Iteration using range() in Python

Introduction

We use range() to repeat actions a certain number of times easily. It helps us count from one number to another without writing many lines.

When you want to print numbers from 0 to 9.

When you need to repeat a task 5 times, like asking a question.

When you want to loop through a list by index.

When you want to create a sequence of numbers for a game score.

When you want to run a timer that counts seconds.

Syntax

Python

range(stop)
range(start, stop)
range(start, stop, step)

start is where counting begins (default is 0).

stop is where counting stops (not included).

step is how much to increase each time (default is 1).

Examples

Counts from 0 up to 4 and prints each number.

Python

for i in range(5):
    print(i)

Counts from 2 up to 5 and prints each number.

Python

for i in range(2, 6):
    print(i)

Counts from 1 to 9, skipping every other number (prints odd numbers).

Python

for i in range(1, 10, 2):
    print(i)

Sample Program

This program prints "Hello" followed by numbers 0, 1, and 2.

Python

for number in range(3):
    print(f"Hello {number}!")

Important Notes

The stop number is never included in the loop.

You can use negative step to count backwards, like range(5, 0, -1).

Summary

range() helps repeat actions by counting numbers.

You can control where to start, stop, and how much to step.

It is useful for loops that need to run a set number of times.

Practice

1. What does range(5) do in a Python for loop?

easy

A. Generates numbers from 0 to 4

B. Generates numbers from 1 to 5

C. Generates numbers from 0 to 5

D. Generates numbers from 1 to 4

Solution

1. Step 1: Understand the range function

   range(5) starts at 0 by default and stops before 5.

2. Step 2: Identify the numbers generated

   The numbers generated are 0, 1, 2, 3, 4.

3. Final Answer:

   Generates numbers from 0 to 4 -> Option A

4. Quick Check:

   range(5) = 0,1,2,3,4 [OK]

Hint: Remember range stops before the end number [OK]

Common Mistakes:

• Thinking range(5) includes 5
• Starting count at 1 by default
• Confusing stop value as inclusive

2. Which of the following is the correct syntax to loop 3 times using range()?

easy

A. for i in range(1, 3):

B. for i in range(3):

C. for i in range(0, 3, 2):

D. for i in range(3, 0):

Solution

1. Step 1: Check each range syntax

   for i in range(1, 3): loops from 1 to 2 (2 times), for i in range(3): loops 0 to 2 (3 times), for i in range(0, 3, 2): loops 0 and 2 (2 times), for i in range(3, 0): runs zero times because start > stop without negative step.

2. Step 2: Identify the correct loop count

   Only for i in range(3): loops exactly 3 times: 0,1,2.

3. Final Answer:

   for i in range(3): -> Option B

4. Quick Check:

   range(3) loops 3 times [OK]

Hint: range(n) loops exactly n times starting at 0 [OK]

Common Mistakes:

• Using range(1, 3) which loops 2 times
• Using invalid range with start > stop
• Confusing step parameter

3. What is the output of this code?

for i in range(2, 7, 2):
    print(i, end=' ')

medium

A. 0 2 4 6

B. 2 3 4 5 6

C. 2 4 6 8

D. 2 4 6

Solution

1. Step 1: Understand the range parameters

   range(2, 7, 2) starts at 2, stops before 7, stepping by 2.

2. Step 2: List the numbers generated

   Numbers are 2, 4, 6.

3. Final Answer:

   2 4 6 -> Option D

4. Quick Check:

   range(2,7,2) = 2,4,6 [OK]

Hint: Start, stop, step control numbers generated [OK]

Common Mistakes:

• Including 7 in output
• Using wrong step size
• Starting from 0 instead of 2

4. Find the error in this code:

for i in range(5, 1):
    print(i)

medium

A. No error, prints 5 to 1

B. SyntaxError due to wrong range syntax

C. Loop does not run because start > stop without step

D. Infinite loop

Solution

1. Step 1: Analyze the range parameters

   range(5, 1) means start at 5, stop before 1, with default step 1.

2. Step 2: Understand loop behavior

   Since start is greater than stop and step is positive, the loop runs zero times.

3. Final Answer:

   Loop does not run because start > stop without step -> Option C

4. Quick Check:

   range(5,1) empty range [OK]

Hint: Positive step needs start < stop to run [OK]

Common Mistakes:

• Expecting loop to run backwards
• Thinking syntax error occurs
• Assuming infinite loop

5. You want to print all odd numbers from 1 to 15 using range(). Which code does this correctly?

hard

A. for i in range(1, 16, 2): print(i)

B. for i in range(0, 15, 2): print(i)

C. for i in range(1, 15): if i % 2 == 1: print(i)

D. for i in range(1, 16): print(i % 2)

Solution

1. Step 1: Identify odd numbers range

   Odd numbers from 1 to 15 are 1,3,5,...,15.

2. Step 2: Check each option

   for i in range(1, 16, 2): print(i) uses range(1,16,2) which generates 1,3,5,...,15 directly. for i in range(0, 15, 2): print(i) starts at 0 (even). for i in range(1, 15): if i % 2 == 1: print(i) misses 15 (range(1,15) goes to 14) and uses extra if check. for i in range(1, 16): print(i % 2) prints 1 or 0, not numbers.

3. Final Answer:

   for i in range(1, 16, 2): print(i) -> Option A

4. Quick Check:

   range(1,16,2) = odd numbers [OK]

Hint: Use step=2 starting at 1 for odd numbers [OK]

Common Mistakes:

• Starting at 0 for odd numbers
• Forgetting to include last number
• Printing modulo instead of number