Nested loop execution in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested loops with break

What is the output of the following C# code?

C Sharp (C#)

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (j == 2) break;
        Console.Write(i * j + " ");
    }
}

A 1 2 3 4 5 6

B 1 3 1 3 1 3

C 1 2 3

D 1 2 2 4 3 6

💡 Hint

Remember that 'break' exits the inner loop immediately when j equals 2.

The inner loop breaks when j == 2, so only j == 1 runs each time, printing i*1 (1, then 2, then 3).

❓ Predict Output

intermediate

Counting iterations in nested loops

How many times will the innermost statement execute in this code?

C Sharp (C#)

int count = 0;
for (int i = 0; i < 4; i++) {
    for (int j = i; j < 4; j++) {
        count++;
    }
}
Console.WriteLine(count);

A 8

B 16

C 6

D 10

💡 Hint

Count how many times the inner loop runs for each i.

For i=0, j runs 4 times; i=1, j runs 3 times; i=2, j runs 2 times; i=3, j runs 1 time. Total = 4+3+2+1=10.

🔧 Debug

advanced

Identify the error in nested loops

What error will this code produce when compiled or run?

C Sharp (C#)

for (int i = 0; i < 3; i++)
    for (int j = 0; j < 3; j++)
        Console.WriteLine(i + j);
    Console.WriteLine("Done");

A Prints sums nine times and then "Done" once

B Compilation error due to missing braces

C Runtime error: NullReferenceException

D No error, prints sums and then "Done" three times

💡 Hint

Check how indentation and braces affect which statements belong to loops.

The outer for loop body is the inner for statement (no braces needed for single statement). Inner for body is the WriteLine(i+j). "Done" executes once after both loops. Prints 9 sums then "Done".

📝 Syntax

advanced

Find the syntax error in nested loops

Which option contains the syntax error in nested loops?

C Sharp (C#)

for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 2; j++)
        Console.WriteLine(i + j);
}

A

for (int i = 0; i < 2; i++)
    for (int j = 0; j < 2; j++)
        Console.WriteLine(i + j);

B

for (int i = 0; i < 2; i++) {
    for int j = 0; j < 2; j++) {
        Console.WriteLine(i + j);
    }
}

C

for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 2; j++) {
        Console.WriteLine(i + j);
    }
}

D

for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 2; j++)
        Console.WriteLine(i + j);
}

💡 Hint

Check the syntax of the inner for loop declaration.

Option B misses the opening parenthesis '(' after 'for' in the inner loop: 'for int j' instead of 'for (int j'.

🚀 Application

expert

Calculate sum of products in nested loops

What is the value of 'total' after running this code?

C Sharp (C#)

int total = 0;
for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if ((i + j) % 2 == 0) {
            total += i * j;
        }
    }
}
Console.WriteLine(total);

A 20

B 25

C 36

D 30

💡 Hint

Add products only when the sum of indices is even.

Pairs where (i+j) even: (1,1):1*1=1, (1,3):1*3=3, (2,2):2*2=4, (3,1):3*1=3, (3,3):3*3=9. Sum=1+3+4+3+9=20.

Practice

1. What happens when you use a nested loop in C#?

easy

A. Both loops run at the same time independently.

B. The outer loop runs only once and the inner loop runs multiple times.

C. The loops run only if a condition outside the loops is true.

D. The inner loop runs completely for each iteration of the outer loop.

Solution

1. Step 1: Understand nested loop structure

   In nested loops, the outer loop controls how many times the inner loop runs fully.

2. Step 2: Analyze loop execution order

   For each iteration of the outer loop, the inner loop runs from start to finish before the outer loop moves to the next iteration.

3. Final Answer:

   The inner loop runs completely for each iteration of the outer loop. -> Option D

4. Quick Check:

   Nested loops = inner loop runs fully each outer loop [OK]

Hint: Inner loop finishes all steps per outer loop step [OK]

Common Mistakes:

• Thinking both loops run simultaneously
• Assuming outer loop runs only once
• Confusing loop execution order

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++) { Console.WriteLine(i + j); } }

B. for i in range(3) { for j in range(2) { Console.WriteLine(i + j); } }

C. for(int i = 0; i < 3; i++) for(int j = 0; j < 2; j++) Console.WriteLine(i + j); }

D. foreach(int i = 0; i < 3; i++) { foreach(int j = 0; j < 2; j++) { Console.WriteLine(i + j); } }

Solution

1. Step 1: Check C# for loop syntax

   The correct for loop uses parentheses with initialization, condition, and increment separated by semicolons.

