Get length of an array in golang and store it in a const

Question

I used the go playground to try this code:

package main

func main() {
    exp := []string{"HELLO"}

    const length = len(exp)
}

But it is throwing me this error:

./prog.go:6:8: const initializer len(exp) is not a constant

I can see that this is happening because the length of exp is changeable. How can i get the current length and store it in an integer constant?

I would think that I need the reflect module to do this, but I don't know how to do it. I tried using reflect.ValueOf but it did not work.

exp := []string{"HELLO"}
const length = len(reflect.ValueOf(exp).Interface().([]string))

Why not store the string itself in a constant? – mkrieger1 Feb 16 '20 at 00:53 — mkrieger1, Feb 16 '20 at 00:53
what string are you referencing? – Ank i zle Feb 16 '20 at 00:58 — Ank i zle, Feb 16 '20 at 00:58
i want the length of the array not the string inside – Ank i zle Feb 16 '20 at 01:16 — Ank i zle, Feb 16 '20 at 01:16
That's not an array, it's a slice. – Jonathan Hall Feb 16 '20 at 10:19 — Jonathan Hall, Feb 16 '20 at 10:19

score 6 · Answer 1 · answered Feb 16 '20 at 01:21

You can indeed get the current length, but that is by definition not a constant. Here is what the language specification has to say about this:

The expression len(s) is constant if s is a string constant. The expressions len(s) and cap(s) are constants if the type of s is an array or pointer to an array and the expression s does not contain channel receives or (non-constant) function calls; in this case s is not evaluated. Otherwise, invocations of len and cap are not constant and s is evaluated.

Since your invocation of len is as len(exp) we must test exp against the text above:

Is it a string constant? No; it is a slice.
Is it an array? No; it is a slice.
Is it a pointer to an array? No, it is a slice.

The three cases where len would produce a constant have been exhausted, so len must necessarily produce a non-constant.

If you would like len to produce a constant, you will have to apply it to a string constant, or an array, or a pointer to an array. Arrays are similar in many ways to slices, so this is possible. For instance, in your example, simply replacing:

exp := []string{"HELLO"}

with:

exp := [1]string{"HELLO"}

suffices. Should you wish the compiler to count the initializers, using ... suffices:

exp := [...]string{"HELLO"}

See example on Go Playground. Note that because exp is now an array, not a slice, some operations cannot be performed on/with exp.

Get length of an array in golang and store it in a const

1 Answers1