CODEWITHSUNDEEP
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L.jerson
L.jerson

Reputation: 973

golang append() evaluated but not used

func main(){
     var array [10]int
     sliceA := array[0:5]
     append(sliceA, 4)
     fmt.Println(sliceA)
}

Error : append(sliceA, 4) evaluated but not used

I don't Know why? The slice append operation is not run...

Upvotes: 97

Views: 67547

Answers (5)

Mohsin Aljiwala
Mohsin Aljiwala

Reputation: 2627

Refer: Appending to and copying slices

In Go, arguments are passed by value.

Typical append usage is:

a = append(a, x)

You need to write:

func main(){
    var array [10]int
    sliceA := array[0:5]
    // append(sliceA, 4)  // discard
    sliceA = append(sliceA, 4)  // keeping return value is mandatory
    fmt.Println(sliceA)
}

Output:

[0 0 0 0 0 4]

Upvotes: 142

榴莲榴莲
榴莲榴莲

Reputation: 161

sliceA = append(sliceA, 4)

append() returns a slice containing one or more new values.
Note that we need to accept a return value from append as we may get a new slice value.

Upvotes: 16

hefeicoder
hefeicoder

Reputation: 143

The key to understand is that slice is just a "view" of the underling array. You pass that view to the append function by value, the underling array gets modified, at the end the return value of append function gives you the different view of the underling array. i.e. the slice has more items in it

your code
sliceA := array[0:5]  // sliceA is pointing to [0,5)
append(sliceA, 4) // sliceA is still the original view [0,5) until you do the following

sliceA = append(sliceA, 4)

reference: https://blog.golang.org/slices

Upvotes: 1

Nevercare
Nevercare

Reputation: 81

you may try this:

sliceA = append(sliceA, 4)

built-in function append([]type, ...type) returns an array/slice of type, which should be assigned to the value you wanted, while the input array/slice is just a source. Simply, outputSlice = append(sourceSlice, appendedValue)

Upvotes: 4

djt
djt

Reputation: 7535

Per the Go docs:

The resulting value of append is a slice containing all the elements of the original slice plus the provided values.

So the return value of 'append', will contain your original slice with the appended portion.

Upvotes: 2

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