Unit 11.1A · Term 1

For Loop & Range

The for loop iterates over a sequence a definite number of times. Combined with the range() function, it gives you precise control over start, stop, and step values — making it perfect for counting, iterating over sequences, and generating number patterns.

Learning Objectives

11.1.2.4 Write program code using a For loop
11.1.2.5 Define a range of values for a loop

Lesson Presentation

11.1A-lesson-06-for-loop.pdf · Slides for classroom use

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Conceptual Anchor

The Measuring Tape Analogy

A for loop with range() is like walking along a measuring tape. range(start, stop, step) tells you: where to start, where to stop (but not including this mark), and how big each step is. Once you reach or pass the stop mark, you're done — you know exactly how many steps you'll take before you begin.

Rules & Theory

range() Function

Form	Meaning	Example	Values Generated
`range(stop)`	0 to stop-1, step 1	`range(5)`	0, 1, 2, 3, 4
`range(start, stop)`	start to stop-1, step 1	`range(2, 6)`	2, 3, 4, 5
`range(start, stop, step)`	start to stop-1, step N	`range(0, 10, 2)`	0, 2, 4, 6, 8
`range(start, stop, -step)`	Counting down	`range(5, 0, -1)`	5, 4, 3, 2, 1

Critical: Stop value is EXCLUDED

range(1, 5) generates 1, 2, 3, 4 — NOT including 5. This is the most common source of off-by-one errors. To include 5, use range(1, 6).

For Loop Patterns

# Pattern 1: Simple counting
for i in range(5):         # i = 0, 1, 2, 3, 4
    print(i)

# Pattern 2: Custom range
for i in range(1, 11):     # i = 1, 2, ..., 10
    print(i)

# Pattern 3: Step
for i in range(0, 21, 5):  # i = 0, 5, 10, 15, 20
    print(i)

# Pattern 4: Countdown
for i in range(10, 0, -1): # i = 10, 9, ..., 1
    print(i)
print("Go!")

# Pattern 5: Iterating over a string
for char in "Python":
    print(char)            # P y t h o n (each on new line)

# Pattern 6: Accumulator with for
total = 0
for i in range(1, 101):
    total += i
print("Sum 1-100:", total) # 5050

For vs While

Aspect	for loop	while loop
Use when	Number of iterations is known	Number of iterations is unknown
Counter	Managed automatically by range()	You must manage manually
Infinite loop risk	Very low	High if condition not updated
Example	Process exactly 10 items	Read until user types "quit"

Worked Examples

1 Multiplication Table

num = int(input("Enter a number: "))   # e.g., 7

for i in range(1, 11):
    result = num * i
    print(f"{num} x {i} = {result}")

# Output for 7:
# 7 x 1 = 7
# 7 x 2 = 14
# ...
# 7 x 10 = 70

2 Even Numbers in a Range

start = int(input("Start: "))   # e.g., 1
end = int(input("End: "))       # e.g., 20

print("Even numbers:")
for i in range(start, end + 1):
    if i % 2 == 0:
        print(i, end=" ")
print()

# Alternative using step:
for i in range(2, end + 1, 2):
    print(i, end=" ")

3 Power Table with Trace

# Print powers of 2 from 2^0 to 2^8
print("n\t2^n")
print("-" * 16)
for n in range(9):
    print(f"{n}\t{2**n}")

# Trace table:
# n | 2**n | Output
# 0 | 1    | 0    1
# 1 | 2    | 1    2
# 2 | 4    | 2    4
# 3 | 8    | 3    8
# ...
# 8 | 256  | 8    256

4 break and continue

# break — exit the loop immediately
for i in range(1, 100):
    if i * i > 50:
        print(f"First square > 50: {i}^2 = {i*i}")
        break

# continue — skip to the next iteration
for i in range(1, 11):
    if i % 3 == 0:
        continue           # Skip multiples of 3
    print(i, end=" ")
# Output: 1 2 4 5 7 8 10

Pitfalls & Common Errors

Modifying the Loop Variable

Changing i inside a for loop has no effect — it resets at the start of each iteration. Unlike while, the for loop controls i automatically.

Empty Range

range(5, 2) produces no values (start > stop with positive step). For counting down, use a negative step: range(5, 2, -1).

range() with Float

range(0.0, 1.0, 0.1) causes a TypeError. range() only works with integers.

Pro-Tips for Exams

Range Counting Formula

range(a, b) produces b - a values
range(a, b, s) produces ceil((b - a) / s) values
If the question says "from 1 to N including N", use range(1, N + 1)
When tracing, write out the sequence generated by range() first, then iterate

Graded Tasks

Remember

Write the values generated by: range(3, 15, 3) and range(10, 0, -2).

Understand

Explain why range(1, 10) does not include 10. How would you include it?

Apply

Write a program that calculates the factorial of N (N!) using a for loop.

Apply

Write a program to print all odd numbers between 1 and 50 using range() with a step.

Analyze

How many times does for i in range(2, 100, 7) execute? List the first 5 and last 2 values of i.

Create

Create a program that prints an alphabet pattern: Line 1: A, Line 2: AB, Line 3: ABC, ..., Line 26: ABC...XYZ. Use chr() and ord().

Self-Check Quiz

1. What values does range(5) generate?

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2. What does range(2, 8, 2) produce?

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3. How do you count from 10 down to 1?

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4. Can you use range() with floats?

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5. What does break do inside a for loop?

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