GoHow-ToBeginner · 3 min read

How to Use make for Slice in Go: Syntax and Examples

In Go, you use make to create a slice by specifying its type, length, and optionally capacity like make([]Type, length, capacity). This allocates the slice with the given length and capacity, ready to use.

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Syntax

The make function creates slices with a specified length and optional capacity.

Type: The slice element type inside square brackets, e.g., []int.
Length: Number of elements the slice initially holds.
Capacity (optional): Total space allocated for the slice, must be >= length.

slice := make([]int, length, capacity)

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Example

This example creates a slice of integers with length 3 and capacity 5, then prints its length, capacity, and contents.

package main

import "fmt"

func main() {
    // Create a slice of int with length 3 and capacity 5
    s := make([]int, 3, 5)

    // Print length and capacity
    fmt.Println("Length:", len(s))
    fmt.Println("Capacity:", cap(s))

    // Print the slice contents (zero values)
    fmt.Println("Slice contents:", s)

    // Assign values to slice elements
    s[0] = 10
    s[1] = 20
    s[2] = 30

    fmt.Println("Updated slice:", s)
}

Output

Length: 3 Capacity: 5 Slice contents: [0 0 0] Updated slice: [10 20 30]

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Common Pitfalls

Common mistakes when using make for slices include:

Setting capacity smaller than length causes a compile-time error.
Accessing elements beyond the length causes a runtime panic.
Confusing length and capacity: length is how many elements are accessible, capacity is total allocated space.

package main

import "fmt"

func main() {
    // Wrong: capacity less than length (compile error)
    // s := make([]int, 5, 3) // Uncommenting causes error

    // Right: capacity >= length
    s := make([]int, 3, 5)

    // Wrong: accessing beyond length causes panic
    // fmt.Println(s[4]) // Uncommenting causes runtime panic

    // Right: access only within length
    fmt.Println(s[2])
}

Output

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Quick Reference

Parameter	Description	Example
Type	Element type of the slice	[]int, []string
Length	Initial number of elements	3
Capacity	Total allocated space (optional)	5
Usage	Create slice with length and capacity	make([]int, 3, 5)

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Key Takeaways

Use make([]Type, length, capacity) to create slices with specified size and space.

Length is how many elements you can use; capacity is total allocated space.

Capacity must be equal or larger than length, or Go will error.

Accessing elements beyond length causes runtime panic.

make initializes slice elements with zero values of the element type.