For loop in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a for loop in C++?

A for loop is a control structure that repeats a block of code a set number of times. It has three parts: initialization, condition, and update.

beginner

Write the basic syntax of a for loop in C++.

for (initialization; condition; update) {
// code to repeat
}

beginner

What happens if the condition in a for loop is false at the start?

The loop body does not run at all, and the program continues after the loop.

beginner

How can you use a for loop to print numbers from 1 to 5?

for (int i = 1; i <= 5; i++) {
std::cout << i << std::endl;
}

beginner

What is the role of the update expression in a for loop?

The update expression changes the loop variable each time the loop runs, usually to move toward the loop ending condition.

Which part of the for loop runs first?

A Initialization

B Condition check

C Update expression

D Loop body

The initialization runs first to set up the loop variable before checking the condition.

What happens if the condition in a for loop is always true?

A The loop runs once

B The loop runs forever unless broken

C The loop never runs

D The program crashes

If the condition never becomes false, the loop repeats endlessly until a break or exit.

Which symbol is used to separate the parts of a for loop?

A ;

B ,

C :

D .

Semicolons separate initialization, condition, and update in a for loop.

How do you increase the loop variable by 1 each time?

A i = i - 1

B i--

C i++

D i = 0

The i++ expression adds 1 to i each time the loop runs.

What will this loop print? for(int i=0; i<3; i++) { std::cout << i; }

A 321

B 123

C 000

D 012

The loop prints 0, then 1, then 2, all together as 012.

Explain how a for loop works in C++ and describe its three main parts.

Think about how the loop starts, checks if it should continue, runs code, and then changes the variable.

Write a for loop that counts down from 5 to 1 and prints each number.

Start with i = 5, stop when i is less than 1, and decrease i each time.

Practice

1.

What does a for loop in C++ do?

easy

A. Defines a new function

B. Runs code only once

C. Stops the program immediately

D. Repeats a block of code a fixed number of times

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop is designed to repeat code multiple times based on a condition.

2. Step 2: Identify the correct description

   Among the options, only repeating code a fixed number of times matches the for loop's behavior.

3. Final Answer:

   Repeats a block of code a fixed number of times -> Option D

4. Quick Check:

   For loop = repeat fixed times [OK]

Hint: For loops repeat code a set number of times [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing for loops with if statements
• Believing for loops stop the program

2.

Which of the following is the correct syntax for a for loop in C++?

for (___; i < 5; i++) {
    // code
}

easy

A. int i = 0

B. i = 0

C. int i

D. int i == 0

Solution

1. Step 1: Check initialization syntax in for loop

   The initialization part must declare and initialize the loop variable, e.g., int i = 0.

2. Step 2: Compare options for correctness

   Only int i = 0 correctly declares and initializes the variable.

3. Final Answer:

   int i = 0 -> Option A

4. Quick Check:

   Initialization needs declaration and assignment [OK]

Hint: Initialization needs declaration and assignment [OK]

Common Mistakes:

• Using 'int i' without initialization
• Using 'int i == 0' which is a comparison
• Omitting the type declaration

3.

What is the output of this code?

for (int i = 1; i <= 3; i++) {
    std::cout << i * 2 << " ";
}

medium

A. 1 2 3

B. 2 4 6

C. 0 2 4

D. 2 3 4

Solution

1. Step 1: Identify loop iterations

   The loop runs with i = 1, 2, 3 because condition is i <= 3.

2. Step 2: Calculate output for each iteration

   Outputs are i*2: 2, 4, 6 printed with spaces.

3. Final Answer:

   2 4 6 -> Option B

4. Quick Check:

   Multiply i by 2 each loop [OK]

Hint: Multiply loop variable by 2 each iteration [OK]

Common Mistakes:

• Confusing loop range (off by one)
• Printing i instead of i*2
• Missing spaces in output

4.

Find the error in this for loop:

for (int i = 0; i > 5; i++) {
    std::cout << i << " ";
}

medium

A. Variable i is not declared

B. Missing semicolon after i++

C. The condition 'i > 5' is incorrect, loop never runs

D. The loop will run infinitely

Solution

1. Step 1: Analyze the loop condition

   The condition is i > 5 but i starts at 0, so it is false immediately.

2. Step 2: Understand loop execution

   Since condition is false at start, loop body never executes.

3. Final Answer:

   The condition 'i > 5' is incorrect, loop never runs -> Option C

4. Quick Check:

   Condition false at start means no loop runs [OK]

Hint: Check if loop condition is true initially [OK]

Common Mistakes:

• Assuming loop runs despite false condition
• Thinking missing semicolon causes error here
• Confusing variable declaration errors

5.

How can you use a for loop to print all even numbers from 2 to 10 inclusive?

Choose the correct code snippet:

hard

A. for (int i = 2; i <= 10; i += 2) { std::cout << i << ' '; }

B. for (int i = 1; i < 10; i++) { if (i % 2 == 0) std::cout << i << ' '; }

C. for (int i = 2; i < 10; i++) { std::cout << i * 2 << ' '; }

D. for (int i = 0; i <= 10; i += 3) { std::cout << i << ' '; }

Solution

1. Step 1: Understand the goal

   We want to print even numbers 2,4,6,8,10 inclusive.

2. Step 2: Check each option

   for (int i = 2; i <= 10; i += 2) { std::cout << i << ' '; } increments by 2 starting at 2, printing 2,4,6,8,10 correctly.
   for (int i = 1; i < 10; i++) { if (i % 2 == 0) std::cout << i << ' '; } prints 2,4,6,8 but misses 10.
   for (int i = 2; i < 10; i++) { std::cout << i * 2 << ' '; } prints 4,6,8,10,12,14,16,18 (wrong range).
   for (int i = 0; i <= 10; i += 3) { std::cout << i << ' '; } increments by 3, not even numbers.

3. Final Answer:

   for (int i = 2; i <= 10; i += 2) { std::cout << i << ' '; } -> Option A

4. Quick Check:

   Increment by 2 from 2 to 10 [OK]

Hint: Increment loop variable by 2 starting at 2 [OK]

Common Mistakes:

• Using wrong increment step
• Forgetting to include 10
• Using condition inside loop unnecessarily