Nested loops in Go

Introduction

Nested loops help you repeat actions inside other repeated actions, like checking every seat in a theater row by row.

When you want to print a grid or table of values.

When you need to compare every item in one list with every item in another list.

When you want to create patterns with rows and columns.

When you process multi-dimensional data like a chessboard or calendar.

Syntax

Go

for outerVariable := start; outerVariable < end; outerVariable++ {
    for innerVariable := start; innerVariable < end; innerVariable++ {
        // code to repeat
    }
}

The outer loop runs first, then the inner loop runs completely for each outer loop step.

You can use any loop variable names and conditions as needed.

Examples

Prints pairs of numbers where i goes from 1 to 3 and j goes from 1 to 2 for each i.

Go

for i := 1; i <= 3; i++ {
    for j := 1; j <= 2; j++ {
        fmt.Println(i, j)
    }
}

Prints a 2 by 3 grid of coordinates.

Go

for row := 1; row <= 2; row++ {
    for col := 1; col <= 3; col++ {
        fmt.Printf("%d,%d ", row, col)
    }
    fmt.Println()
}

Sample Program

This program prints numbers 1 to 3 in three rows, showing how nested loops create rows and columns.

Go

package main

import "fmt"

func main() {
    for i := 1; i <= 3; i++ {
        for j := 1; j <= 3; j++ {
            fmt.Printf("%d ", j)
        }
        fmt.Println()
    }
}

Important Notes

Remember to indent inner loops to keep your code clear.

Too many nested loops can make your program slow, so use them wisely.

You can nest more than two loops if needed, but keep it simple for beginners.

Summary

Nested loops run one loop inside another to repeat actions multiple times.

They are useful for working with grids, tables, and multi-step tasks.

In Go, use the for loop inside another for loop with clear variable names.

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