Iteration using range() in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the range() function do in Python?

The range() function generates a sequence of numbers, which is often used to repeat actions a certain number of times in loops.

beginner

How do you use range() to count from 0 to 4?

Use range(5). It creates numbers starting at 0 up to but not including 5, so 0, 1, 2, 3, 4.

intermediate

What are the three parameters of range() and what do they mean?

The three parameters are start (where to begin), stop (where to end, not included), and step (how much to jump each time). Example: range(1, 10, 2) counts 1, 3, 5, 7, 9.

intermediate

What happens if you use a negative step in range()?

Using a negative step makes the numbers count down. For example, range(5, 0, -1) counts 5, 4, 3, 2, 1.

beginner

Why is range() useful in a for loop?

range() tells the loop how many times to run by giving a sequence of numbers to go through. This helps repeat tasks easily without writing extra code.

What does range(3) produce?

A 3

B 1, 2, 3

C 0, 1, 2, 3

D 0, 1, 2

range(3) starts at 0 and goes up to but not including 3, so it produces 0, 1, 2.

Which range() call counts backwards from 5 to 1?

A range(1, 6, 1)

B range(5, 0, -1)

C range(5, 1)

D range(0, 5, -1)

range(5, 0, -1) starts at 5 and counts down by 1 until it reaches 1 (stop is not included).

What is the output of this code?

for i in range(2, 7, 2): print(i)

A 2 3 4 5 6

B 2 4

C 2 4 6

D 2 3 5

The loop starts at 2, adds 2 each time, and stops before 7, so it prints 2, 4, 6.

What happens if you call range() with no arguments?

A It produces an error

B It produces an empty sequence

C It counts from 0 to 10

D It counts from 1 to 10

range() requires at least one argument; calling it without arguments causes an error.

Which of these is a valid way to use range()?

A range(1, 10, 3)

B range(5, 1, 1)

C range(0, 5, -1)

D range()

range(1, 10, 3) counts from 1 to 9 in steps of 3. The others produce empty sequences or an error.

Explain how to use range() in a for loop to repeat an action 5 times.

Think about how range creates numbers and how the loop uses them.

Describe the role of the start, stop, and step parameters in range().

Imagine walking on steps: where you start, where you stop, and how big your steps are.

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