Golang concatenate or merge two or more slices [SOLVED]

In this tutorial, we will go through some examples of concatenating two or multiple slices in Golang. We will use the append() function, which takes a slice as the first argument and the values to append as the second.

func append(slice []Type, elems ...Type) []Type: The append built-in function appends elements to the end of a slice. If it has sufficient capacity, the destination is resliced to accommodate the new elements. If it does not, a new underlying array will be allocated. Append returns the updated slice. It is therefore necessary to store the result of append, often in the variable holding the slice itself:

slice = append(slice, elem1, elem2)
slice = append(slice, anotherSlice...)

We need to use the variadic function signature dots to merge 2 slices. In variadic functions in Golang, we can pass a variable number of arguments to the function and all the numbers can be accessed with the help of the argument name.

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Example 1: Merge slices using append() function

Let’s consider a simple example, where we will append an integer to an integer slice using append() function

package main

import "fmt"

func main() {
    intSlice := []int{2, 4, 6}
    fmt.Println("Slice before append:", intSlice)

    intSlice = append(intSlice, []int{8, 10}...)
    fmt.Println("Slice after append:", intSlice)

    //new integer slice
    intSliceNew := []int{1, 3, 5}
    intSlice = append(intSlice, intSliceNew...)
    fmt.Println("Slice after 2 append:", intSlice)
}

Output:

Slice before append: [2 4 6]
Slice after append: [2 4 6 8 10]
Slice after 2 append: [2 4 6 8 10 1 3 5]

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Example 2: Merge slices using copy() function

If you have the target slice, “appending” multiple slices is simply copying them to the target in the proper position. You can do this by using the built-in copy() function.

func copy(dst, src []Type) int: The copy built-in function copies elements from a source slice into a destination slice. (As a special case, it also will copy bytes from a string to a slice of bytes.) The source and destination may overlap. Copy returns the number of elements copied, which will be the minimum of len(src) and len(dst).

Let’s consider below example, where we append an integer to a slice using copy() function

package main

import "fmt"

func main() {
        // create 2 slice we want to merge
    intSlice1 := [5]int{2, 4, 6, 8, 10}
    intSlice2 := [4]int{3, 5, 7, 9}

        // create a destination slice
    var mergeSlice [len(intSlice1) + len(intSlice2)]int

        // copy all elements from slice 1 to destination slice
    copy(mergeSlice[:], intSlice1[:])

        // copy all elements from slice 2 to destination slice
    copy(mergeSlice[len(intSlice1):], intSlice2[:])
    fmt.Printf("%v\n", mergeSlice)
}

Output:

[2 4 6 8 10 3 5 7 9]

It should be noted that we can only use this method to append two fixed-length slices.

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Example 3: Concatenate multiple slices using append() function

In this example below, we demonstrate how to concatenate multiple string slices.

package main

import "fmt"

func main() {
    strSlice := []string{"This", "is", "Go", "Linux", "Cloud!"}
    fmt.Println("Slice before append:", strSlice)
    strSliceAppend1 := []string{"Welcome", "to", "our", "page."}
    strSliceAppend2 := []string{"Have", "a", "nice", "day!"}

    //append 2 string slices
    strSlice = append(strSlice, strSliceAppend1...)

    //append 2 string slices
    strSlice = append(strSlice, strSliceAppend2...)
    fmt.Println("Slice after append:", strSlice)

}

Output:

Slice before append: [This is Go Linux Cloud!]
Slice after append: [This is Go Linux Cloud! Welcome to our page. Have a nice day!]

You cannot concatenate more than two slices at a time with append. If you try to do so, it will throw atoo many arguments to append error at compile time. If you want to concatenate multiple slices, you have to call the append function multiple times.

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Summary

We have shown 3 examples of combining 2 or more slices using copy() and append() function in this article. We can also concatenate multiple slices using golang append function but the limitation is that at a time we can merge only 2 slices so we need to use append function multiple times to merge individual slice elements.

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Reference

https://pkg.go.dev/builtin#append
https://en.wikipedia.org/wiki/Variadic_function
https://pkg.go.dev/builtin#copy