Nested loops in Go - Step-by-Step Execution

Concept Flow - Nested loops

Start outer loop i=0

↓

Start inner loop j=0

↓

Execute inner loop body

↓

Update inner loop j=j+1

↓

Check inner loop j < limit?

Yes→Repeat inner loop body

No↓

Update outer loop i=i+1

↓

Check outer loop i < limit?

Yes→Repeat inner loop

No↓

End

The outer loop runs first. For each outer loop step, the inner loop runs completely. Then the outer loop moves to the next step.

Execution Sample

Go

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

Print pairs of i and j where i goes 0 to 1 and j goes 0 to 2 for each i.

Execution Table

Step	i	j	Condition i<2	Condition j<3	Action	Output
1	0	0	true	true	Print (0,0)	(0 0)
2	0	1	true	true	Print (0,1)	(0 1)
3	0	2	true	true	Print (0,2)	(0 2)
4	0	3	true	false	Inner loop ends
5	1	0	true	true	Print (1,0)	(1 0)
6	1	1	true	true	Print (1,1)	(1 1)
7	1	2	true	true	Print (1,2)	(1 2)
8	1	3	true	false	Inner loop ends
9	2	0	false	true	Outer loop ends

💡 Outer loop i=2 fails condition i<2, so loops stop.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	Final
i	0	0	0	0	0	1	1	1	1	2
j	0	1	2	3	0	1	2	3	0	0

Key Moments - 3 Insights

Why does j reset to 0 after inner loop ends?

Because j is declared inside the inner loop, it starts fresh each time the inner loop begins (see rows 4 and 5 in execution_table).

Why does the outer loop increment only after inner loop finishes?

The outer loop waits for the inner loop to complete all its steps before increasing i (see transition from row 4 to 5).

What stops the loops from running forever?

The conditions i<2 and j<3 become false eventually, stopping loops (see rows 4 and 9 where conditions fail).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of j at step 6?

A 2

B 1

C 0

D 3

💡 Hint

Check the 'j' column in row 6 of the execution_table.

At which step does the outer loop condition become false?

A Step 9

B Step 8

C Step 4

D Step 7

💡 Hint

Look at the 'Condition i<2' column and find where it is false.

If the inner loop condition changed to j<2, how many times would the inner loop print per outer loop iteration?

A 3 times

B 1 time

C 2 times

D 4 times

💡 Hint

Check how many times j runs from 0 while j<2 in the variable_tracker.

Concept Snapshot

Nested loops run one loop inside another.
Outer loop runs first.
Inner loop runs fully for each outer loop step.
Inner loop variable resets each time.
Loops stop when conditions fail.

Full Transcript

This example shows nested loops in Go. The outer loop variable i starts at 0 and runs while less than 2. For each i, the inner loop variable j runs from 0 to less than 3. Each pair (i,j) is printed. The inner loop finishes all its steps before the outer loop increments i. Variables i and j change as loops run. The loops stop when conditions become false. This helps understand how loops inside loops work step-by-step.

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