by a computer can be solved using these three constructs alone — a theorem first proved in the 1960s and still the foundation of every language written since.

Selection — if, elif, else

Selection lets a program make decisions. Python uses if, elif (else if), and else to branch between different blocks of code depending on conditions. Only the block whose condition is True executes; all others are skipped.

# Classify an exam score
score = int(input("Enter score: "))

if score >= 70:
    print("Distinction")
elif score >= 50:
    print("Pass")
else:
    print("Fail")

Python checks conditions from top to bottom and executes the first block whose condition is True. Once a match is found, the remaining branches are skipped. If no condition is true, the else block (if present) executes.

Count-Controlled Iteration — for with range()

Definite iteration uses a for loop with range(). You know in advance how many times the loop will run. The loop variable takes each value in the sequence, one at a time.

# Print the 3 times table from 1 to 10
for i in range(1, 11):
    print(i, "x 3 =", i * 3)

range(1, 11) produces the integers 1, 2, 3…10. The end value (11) is excluded — this is one of the most common mistakes students make. range(5) gives 0, 1, 2, 3, 4 (starting from 0). range(1, 6) gives 1, 2, 3, 4, 5.

total = 0
for i in range(1, 5):
    total = total + i
print(total)

Iterating Over a Collection

A for loop can also iterate directly over the items in a list or the characters in a string, without needing range():

grades = ["Pass", "Fail", "Pass", "Distinction"]
for grade in grades:
    print(grade)

Each time the loop runs, grade takes the next value from the list. When all items have been processed, the loop ends automatically. This pattern is used constantly when working with lists of student records, sensor readings, or transaction histories.

Condition-Controlled Iteration — while

Indefinite iteration uses a while loop. The condition is tested before each pass. If the condition is False on first entry, the body never runs.

# Keep asking until a valid score is entered
score = -1
while score < 0 or score > 100:
    score = int(input("Enter a score between 0 and 100: "))
print("Valid score:", score)

The number of iterations is not fixed — it depends entirely on what the user enters. This is indefinite iteration. A while loop is the right choice when you cannot know in advance how many passes will be needed.

Nested Structures

Nesting means placing one construct inside another. Loops can be nested inside loops; if statements can be nested inside loops; and conditions can contain multiple levels of elif. Nested structures are used for processing 2D data, multi-condition menus, and grid operations.

scores = [45, 72, 88, 30, 65]
count = 0
for score in scores:
    if score > 60:
        count = count + 1
print("Scores above 60:", count)

Single Entry and Exit Points

Well-structured code has a single entry point and a single exit point for each block. In Python, each loop or selection block has one entry (the first line) and one exit (when the condition fails or the block ends). Avoid designs that “break out” of the middle of multiple nested loops — these make code harder to trace, test and maintain.