Loops (repeating actions) in Intro to Computing - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a loop in programming?

A loop is a way to repeat a set of instructions multiple times until a condition is met, like doing the same chore over and over until it's done.

beginner

Name two common types of loops.

Two common loops are:
1. For loop: repeats a set number of times.
2. While loop: repeats while a condition is true.

intermediate

What happens if a loop's condition never becomes false?

The loop runs forever, called an infinite loop. It's like a chore that never ends and can freeze a program.

beginner

Explain a real-life example of a loop.

Imagine washing dishes: you wash one dish, then the next, and keep going until no dishes are left. This is like a loop repeating an action until a condition (no dishes) is met.

beginner

What is the purpose of a loop counter?

A loop counter keeps track of how many times the loop has run, helping to stop the loop after a set number of repeats.

Which loop repeats a fixed number of times?

A For loop

B While loop

C If statement

D Switch case

A for loop repeats a set number of times, like counting from 1 to 5.

What is an infinite loop?

A A loop that never stops

B A loop that runs once

C A loop that counts to zero

D A loop that skips steps

An infinite loop keeps running because its stop condition is never met.

Which part of a loop decides when to stop?

A Counter

B Function

C Variable

D Condition

The condition is checked each time to decide if the loop should continue or stop.

What does a loop counter do?

A Changes the loop condition

B Counts how many times the loop runs

C Stops the program

D Starts the loop

The counter tracks the number of repetitions to help control the loop.

Which loop type repeats while a condition is true?

A Do-while loop

B For loop

C While loop

D Switch loop

A while loop continues as long as its condition remains true.

Describe how a loop works using a real-life example.

Think about doing the same task repeatedly until something changes.

Explain the difference between a for loop and a while loop.

One counts repeats, the other checks a condition each time.

Practice

1. What is the main purpose of a loop in programming?

easy

A. To display text on the screen

B. To store data permanently

C. To repeat a set of actions multiple times

D. To create a new variable

Solution

1. Step 1: Understand the role of loops

   Loops are used to repeat actions without writing the same code again and again.

2. Step 2: Identify the correct purpose

   Repeating a set of actions multiple times matches the purpose of loops.

3. Final Answer:

   To repeat a set of actions multiple times -> Option C

4. Quick Check:

   Loops = Repeat actions [OK]

Hint: Loops repeat tasks to save writing code again [OK]

Common Mistakes:

• Confusing loops with data storage
• Thinking loops create variables
• Believing loops only display text

2. Which of the following is the correct syntax for a for loop in Python to repeat 5 times?

easy

A. for i to 5:

B. for i in range(5):

C. loop i from 1 to 5:

D. repeat 5 times:

Solution

1. Step 1: Recall Python for loop syntax

   Python uses for variable in range(number): to repeat actions a set number of times.

2. Step 2: Match the correct syntax

   for i in range(5): correctly repeats 5 times from 0 to 4.

3. Final Answer:

   for i in range(5): -> Option B

4. Quick Check:

   Python for loop = for i in range(5): [OK]

Hint: Python for loops use 'for variable in range():' [OK]

Common Mistakes:

• Using 'for i to 5:' which is invalid syntax
• Confusing loop syntax with other languages
• Missing colon at the end of the loop line

3. What will be the output of this Python code?

sum = 0
for i in range(3):
    sum += i
print(sum)

medium

A. 6

B. 3

C. 0

D. SyntaxError

Solution

1. Step 1: Trace the loop iterations

   The loop runs with i = 0, 1, 2. sum starts at 0.

2. Step 2: Calculate sum after each iteration

   Iteration 1: sum = 0 + 0 = 0
   Iteration 2: sum = 0 + 1 = 1
   Iteration 3: sum = 1 + 2 = 3

3. Step 3: Sum all values

   Sum of 0 + 1 + 2 = 3, but the code adds i each iteration, so final sum is 3.

4. Final Answer:

   3 -> Option A

5. Quick Check:

   0+1+2 = 3 [OK]

Hint: Add all numbers from 0 up to 2 [OK]

Common Mistakes:

• Adding 3 instead of stopping at 2
• Confusing range(3) with range(1,3)
• Expecting SyntaxError due to indentation

4. Identify the error in this loop code:

i = 1
while i < 4
    print(i)
    i += 1

medium

A. Missing colon after while condition

B. Variable i should start at 0

C. print() should be outside the loop

D. i += 1 should be before print()

Solution

1. Step 1: Check while loop syntax

   Python requires a colon ':' after the while condition to start the loop block.

2. Step 2: Identify the missing colon

   The code misses ':' after while i < 4, causing a syntax error.

3. Final Answer:

   Missing colon after while condition -> Option A

4. Quick Check:

   while condition needs ':' [OK]

Hint: Always put ':' after while condition [OK]

Common Mistakes:

• Ignoring missing colon error
• Changing variable start unnecessarily
• Moving print() outside loop incorrectly

5. You want to print all even numbers from 2 to 10 using a loop. Which code snippet correctly does this?

hard

A. for i in range(2, 11, 2):
print(i)

B. for i in range(1, 11):
if i % 2 == 0:
print(i)

C. i = 2
while i <= 10:
print(i)
i += 2

D. All of the above

Solution

1. Step 1: Analyze each code snippet

   for i in range(2, 11, 2): print(i) uses for loop with step 2 from 2 to 10.
   for i in range(1, 11): if i % 2 == 0: print(i) uses for loop with condition to print even numbers.
   i = 2 while i <= 10: print(i) i += 2 uses while loop increasing by 2 starting at 2.

2. Step 2: Confirm all snippets print even numbers 2 to 10

   All three snippets correctly print 2, 4, 6, 8, 10.

3. Final Answer:

   All of the above -> Option D

4. Quick Check:

   All methods print even numbers 2 to 10 [OK]

Hint: Multiple loop types can print even numbers correctly [OK]

Common Mistakes:

• Using wrong range end value
• Forgetting to increment in while loop
• Not checking condition for even numbers