Nested loops in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested loops with break

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    for i := 1; i <= 3; i++ {
        for j := 1; j <= 3; j++ {
            if j == 2 {
                break
            }
            fmt.Printf("%d%d ", i, j)
        }
    }
}

A 11 12 21 22 31 32

B 11 21 31

C 11 12 13 21 22 23 31 32 33

D 12 22 32

💡 Hint

Remember that break exits the inner loop immediately.

The inner loop breaks when j == 2, so only j == 1 prints for each i. The output is 11 21 31 .

❓ Predict Output

intermediate

Counting iterations in nested loops

How many times will the print statement execute in this Go code?

Go

package main
import "fmt"
func main() {
    count := 0
    for i := 0; i < 4; i++ {
        for j := 0; j < 3; j++ {
            count++
        }
    }
    fmt.Println(count)
}

A 12

B 10

C 15

D 7

💡 Hint

Multiply the number of outer loop runs by the inner loop runs.

The outer loop runs 4 times, inner loop 3 times each. Total prints = 4 * 3 = 12.

🔧 Debug

advanced

Identify the error in nested loops

What error does this Go code produce?

Go

package main
import "fmt"
func main() {
    for i := 0; i < 3; i++ {
        for j := 0; j < 3; j++
            fmt.Println(i, j)
    }
}

A Syntax error: missing braces for inner loop

B Syntax error: missing semicolon

C No error, prints pairs of i and j

D Runtime error: index out of range

💡 Hint

Check the syntax for loops in Go.

The inner loop lacks braces { }, which are required for multiple statements or to define the loop body properly. This causes a syntax error.

❓ Predict Output

advanced

Output of nested loops with continue

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    for i := 1; i <= 2; i++ {
        for j := 1; j <= 3; j++ {
            if j == 2 {
                continue
            }
            fmt.Printf("%d%d ", i, j)
        }
    }
}

A 12 22

B 11 12 13 21 22 23

C 11 13 21 23

D 11 21

💡 Hint

continue skips the current iteration of the inner loop.

When j == 2, the loop skips printing. So only j == 1 and j == 3 print for each i.

🧠 Conceptual

expert

Number of iterations in nested loops with variable limits

Consider this Go code snippet. What is the total number of times the inner loop's print statement executes?

Go

package main
import "fmt"
func main() {
    total := 0
    for i := 1; i <= 4; i++ {
        for j := 1; j <= i; j++ {
            total++
        }
    }
    fmt.Println(total)
}

A 20

B 16

C 8

D 10

💡 Hint

Sum the counts of inner loop runs for each outer loop iteration.

The inner loop runs 1 + 2 + 3 + 4 times = 10 times total.

Practice

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

easy

A. Runs one loop inside another to repeat code multiple times

B. Runs two loops one after another without overlap

C. Runs only the inner loop once

D. Creates an infinite loop automatically

Solution

1. Step 1: Understand nested loops structure

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

2. Step 2: Explain the repetition effect

   This means the inner code runs 3 * 2 = 6 times, repeating actions multiple times inside another loop.

3. Final Answer:

   Runs one loop inside another to repeat code multiple times -> Option A

4. Quick Check:

   Nested loops repeat code multiple times [OK]

Hint: Nested loops repeat inner code for each outer loop cycle [OK]

Common Mistakes:

• Thinking loops run sequentially without nesting
• Assuming inner loop runs only once
• Believing nested loops cause infinite loops automatically

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

easy

A. for i := 0; i < 3; i++; for j := 0; j < 2; j++ { fmt.Println(i, j) }

B. for i := 0; i < 3; i++ { for j < 2 { fmt.Println(i, j); j++ } }

C. for i := 0; i < 3; i++ { for j := 0; j < 2; j++ } fmt.Println(i, j)

D. for i := 0; i < 3; i++ { for j := 0; j < 2; j++ { fmt.Println(i, j) } }

Solution

1. Step 1: Check correct nested loop syntax

   for i := 0; i < 3; i++ { for j := 0; j < 2; j++ { fmt.Println(i, j) } } correctly nests the inner for loop inside the outer loop with braces and increments.

2. Step 2: Identify syntax errors in other options

   for i := 0; i < 3; i++ { for j < 2 { fmt.Println(i, j); j++ } } uses a while-style loop without declaring j, causing an undefined variable error in Go. for i := 0; i < 3; i++; for j := 0; j < 2; j++ { fmt.Println(i, j) } has two for loops separated by a semicolon, which is invalid. for i := 0; i < 3; i++ { for j := 0; j < 2; j++ } fmt.Println(i, j) places the print statement outside the inner loop braces, causing scope issues.

