How to Find Capacity of Slice in Go: Simple Guide

In Go, you can find the capacity of a slice using the built-in cap() function. This function returns the total number of elements the slice can hold without allocating more memory.

Syntax

The syntax to find the capacity of a slice is simple:

• cap(slice): Returns the capacity of the given slice.

The slice is the variable holding your slice data.

go

capacity := cap(slice)

Example

This example shows how to create a slice and find its capacity using cap(). It prints the length and capacity of the slice.

go

package main

import "fmt"

func main() {
    numbers := make([]int, 3, 5) // length 3, capacity 5
    fmt.Println("Slice:", numbers)
    fmt.Println("Length:", len(numbers))
    fmt.Println("Capacity:", cap(numbers))
}

Output

Slice: [0 0 0]
Length: 3
Capacity: 5

Common Pitfalls

Some common mistakes when working with slice capacity:

• Confusing len() with cap(). len() gives the number of elements currently in the slice, while cap() gives the total space allocated.
• Assuming capacity changes automatically when appending. Capacity only grows when needed, not on every append.
• Using cap() on arrays returns the array length, which might be confusing.

go

package main

import "fmt"

func main() {
    s := []int{1, 2, 3}
    fmt.Println("Length:", len(s))   // 3
    fmt.Println("Capacity:", cap(s)) // 3

    s = append(s, 4)
    fmt.Println("After append - Length:", len(s))   // 4
    fmt.Println("After append - Capacity:", cap(s)) // Capacity may increase
}

Output

Length: 3
Capacity: 3
After append - Length: 4
After append - Capacity: 6

Quick Reference

Remember these quick tips when working with slice capacity in Go:

• cap(slice) returns the total allocated space.
• len(slice) returns the current number of elements.
• Capacity can be larger than length.
• Appending beyond capacity increases capacity automatically.

Key Takeaways

Use the built-in cap() function to get a slice's capacity in Go.

Capacity shows how many elements the slice can hold without resizing.

Length and capacity are different; length is current size, capacity is total space.

Appending to a slice may increase its capacity automatically.

Always check capacity when optimizing memory or performance.