Accessing array elements in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing array element by index

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    arr := [4]int{10, 20, 30, 40}
    fmt.Println(arr[2])
}

A 20

B 40

C 30

D Index out of range error

💡 Hint

Remember that Go arrays are zero-indexed, so the first element is at index 0.

The array has elements 10, 20, 30, 40 at indices 0, 1, 2, 3 respectively. Accessing arr[2] returns 30.

❓ Predict Output

intermediate

Accessing last element of an array

What will this Go program print?

Go

package main
import "fmt"
func main() {
    arr := [5]string{"a", "b", "c", "d", "e"}
    fmt.Println(arr[len(arr)-1])
}

A e

B a

C d

D Index out of range error

💡 Hint

len(arr) gives the number of elements. The last index is length minus one.

The last element is at index 4 (5-1), which is "e".

❓ Predict Output

advanced

What error occurs when accessing out-of-bounds index?

What error does this Go program produce when run?

Go

package main
func main() {
    arr := [3]int{1, 2, 3}
    _ = arr[3]
}

A no error, prints 0

B panic: runtime error: index out of range [3] with length 3

C compile error: invalid array index

D panic: nil pointer dereference

💡 Hint

Array indices must be within 0 to length-1. Accessing index 3 in a 3-element array is invalid.

Go panics at runtime with an index out of range error when accessing arr[3].

🧠 Conceptual

advanced

Understanding array element access with variables

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    arr := [3]int{5, 10, 15}
    i := 1
    fmt.Println(arr[i])
    i = 2
    fmt.Println(arr[i])
}

A 15\n10

B 5\n10

C runtime error: index out of range

D 10\n15

💡 Hint

The variable i changes before the second print, so the index changes.

First print accesses arr[1] which is 10, second print accesses arr[2] which is 15.

🔧 Debug

expert

Identify the cause of the runtime error

This Go program crashes with a runtime error. What is the cause?

Go

package main
import "fmt"
func main() {
    arr := [2]int{7, 8}
    for i := 0; i <= len(arr); i++ {
        fmt.Println(arr[i])
    }
}

A The loop condition uses <= instead of < causing out-of-bounds access

B Array is not initialized properly

C fmt.Println cannot print array elements

D Variable i is not declared

💡 Hint

Check the loop boundary and valid indices for the array.

The loop runs from i=0 to i=2 inclusive, but arr has length 2, so arr[2] is out of range causing runtime panic.

Practice

1. What is the index of the first element in a Go array?

easy

A. 1

B. Depends on the array size

C. -1

D. 0

Solution

1. Step 1: Understand Go array indexing

   In Go, arrays start indexing from 0, meaning the first element is at index 0.

2. Step 2: Confirm the first element's index

   The first element is always accessed with index 0, not 1 or any other number.

3. Final Answer:

   0 -> Option D

4. Quick Check:

   First element index = 0 [OK]

Hint: Remember, Go arrays start counting at zero [OK]

Common Mistakes:

• Thinking indexing starts at 1
• Assuming negative indexes are valid
• Believing index depends on array size

2. Which of the following is the correct way to access the third element of an array named arr in Go?

easy

A. arr[3]

B. arr[2]

C. arr(2)

D. arr{2}

Solution

1. Step 1: Recall Go array indexing syntax

   Go uses square brackets with zero-based indexing to access elements, so the third element is at index 2.

2. Step 2: Identify correct syntax

   arr[2] accesses the third element; arr[3] accesses the fourth, arr(2) and arr{2} are invalid syntax.

3. Final Answer:

   arr[2] -> Option B

4. Quick Check:

   Third element = arr[2] [OK]

Hint: Use square brackets and zero-based index [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based index
• Using curly braces for access

3. What will be the output of this Go code?

arr := [4]int{10, 20, 30, 40}
fmt.Println(arr[1])

medium

A. 10

B. 40

C. 20

D. 30

Solution

1. Step 1: Identify the array elements and their indexes

   The array is [10, 20, 30, 40], indexed 0 to 3. Index 1 corresponds to the second element, 20.

2. Step 2: Determine the printed value

   arr[1] accesses the element 20, so fmt.Println prints 20.

3. Final Answer:

   20 -> Option C

4. Quick Check:

   arr[1] = 20 [OK]

Hint: Remember index 1 is second element [OK]

Common Mistakes:

• Printing arr[0] instead
• Confusing index 1 with 2
• Expecting output as 30 or 40

4. What is wrong with this Go code snippet?

arr := [3]int{1, 2, 3}
fmt.Println(arr[3])

medium

A. Index 3 is out of bounds for the array

B. Array declaration syntax is incorrect

C. fmt.Println cannot print array elements

D. The array size should be 4

Solution

1. Step 1: Check array size and valid indexes

   The array has size 3, so valid indexes are 0, 1, and 2 only.

2. Step 2: Identify the error in accessing arr[3]

   Accessing arr[3] tries to get the fourth element, which does not exist, causing an out-of-bounds runtime error.

3. Final Answer:

   Index 3 is out of bounds for the array -> Option A

4. Quick Check:

   Valid indexes: 0 to 2 only [OK]

Hint: Array indexes go from 0 to length-1 [OK]

Common Mistakes:

• Using index equal to array length
• Thinking fmt.Println can't print elements
• Assuming array size auto-expands

5. Given the array arr := [5]int{5, 10, 15, 20, 25}, which code snippet correctly sums the first three elements?

hard

A. sum := arr[0] + arr[1] + arr[2]

B. sum := arr[1] + arr[2] + arr[3]

C. sum := arr[0] + arr[2] + arr[3]

D. sum := arr[2] + arr[3] + arr[4]

Solution

1. Step 1: Identify the first three elements

   The first three elements are at indexes 0, 1, and 2, which are 5, 10, and 15 respectively.

2. Step 2: Check which option sums these elements

   sum := arr[0] + arr[1] + arr[2] sums arr[0], arr[1], and arr[2], which is correct. Other options include elements beyond the first three.

3. Final Answer:

   sum := arr[0] + arr[1] + arr[2] -> Option A

4. Quick Check:

   Sum first three = arr[0]+arr[1]+arr[2] [OK]

Hint: Sum elements at indexes 0, 1, and 2 for first three [OK]

Common Mistakes:

• Starting sum at index 1
• Skipping elements
• Including elements beyond first three