2. Step 2: Verify nested loop braces and statements

   for(int i = 0; i < 3; i++) { for(int j = 0; j < 2; j++) { Console.WriteLine(i + j); } } correctly nests two for loops with braces and valid syntax. Other options use incorrect syntax or keywords.

3. Final Answer:

   for(int i = 0; i < 3; i++) { for(int j = 0; j < 2; j++) { Console.WriteLine(i + j); } } -> Option A

4. Quick Check:

   C# for loop syntax = for(int i = 0; i < 3; i++) { for(int j = 0; j < 2; j++) { Console.WriteLine(i + j); } } [OK]

Hint: Remember C# for loop uses semicolons inside parentheses [OK]

Common Mistakes:

• Using Python-like syntax in C#
• Missing semicolons in for loop
• Using foreach incorrectly for numeric loops

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

for(int i = 1; i <= 2; i++) {
  for(int j = 1; j <= 3; j++) {
    Console.Write(i * j + " ");
  }
  Console.WriteLine();
}

medium

A. 6 12

B. 1 2 3
2 4 6

C. 1 1 1
2 2 2

D. 1 2 3 4 5 6

Solution

1. Step 1: Trace outer loop values

   Outer loop i runs from 1 to 2 (two iterations).

2. Step 2: Calculate inner loop outputs per i

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

3. Final Answer:

   1 2 3 2 4 6 -> Option B

4. Quick Check:

   Nested loops multiply i and j = output lines [OK]

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

Common Mistakes:

• Confusing loop ranges
• Printing without spaces or newlines
• Mixing up i and j in multiplication

4. Identify the error in this nested loop code:

for(int i = 0; i < 3; i++) {
  for(int j = 0; j < 2; j++)
    Console.WriteLine(i + j);
  }
}

medium

A. Extra closing brace '}' after the inner loop.

B. Missing semicolon after Console.WriteLine statement.

C. Incorrect loop condition in the outer loop.

D. Variable j is not declared.

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 elements

   Console.WriteLine has correct syntax, loops have valid conditions, and variables are declared properly.

3. Final Answer:

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

4. Quick Check:

   Brace mismatch = syntax error [OK]

Hint: Count opening and closing braces carefully [OK]

Common Mistakes:

• Ignoring extra or missing braces
• Assuming missing semicolon error
• Confusing variable declaration errors

5. You want to print a multiplication table from 1 to 3 using nested loops in C#. Which code correctly prints the table with rows and columns aligned?

hard

A. for(int i = 1; i <= 3; i++) { for(int j = 1; j <= 3; j++) { Console.Write(i*j); } Console.WriteLine(); }

B. for(int i = 1; i <= 3; i++) { for(int j = 1; j <= 3; j++) { Console.WriteLine(i*j); } }

C. for(int i = 1; i <= 3; i++) { for(int j = 1; j <= 3; j++) { Console.Write($"{i*j,4}"); } Console.WriteLine(); }

D. for(int i = 1; i <= 3; i++) { for(int j = 1; j <= 3; j++) { Console.WriteLine($"{i}*{j}={i*j}"); } Console.WriteLine(); }

Solution

1. Step 1: Understand formatting for aligned output

   Using Console.Write with format specifier {value,4} aligns numbers in columns with 4 spaces width.

2. Step 2: Check nested loop and output structure

   for(int i = 1; i <= 3; i++) { for(int j = 1; j <= 3; j++) { Console.Write($"{i*j,4}"); } Console.WriteLine(); } prints products in a grid with spaces, then moves to next line after inner loop finishes, creating a table.

3. Final Answer:

   for(int i = 1; i <= 3; i++) { for(int j = 1; j <= 3; j++) { Console.Write($"{i*j,4}"); } Console.WriteLine(); } -> Option C

4. Quick Check:

   Formatted nested loops print aligned multiplication table [OK]

Hint: Use Console.Write with alignment specifier for columns [OK]

Common Mistakes:

• Using Console.WriteLine inside inner loop causing new lines
• Not formatting output for alignment
• Missing Console.WriteLine after inner loop