Array length in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Go code?

Consider the following Go program. What will it print when run?

Go

package main
import "fmt"
func main() {
    arr := [5]int{1, 2, 3, 4, 5}
    fmt.Println(len(arr))
}

A 0

B 5

C 6

D Compilation error

💡 Hint

Remember, len() returns the number of elements in an array.

The array has 5 elements, so len(arr) returns 5.

❓ Predict Output

intermediate

What is the output of this Go code with slice?

What will this Go program print?

Go

package main
import "fmt"
func main() {
    slice := []int{10, 20, 30}
    fmt.Println(len(slice))
}

A Compilation error

B 0

C 3

D Runtime panic

💡 Hint

Slices also have a length accessible by len().

The slice has 3 elements, so len(slice) returns 3.

❓ Predict Output

advanced

What is the output of this Go code with multidimensional array?

What will this program print?

Go

package main
import "fmt"
func main() {
    arr := [2][3]int{{1,2,3},{4,5,6}}
    fmt.Println(len(arr))
    fmt.Println(len(arr[0]))
}

A Compilation error

B 3\n2

C 6\n0

D 2\n3

💡 Hint

len(arr) returns the number of rows, len(arr[0]) returns the number of columns.

The outer array has length 2, each inner array has length 3.

❓ Predict Output

advanced

What error does this Go code produce?

What happens when you run this Go code?

Go

package main
import "fmt"
func main() {
    var arr [3]int
    fmt.Println(len(arr))
    fmt.Println(arr[3])
}

A Runtime panic: index out of range

B 0\n0

C Compilation error: invalid array index

D 3\n0

💡 Hint

Accessing arr[3] is out of bounds for an array of length 3.

The program compiles but panics at runtime due to out-of-range index.

🧠 Conceptual

expert

How many elements are in this Go array?

Given this Go array declaration, how many elements does it contain? var arr = [...]int{1, 2, 3, 4, 5, 6, 7}

A 7

B 0

C Compilation error

D Depends on runtime

💡 Hint

The ... tells Go to count the elements automatically.

The ... means Go counts the elements in the initializer, which are 7.

Practice

1. What does the len() function return when used with an array in Go?

easy

A. The memory size of the array in bytes

B. The number of elements in the array

C. The capacity of the array's underlying slice

D. The last element of the array

Solution

1. Step 1: Understand the purpose of len() in Go

   The len() function returns the count of elements in arrays, slices, maps, and strings.

2. Step 2: Apply len() to an array

   When used on an array, len() returns the fixed number of elements it holds.

3. Final Answer:

   The number of elements in the array -> Option B

4. Quick Check:

   len(array) = number of elements [OK]

Hint: len() always gives count of elements in an array [OK]

Common Mistakes:

• Confusing len() with capacity
• Thinking len() returns memory size
• Assuming len() returns last element

2. Which of the following is the correct way to get the length of an array named arr in Go?

easy

A. len(arr)

B. arr.length

C. size(arr)

D. length(arr)

Solution

1. Step 1: Recall Go syntax for length

   In Go, the built-in function len() is used to get the length of arrays and slices.

2. Step 2: Check each option

   Only len(arr) is valid syntax; others are invalid or do not exist in Go.

3. Final Answer:

   len(arr) -> Option A

4. Quick Check:

   Use len() function for array length [OK]

Hint: Use len(array) to get length, not dot or other functions [OK]

Common Mistakes:

• Using dot notation like arr.length
• Using non-existent functions like length() or size()
• Confusing with other languages' syntax

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

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

medium

A. 0

B. 3

C. 4

D. Compilation error

Solution

1. Step 1: Analyze the array declaration

   The array arr is declared with 4 elements: 10, 20, 30, 40.

2. Step 2: Understand len(arr)

   The len() function returns the number of elements, which is 4.

3. Final Answer:

   4 -> Option C

4. Quick Check:

   len([4]int{...}) = 4 [OK]

Hint: Count elements inside [ ] to find length [OK]

Common Mistakes:

• Counting elements incorrectly
• Thinking len() returns last index (3)
• Expecting runtime error

4. Identify the error in this Go code snippet:

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

medium

A. fmt.Println cannot print arrays

B. len(arr) is invalid for arrays

C. Array declaration syntax is wrong

D. Accessing arr[3] causes an index out of range error

Solution

1. Step 1: Check array length and indexing

   The array arr has length 3, so valid indices are 0, 1, and 2.

2. Step 2: Identify invalid index access

   Accessing arr[3] is out of range and causes a runtime panic.

3. Final Answer:

   Accessing arr[3] causes an index out of range error -> Option D

4. Quick Check:

   Array indices go from 0 to len-1 [OK]

Hint: Array indices start at 0, max index = length-1 [OK]

Common Mistakes:

• Assuming len(arr) is invalid
• Thinking fmt.Println can't print arrays
• Confusing array length with capacity

5. Given two arrays in Go:

arr1 := [5]int{1, 2, 3, 4, 5}
arr2 := [3]int{10, 20, 30}

Which expression correctly checks if arr2 has fewer elements than arr1?

hard

A. len(arr2) < len(arr1)

B. len(arr1) < len(arr2)

C. arr2.length < arr1.length

D. size(arr2) < size(arr1)

Solution

1. Step 1: Identify lengths of both arrays

   arr1 has length 5, arr2 has length 3.

2. Step 2: Write correct comparison syntax

   Use len() to get lengths. To check if arr2 is smaller, compare len(arr2) < len(arr1).

3. Final Answer:

   len(arr2) < len(arr1) -> Option A

4. Quick Check:

   len(arr2) = 3 < 5 = len(arr1) [OK]

Hint: Use len() and compare lengths directly [OK]

Common Mistakes:

• Using wrong function names like length or size
• Reversing comparison order
• Using dot notation for length