Nested loops in C++

Introduction

Nested loops let you repeat actions inside other repeated actions. This helps when you work with tables or grids.

When you want to print a grid of stars or numbers.

When you need to check every pair of items in a list.

When you want to create patterns with rows and columns.

When you process multi-dimensional arrays like tables.

When you want to compare elements in two different lists.

Syntax

C++

for (initialization; condition; update) {
    for (initialization; condition; update) {
        // inner loop code
    }
    // outer loop code
}

The inner loop runs completely every time the outer loop runs once.

You can have more than two loops nested, but keep it simple to avoid confusion.

Examples

This prints pairs of numbers where the first number goes from 1 to 3 and the second from 1 to 2.

C++

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

This prints a 4x4 square of stars.

C++

for (int row = 1; row <= 4; row++) {
    for (int col = 1; col <= 4; col++) {
        std::cout << "* ";
    }
    std::cout << std::endl;
}

Sample Program

This program prints pairs of numbers with a star between them, showing how nested loops work together.

C++

#include <iostream>

int main() {
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            std::cout << i << "*" << j << " ";
        }
        std::cout << std::endl;
    }
    return 0;
}

Important Notes

Remember that the inner loop finishes all its cycles before the outer loop moves to the next step.

Too many nested loops can make your program slow and hard to read.

Summary

Nested loops run one loop inside another to handle repeated tasks in rows and columns.

They are useful for grids, tables, and comparing pairs of items.

Keep loops simple and avoid too many levels to stay clear and fast.

Practice

1. What does a nested loop in C++ mean?
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 2; j++) {
// code
}
}

easy

A. Two loops running one after another

B. A loop inside another loop

C. A loop that runs only once

D. A loop that never ends

Solution

1. Step 1: Understand the code structure

   The code shows one for loop inside another for loop, which means nested loops.

2. Step 2: Define nested loops

   Nested loops mean one loop runs inside another loop's block of code.

3. Final Answer:

   A loop inside another loop -> Option B

4. Quick Check:

   Nested loop = loop inside loop [OK]

Hint: Look for one loop inside another's braces [OK]

Common Mistakes:

• Thinking loops run one after another, not inside
• Confusing nested loops with single loops
• Assuming nested loops run only once

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

easy

A. for(int i=0; i<3; i++) { for(int j=0; j<2; j++) { /* code */ } }

B. for i in range(3): for j in range(2): /* code */

C. while(i<3) { while(j<2) /* code */ }

D. loop(i=0; i<3; i++) { loop(j=0; j<2; j++) { /* code */ } }

Solution

1. Step 1: Identify C++ loop syntax

   C++ uses for(initialization; condition; increment) syntax for loops.

2. Step 2: Check each option

   for i in range(3): for j in range(2): /* code */ uses Python syntax, invalid in C++.
   for(int i=0; i<3; i++) { for(int j=0; j<2; j++) { /* code */ } } uses correct C++ syntax with nested for loops.
   while(i<3) { while(j<2) /* code */ } uses while loops missing initializations and increments.
   loop(i=0; i<3; i++) { loop(j=0; j<2; j++) { /* code */ } } uses invalid loop keyword.

3. Final Answer:

   for(int i=0; i<3; i++) { for(int j=0; j<2; j++) { /* code */ } } -> Option A

4. Quick Check:

   C++ nested loops use for(init; cond; inc) [OK]

Hint: Remember C++ loops use for(init; cond; inc) syntax [OK]

Common Mistakes:

• Using Python or other language syntax in C++
• Missing semicolons in for loop header
• Using wrong keywords like 'loop'

3. What is the output of this C++ code?

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

medium

A. 3 6 9 \n 6 12 18

B. 1 2 3 4 5 6

C. 1 2 3 \n 2 4 6

D. 1 1 1 \n 2 2 2

Solution

1. Step 1: Trace outer loop values

   Outer loop runs i=1 and i=2.

2. Step 2: Calculate inner loop outputs

   For i=1, j=1 to 3: outputs 1*1=1, 1*2=2, 1*3=3.
   For i=2, j=1 to 3: outputs 2*1=2, 2*2=4, 2*3=6.

3. Final Answer:

   1 2 3 2 4 6 -> Option C

4. Quick Check:

   Multiply i and j in nested loops [OK]

Hint: Multiply outer and inner loop counters for output [OK]

Common Mistakes:

• Ignoring the newline after inner loop
• Adding all numbers instead of printing products
• Confusing loop ranges

4. Find the error in this nested loop code:

for(int i = 0; i < 3; i++) {
  for(int j = 0; j < 3; i++) {
    std::cout << i+j << " ";
  }
  std::cout << std::endl;
}

medium

A. Inner loop increments i instead of j

B. Outer loop condition is wrong

C. Missing semicolon after std::cout

D. No error, code runs fine

Solution

1. Step 1: Check inner loop increment

   Inner loop header uses i++ instead of j++, causing infinite loop or wrong behavior.

2. Step 2: Identify correct increment variable

   Inner loop should increment j to progress through its range.

3. Final Answer:

   Inner loop increments i instead of j -> Option A

4. Quick Check:

   Inner loop must increment its own variable [OK]

Hint: Check loop increments match loop variables [OK]

Common Mistakes:

• Incrementing wrong variable in inner loop
• Assuming code runs without checking increments
• Confusing outer and inner loop variables

5. You want to print a 4x4 multiplication table using nested loops in C++. Which code snippet correctly achieves this?

hard

A. for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i-j << " "; } std::cout << std::endl; }

B. for(int i=0; i<4; i++) { for(int j=0; j<4; j++) { std::cout << i+j << " "; } std::cout << std::endl; }

C. for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i+j << " "; } std::cout << std::endl; }

D. for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i*j << " "; } std::cout << std::endl; }

Solution

1. Step 1: Understand multiplication table logic

   Each cell should print the product of row and column numbers (i*j).

2. Step 2: Check each option's output logic

   for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i*j << " "; } std::cout << std::endl; } prints i*j correctly.
   for(int i=0; i<4; i++) { for(int j=0; j<4; j++) { std::cout << i+j << " "; } std::cout << std::endl; } prints sum with zero-based indices.
   for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i+j << " "; } std::cout << std::endl; } prints sum with one-based indices.
   for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i-j << " "; } std::cout << std::endl; } prints difference.

3. Final Answer:

   for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i*j << " "; } std::cout << std::endl; } -> Option D

4. Quick Check:

   Multiplication table = print i*j in nested loops [OK]

Hint: Multiply loop counters for multiplication table [OK]

Common Mistakes:

• Using addition or subtraction instead of multiplication
• Starting loops at 0 instead of 1 for table
• Not printing newline after each row