Array length in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the len() function do when used with an array in Go?

The len() function returns the number of elements in the array. It tells you how many items the array holds.

beginner

How do you declare an array of 5 integers in Go?

You write: var arr [5]int. This creates an array named arr that can hold exactly 5 integers.

beginner

If you have arr := [3]string{"a", "b", "c"}, what will len(arr) return?

It will return 3 because the array has 3 elements: "a", "b", and "c".

beginner

Can the length of an array in Go change after it is created?

No, the length of an array in Go is fixed when you declare it. You cannot add or remove elements to change its size.

intermediate

What is the difference between an array and a slice in Go regarding length?

An array has a fixed length known at compile time, while a slice is a flexible, dynamic view over an array and its length can change during runtime.

What does len(arr) return if arr := [4]int{1, 2, 3, 4}?

A 3

B Undefined

C 0

D 4

len() returns the number of elements in the array, which is 4 here.

Can you change the length of an array after it is declared in Go?

A No, arrays have fixed length

B Yes, by using <code>len()</code>

C Yes, by adding elements

D Only if the array is empty

Arrays in Go have a fixed length that cannot be changed after declaration.

Which Go type allows flexible length that can change during runtime?

A Slice

B Array

C Map

D Struct

Slices are dynamic and can grow or shrink, unlike arrays.

What is the output of len([3]string{"x", "y", "z"})?

A 1

B 3

C 0

D Error

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

How do you get the length of an array named numbers in Go?

A <code>numbers.length</code>

B <code>numbers.len()</code>

C <code>len(numbers)</code>

D <code>length(numbers)</code>

The correct way is to use the built-in function len().

Explain how to find the length of an array in Go and why the length is fixed.

Think about how arrays are declared and how len() works.

Describe the difference between an array and a slice in Go regarding their length and flexibility.

Consider how each type handles size changes.

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