Iterating over arrays in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of range loop with index and value

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
  arr := [3]int{10, 20, 30}
  for i, v := range arr {
    fmt.Printf("%d:%d ", i, v)
  }
}

A 0:10 1:20 2:30

B 10:0 20:1 30:2

C 0 10 1 20 2 30

D 10 20 30

💡 Hint

Remember that range returns index and value in that order.

The range keyword returns two values: the index and the element at that index. The fmt.Printf prints them as index:value pairs separated by spaces.

❓ Predict Output

intermediate

Output when ignoring index in range loop

What will this Go program print?

Go

package main
import "fmt"
func main() {
  arr := [4]string{"a", "b", "c", "d"}
  for _, v := range arr {
    fmt.Print(v)
  }
}

A abcd

B 0123

C a b c d

D [a b c d]

💡 Hint

The underscore _ ignores the index, so only values are printed.

The loop ignores the index and prints each string value directly without spaces or newlines.

🔧 Debug

advanced

Identify the error in this array iteration

What error will this Go code produce when compiled?

Go

package main
import "fmt"
func main() {
  arr := [3]int{1, 2, 3}
  for i, v := range arr {
    fmt.Println(i + v)
  }
  fmt.Println(arr[3])
}

A compile error: invalid array index 3 (out of bounds)

B no error, prints 1 3 5

C prints 1 3 5 and then 0

D runtime panic: index out of range

💡 Hint

Check the array length and the index used after the loop.

The array has length 3, so valid indices are 0,1,2. Accessing arr[3] causes a runtime panic: index out of range.

❓ Predict Output

advanced

Output of modifying array elements inside range loop

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
  arr := [3]int{1, 2, 3}
  for i, v := range arr {
    v = v * 2
    fmt.Print(v, " ")
  }
  fmt.Println()
  fmt.Println(arr)
}

A

1 2 3 
[1 2 3]

B

2 4 6 
[2 4 6]

C

2 4 6 
[1 2 3]

D compile error

💡 Hint

Remember that v is a copy of the element, not the element itself.

The range loop copies each element into v. Modifying v does not change the original array. So printing v shows doubled values, but the array remains unchanged.

🧠 Conceptual

expert

Number of iterations in nested array loops

Given this Go code, how many times will the innermost print statement execute?

Go

package main
import "fmt"
func main() {
  arr := [2][3]int{{1,2,3},{4,5,6}}
  count := 0
  for _, inner := range arr {
    for i := range inner {
      fmt.Println(inner[i])
      count++
    }
  }
  fmt.Println("Total:", count)
}

A 5

B 6

C 3

D 2

💡 Hint

Count elements in each inner array and multiply by number of outer arrays.

The outer array has 2 elements, each inner array has 3 elements. The inner loop runs 3 times per outer iteration, total 2*3=6 times.