3. Final Answer:

   for i := 0; i < 3; i++ { for j := 0; j < 2; j++ { fmt.Println(i, j) } } -> Option D

4. Quick Check:

   Correct nested loop syntax = for i := 0; i < 3; i++ { for j := 0; j < 2; j++ { fmt.Println(i, j) } } [OK]

Hint: Remember Go uses braces and semicolons inside for loops [OK]

Common Mistakes:

• Using semicolon between two for loops instead of nesting
• Trying while-style loops without proper syntax
• Placing code outside loop braces incorrectly

3. What is the output of this Go program?

package main
import "fmt"
func main() {
  for i := 1; i <= 2; i++ {
    for j := 1; j <= 3; j++ {
      fmt.Print(i * j, " ")
    }
    fmt.Println()
  }
}

medium

A. 2 4 6
1 2 3

B. 1 1 1
2 2 2

C. 1 2 3
2 4 6

D. 1 2 3 4 5 6

Solution

1. Step 1: Analyze outer loop iterations

   The outer loop runs for i = 1 and i = 2.

2. Step 2: Calculate inner loop outputs for each i

   For i=1, inner loop prints 1*1=1, 1*2=2, 1*3=3. For i=2, inner loop prints 2*1=2, 2*2=4, 2*3=6.

3. Final Answer:

   1 2 3 2 4 6 -> Option C

4. Quick Check:

   Multiplying i and j in nested loops = 1 2 3 2 4 6 [OK]

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

Common Mistakes:

• Confusing loop ranges or order
• Printing all numbers in one line without newline
• Mixing up i and j in multiplication

4. Find the error in this nested loop code snippet:

for i := 0; i < 3; i++ {
  for j := 0; j < 2; j++
    fmt.Println(i, j)
}

medium

A. Missing braces {} for inner loop body

B. Outer loop condition is incorrect

C. Inner loop variable j is not declared

D. fmt.Println cannot be used inside loops

Solution

1. Step 1: Check inner loop syntax

   The inner loop lacks braces {} around its body, which is required for multiple statements or to clearly define the loop block in Go.

2. Step 2: Understand Go syntax rules

   Without braces, only the next statement is considered inside the loop. Here, fmt.Println is intended as the loop body but braces improve clarity and prevent errors.

3. Final Answer:

   Missing braces {} for inner loop body -> Option A

4. Quick Check:

   Braces needed for loop body in Go [OK]

Hint: Always use braces {} for nested loop bodies [OK]

Common Mistakes:

• Assuming braces are optional for loops
• Thinking variable declaration is missing
• Believing fmt.Println can't be inside loops

5. You want to print a multiplication table from 1 to 4 using nested loops in Go. Which code snippet correctly achieves this?

hard

A. for i := 1; i < 4; i++ {
for j := 1; j < 4; j++ {
fmt.Println(i*j)
}
}

B. for i := 1; i <= 4; i++ {
for j := 1; j <= 4; j++ {
fmt.Printf("%d ", i*j)
}
fmt.Println()
}

C. for i := 1; i <= 4; i++ {
for j := 1; j <= 4; j++ {
fmt.Println(i+j)
}
fmt.Println()
}

D. for i := 1; i <= 4; i++ {
for j := 1; j <= 4; j++ {
fmt.Printf("%d ", i+j)
}
fmt.Println()
}

Solution

1. Step 1: Identify correct loop ranges

   To print from 1 to 4, loops must run from 1 to 4 inclusive, so conditions use <= 4.

2. Step 2: Check output format and calculation

   for i := 1; i <= 4; i++ { for j := 1; j <= 4; j++ { fmt.Printf("%d ", i*j) } fmt.Println() } multiplies i and j and prints in one line with spaces, then prints a newline after inner loop, forming a table.

3. Step 3: Compare other options

   for i := 1; i < 4; i++ { for j := 1; j < 4; j++ { fmt.Println(i*j) } } uses < 4, missing 4. Options C and D add i+j instead of multiplication, which is incorrect for multiplication table.

4. Final Answer:

   for i := 1; i <= 4; i++ { for j := 1; j <= 4; j++ { fmt.Printf("%d ", i*j) } fmt.Println() } -> Option B

5. Quick Check:

   Correct ranges and multiplication print = for i := 1; i <= 4; i++ { for j := 1; j <= 4; j++ { fmt.Printf("%d ", i*j) } fmt.Println() } [OK]

Hint: Use <= for inclusive range and multiply counters [OK]

Common Mistakes:

• Using < instead of <= to miss last number
• Adding instead of multiplying counters
• Printing each number on a new line instead of a table