Nested loops in C - Mini Project: Build & Apply

Nested loops

📖 Scenario: You are organizing a small event where you want to print a seating chart. The chart shows rows and seats in each row.

🎯 Goal: Build a program that uses nested loops to print seat numbers for each row.

📋 What You'll Learn

Create two variables for rows and seats per row

Use nested for loops to print seat numbers

Print the seat numbers in the format: Row X Seat Y

💡 Why This Matters

🌍 Real World

Nested loops are useful when dealing with tables, grids, or any situation where you have rows and columns, like seating charts, calendars, or pixel grids.

💼 Career

Understanding nested loops is essential for programming tasks in software development, data processing, and game development where multi-dimensional data is common.

1

Create variables for rows and seats

Create two integer variables called rows and seats_per_row. Set rows to 3 and seats_per_row to 4.

C

// Your code here

Hint

Use int to declare variables and assign the exact values.

2

Set up the outer loop for rows

Add a for loop with variable row that runs from 1 to rows inclusive.

C

int rows = 3;
int seats_per_row = 4;

// Your code here

Hint

Use a for loop starting at 1 and ending at rows.

3

Add inner loop for seats and print seat numbers

Inside the for loop for row, add another for loop with variable seat that runs from 1 to seats_per_row inclusive. Inside the inner loop, print the seat number in the format Row X Seat Y\n using printf.

C

int rows = 3;
int seats_per_row = 4;

for (int row = 1; row <= rows; row++) {
    // Your code here
}

Hint

Use nested loops and printf to print each seat in each row.

4

Print the seating chart

Run the program to print all seat numbers for each row. Use printf as in the previous step to display the output.

C

int rows = 3;
int seats_per_row = 4;

for (int row = 1; row <= rows; row++) {
    for (int seat = 1; seat <= seats_per_row; seat++) {
        // Your code here
    }
}

Hint

Make sure your program prints all rows and seats exactly as shown.

Practice

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

easy

A. Runs one loop inside another, repeating inner loop fully each outer loop

B. Runs two loops side by side without interaction

C. Runs only the outer loop multiple times

D. Runs only the inner loop once

Solution

1. Step 1: Understand nested loops structure

   The outer loop runs 3 times, and for each iteration, the inner loop runs fully 2 times.

2. Step 2: Explain the repetition

   Each time the outer loop runs once, the inner loop runs completely, repeating the inner code multiple times.

3. Final Answer:

   Runs one loop inside another, repeating inner loop fully each outer loop -> Option A

4. Quick Check:

   Nested loops repeat inner loop per outer loop [OK]

Hint: Inner loop runs fully for each outer loop iteration [OK]

Common Mistakes:

• Thinking loops run side by side without nesting
• Assuming inner loop runs only once
• Confusing outer and inner loop roles

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

easy

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

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

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

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

Solution

1. Step 1: Check syntax correctness

   for (int i = 0; i < 3; i++) for (int j = 0; j < 2; j++) { /* code */ } uses correct C syntax with parentheses and braces properly placed.

2. Step 2: Identify syntax errors in other options

   for int i = 0; i < 3; i++ { for int j = 0; j < 2; j++ { /* code */ } } misses parentheses and braces; for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) /* code */ } misses braces for inner loop; for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) { /* code */ } misses closing brace for outer loop.

3. Final Answer:

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

4. Quick Check:

   Correct for loop syntax uses parentheses and braces [OK]

Hint: Remember for loops need parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses around loop conditions
• Missing braces for loop bodies
• Incorrect variable declaration syntax

3. What is the output of this C code?

for (int i = 1; i <= 2; i++) {
  for (int j = 1; j <= 3; j++) {
    printf("%d%d ", i, j);
  }
}

medium

A. 12 13 21 23

B. 11 12 13 21 22 23

C. 123 123

D. 11 21 12 22 13 23

Solution

1. Step 1: Trace outer loop iterations

   Outer loop runs i=1 and i=2.

2. Step 2: Trace inner loop for each outer iteration

   For i=1, inner loop prints 11 12 13; for i=2, inner loop prints 21 22 23.

3. Final Answer:

   11 12 13 21 22 23 -> Option B

4. Quick Check:

   Nested loops print pairs in order [OK]

Hint: Outer loop controls first digit, inner loop second digit [OK]

Common Mistakes:

• Mixing order of printed pairs
• Printing numbers without spaces
• Confusing loop limits

4. Find the error in this nested loop code:

for (int i = 0; i < 3; i++) {
  for (int j = 0; j < 3; j++)
    printf("%d%d\n", i, j);
  }
}

medium

A. Extra closing brace '}' after inner loop

B. Missing semicolon after printf

C. Inner loop missing braces causing syntax error

D. Outer loop condition is incorrect

Solution

1. Step 1: Check braces matching

   There is an extra closing brace after the inner loop which causes a syntax error.

2. Step 2: Verify other syntax parts

   Printf has semicolon; inner loop without braces is allowed for single statement; outer loop condition is correct.

3. Final Answer:

   Extra closing brace '}' after inner loop -> Option A

4. Quick Check:

   Braces must match exactly [OK]

Hint: Count opening and closing braces carefully [OK]

Common Mistakes:

• Adding extra braces accidentally
• Assuming missing braces always cause error
• Ignoring semicolon after statements

5. How many times will the innermost statement run in this nested loop?

for (int i = 1; i <= 4; i++) {
  for (int j = i; j <= 4; j++) {
    for (int k = 1; k <= j; k++) {
      printf("*");
    }
  }
}

hard

A. 70

B. 50

C. 40

D. 30

Solution

1. Step 1: Analyze loop ranges

   i runs 1 to 4; for each i, j runs from i to 4; for each j, k runs 1 to j.

2. Step 2: Calculate total iterations

   Calculate sum over i=1 to 4 of sum over j=i to 4 of j:
   i=1: j=1..4 sum=1+2+3+4=10
   i=2: j=2..4 sum=2+3+4=9
   i=3: j=3..4 sum=3+4=7
   i=4: j=4..4 sum=4
   Total = 10+9+7+4=30

3. Final Answer:

   30 -> Option D

4. Quick Check:

   Total iterations = 10+9+7+4=30 [OK]

Hint: Sum inner loop counts carefully per outer loops [OK]

Common Mistakes:

• Adding loop counts incorrectly
• Ignoring changing start of inner loops
• Confusing loop variables