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Golang cap() Function with Examples

Golang | cap() Function: Here, we are going to learn about the built-in cap() function with its usages, syntax, and examples.
Submitted by IncludeHelp, on October 13, 2021

cap() Function

In the Go programming language, the cap() is a built-in function that is used to get the capacity of the given slice.

It accepts one parameter (v Type) and returns the capacity of v.

Syntax:

func cap(v Type) int

Parameter(s):

• v : Slice of type Type whose capacity is to be found.

Return Value:

The return type of the cap() function is an int, it returns the capacity of the given slice.

Special cases:

• Array: In the case of an array the number of elements in v (same as len(v)).
• Pointer to array: In the case of pointer to an array the number of elements in *v (same as len(v)).
• Slice: In the case of a slice the maximum length the slice can reach when resliced;
• if the v (value of the given parameter) is nil, cap(v) is zero.
• Channel: In the case of a channel the channel buffer capacity, in units of elements;

Example 1:

// Golang program to demonstrate the
// example of cap() function

package main

import (
	"fmt"
)

func main() {
	// Declaring & assigning some slices
	a := []int{10, 20, 30}
	b := []float32{20.23, 1.234, 10.20}
	c := []string{"Hello", "world"}

	// Printing their capacities
	fmt.Println("cap(a):", cap(a))
	fmt.Println("cap(b):", cap(b))
	fmt.Println("cap(c):", cap(c))
}

Output:

cap(a): 3
cap(b): 3
cap(c): 2

Example 2:

// Golang program to demonstrate the
// example of cap() function

package main

import (
	"fmt"
)

func main() {
	// Creating int and string slices
	s1 := []int{10, 20, 30}
	s2 := []string{"Hello", "World"}

	// Printing types and values of slices
	fmt.Printf("%T, %v\n", s1, s1)
	fmt.Printf("%T, %q\n", s2, s2)

	// Printing the capacity
	fmt.Println("Capacity of s1:", cap(s1))
	fmt.Println("Capacity of s2:", cap(s2))

	// Appending some elements
	s1 = append(s1, 40, 50)
	s2 = append(s2, "How are you?", "Boys")

	// After appending,
	// Printing types and values of slices
	fmt.Println("After appending...")
	fmt.Printf("%T, %v\n", s1, s1)
	fmt.Printf("%T, %q\n", s2, s2)

	// Printing the capacity
	fmt.Println("Capacity of s1:", cap(s1))
	fmt.Println("Capacity of s2:", cap(s2))
}

Output:

[]int, [10 20 30]
[]string, ["Hello" "World"]
Capacity of s1: 3
Capacity of s2: 2
After appending...
[]int, [10 20 30 40 50]
[]string, ["Hello" "World" "How are you?" "Boys"]
Capacity of s1: 6
Capacity of s2: 4

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