6.1b Sequence, Selection & Iteration — Edexcel 1CP2

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The Three Programming Constructs

All programs — no matter how complex — are built from just three fundamental constructs: sequence, selection, and iteration.

1. Sequence

Sequence means instructions are executed one after another, in order, from top to bottom. This is the default in Python.

# Sequence example
name = input("Name: ")
age = int(input("Age: "))
year_of_birth = 2025 - age
print(name + " was born in " + str(year_of_birth))

2. Selection (if/elif/else)

Selection allows the program to make decisions and execute different code depending on a condition. Python uses if, elif (else if), and else.

# Selection: if / elif / else
score = int(input("Enter score: "))
if score >= 70:
    print("Grade: A")
elif score >= 60:
    print("Grade: B")
elif score >= 50:
    print("Grade: C")
else:
    print("Grade: F")

Key rules for selection:

Conditions use comparison operators: ==, !=, >, <, >=, <=
Indentation (4 spaces or 1 tab) is mandatory in Python — it defines the block
elif is optional and can be repeated; else is optional and covers all remaining cases

3. Iteration (loops)

Iteration allows code to repeat. Python has two types of loop:

for loop — count-controlled iteration

Used when you know exactly how many times to repeat:

# for loop — repeats a fixed number of times
for i in range(5):       # i = 0, 1, 2, 3, 4
    print("Count:", i)

# range(start, stop, step)
for i in range(1, 11):    # 1 to 10 (not 11)
    print(i)

for i in range(0, 10, 2): # 0, 2, 4, 6, 8
    print(i)

while loop — condition-controlled iteration

Used when you don't know how many times to repeat — loop continues as long as a condition is True:

# while loop — repeats while condition is True
password = ""
while password != "secret123":
    password = input("Enter password: ")
print("Access granted!")

Comparison: for vs while

for loop	while loop
Count-controlled (fixed number of iterations)	Condition-controlled (unknown iterations)
Uses range() or iterates over a collection	Continues while condition is True
Loop variable managed automatically	Must manually manage the condition variable
Example: repeat 10 times	Example: keep asking until valid input

Nested Structures

You can put loops inside loops, and if statements inside loops:

# Nested loop example — multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(i * j, end=" ")
    print()

# Selection inside a loop
for i in range(1, 11):
    if i % 2 == 0:
        print(i, "is even")
    else:
        print(i, "is odd")

Exam tip: The Edexcel paper 2 is a practical Python exam on computer. You'll need to write working code. Practice tracing through loops mentally — know what each iteration does. Common exam tasks: count values in a list, find maximum, validate user input using while.

⚠️ Common Mistakes

Missing the colon after if/elif/else/for/while — causes SyntaxError
Wrong indentation — Python uses indentation to define blocks; inconsistent indentation causes IndentationError
Using = (assignment) instead of == (comparison) in an if condition
range(10) gives 0–9 (not 1–10); range(1, 11) gives 1–10
Infinite while loop — forgetting to update the condition variable inside the loop

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Worksheet — 6.1b Sequence, Selection & Iteration

8 Edexcel-style questions · instantly marked

Q1Name the three fundamental programming constructs and briefly describe each.[6]

✅ Mark scheme

Sequence: instructions executed one after another, in order [1]; Selection: a decision/condition that determines which code block executes [1]; Iteration: code that repeats (loops) [1]. 1 mark each for correct name, 1 mark each for correct description. Max 6.

Q2Write Python code using a while loop that keeps asking the user to enter a number between 1 and 10 until they do so. When valid, print "Valid input!"[4]

✅ Mark scheme

num = int(input("Enter a number 1-10: ")) [1]; while num < 1 or num > 10: [1]; (indented) num = int(input("Enter a number 1-10: ")) [1]; print("Valid input!") [1].

Q3Trace through this code and state what is printed: for i in range(2, 10, 3): print(i)[2]

✅ Mark scheme

Prints: 2 [1]; 5 [1]; 8 [1] — range(2, 10, 3) generates 2, 5, 8 (stops before 10). [Award 2 marks for all three correct, 1 mark for two correct.]

Q4A student writes this code. Identify ALL errors: if score > 50 print("Pass") else print("Fail")[2]

✅ Mark scheme

Missing colon after condition (should be if score > 50:) [1]; missing colon after else (should be else:) [1]; also missing indentation on both print statements (accept this as a third error).

Q5Explain when you would use a for loop rather than a while loop.[2]

✅ Mark scheme

Use a for loop when the number of iterations is known in advance/fixed [1]; e.g. counting from 1 to 10, or processing each item in a list. Use a while loop when the number of iterations depends on a condition that may change at runtime [1].

Q6Write Python code to print all even numbers from 0 to 20 using a for loop and selection (if statement).[3]

✅ Mark scheme

for i in range(0, 21): [1] (accept range(21) or range(0,22,2) — give full marks for range(0,22,2) without if statement); if i % 2 == 0: [1]; print(i) [1]. (Alt: for i in range(0, 21, 2): print(i) — award 3 marks.)

Q7What is an infinite loop? Give an example of code that would cause one and explain how to fix it.[3]

✅ Mark scheme

A loop that never terminates because its exit condition is never met [1]; Example: count = 1; while count > 0: print(count) — count stays positive forever [1]; Fix: update the condition variable inside the loop so it eventually becomes False, e.g. count -= 1 [1].

Q8Write a Python program that asks a user to enter 5 numbers one at a time, then prints the largest number entered.[5]

✅ Mark scheme

largest = None (or ask first number before loop) [1]; for i in range(5): [1]; num = int(input("Enter number: ")) [1]; if largest is None or num > largest: [1]; largest = num [1]; print("Largest:", largest) — awarded even if outside loop [1]. Max 5.

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Term	Definition

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The Three Programming Constructs

1. Sequence

2. Selection (if/elif/else)

3. Iteration (loops)

for loop — count-controlled iteration

while loop — condition-controlled iteration

Comparison: for vs while

Nested Structures

Worksheet — 6.1b Sequence, Selection & Iteration

Mini Test — Sequence, Selection & Iteration