Iteration using range() in Python - Step-by-Step Execution

Concept Flow - Iteration using range()

Start: i = 0

↓

Check: i < stop?

No→Exit loop

Yes↓

Execute loop body

↓

Update: i = i + 1

↩Back to Check

The loop starts with i at 0, checks if i is less than stop value, runs the loop body if yes, then increases i by 1 and repeats until the condition is false.

Execution Sample

Python

for i in range(3):
    print(i)

This code prints numbers 0, 1, and 2 by looping over range(3).

Execution Table

Iteration	i value	Condition (i < 3)	Action	Output
1	0	True	Print i	0
2	1	True	Print i	1
3	2	True	Print i	2
4	3	False	Exit loop

💡 i reaches 3, condition 3 < 3 is False, loop ends

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	0 (initial)	1	2	3	3 (loop ends)

Key Moments - 3 Insights

Why does the loop stop when i equals 3?

Because the condition i < 3 becomes False at i = 3, so the loop exits as shown in execution_table row 4.

Does the loop include the stop value 3 in the output?

No, the loop runs while i is less than 3, so it stops before reaching 3, as seen in the outputs 0, 1, 2.

What happens if range(3) is changed to range(5)?

The loop will run 5 times with i from 0 to 4, increasing the number of rows in execution_table accordingly.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of i at iteration 2?

A 0

B 1

C 2

D 3

💡 Hint

Check the 'i value' column in execution_table row for iteration 2.

At which iteration does the condition become false and the loop stops?

A 4

B 3

C 2

D 1

💡 Hint

Look at the 'Condition' and 'Action' columns in execution_table to find when the loop exits.

If we change range(3) to range(2), how many times will the loop run?

A 1 time

B 3 times

C 2 times

D 0 times

💡 Hint

Refer to variable_tracker to see how i changes with different range values.

Concept Snapshot

for i in range(stop):
    # loop body

- i starts at 0
- runs while i < stop
- increments i by 1 each loop
- stop value is NOT included
- prints 0 to stop-1

Full Transcript

This visual trace shows how a for loop with range(stop) works in Python. The loop starts with i at 0 and runs as long as i is less than the stop value. Each iteration prints the current i and then increases i by 1. When i reaches the stop value, the condition becomes false and the loop ends. The execution table tracks each iteration's i value, condition check, action, and output. The variable tracker shows how i changes step by step. Key moments clarify why the loop stops before reaching the stop value and what happens if the range changes. The quiz tests understanding of i values and loop stopping conditions.

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