Dynamically initialize array size in go

I try to write a small application in go that takes 'x' numbers of integers from standard input, calculates the mean and gives it back. I have only gotten so far:

func main() {
var elems, mean int
sum := 0

fmt.Print("Number of elements? ")

fmt.Scan(&elems)

var array = new([elems]int)

for i := 0; i < elems; i++ {
	fmt.Printf("%d . Number? ", i+1)
	fmt.Scan(&array[i])
	sum += array[i];
}............

When trying to compile this I get the following error message:

> invalid array bound elems

What is wrong here?

You should use a slice instead of an array:

//var array = new([elems]int) - no, arrays are not dynamic
var slice = make([]int,elems) // or slice := make([]int, elems)

See "go slices usage and internals". Also you may want to consider using range for your loop:

// for i := 0; i < elems; i++ { - correct but less idiomatic
for i, v := range slice {

In my opinion, this results from confusion over the usage of the new and make functions. This is a known issue/feature in the Go language, as evidenced by several discussions about new vs make at golang-nuts.

The difference between new and make may become clearer by letting Go print out the type of the value created by new and make:

package main

import "fmt"
                                               
func main() {
    fmt.Printf("%T  %v\n", new([10]int), new([10]int))
    fmt.Printf("%T  %v\n", make([]int, 10), make([]int, 10))
}

The output:

*[10]int  &[0 0 0 0 0 0 0 0 0 0]
[]int  [0 0 0 0 0 0 0 0 0 0]

As can be seen from the type, to access an array element of new([10]int) we would first need to dereference the pointer.

Both new and make require a Go type as their 1st argument. However, the expression [elems]int is not a Go type (unless elems is a Go constant, which isn't the case here).

For further reference, see http://golang.org/doc/go_spec.html#Allocation and http://golang.org/doc/go_spec.html#The_zero_value.

To get a better understanding of whether the result of new is usable, it may be helpful to lookup whether len and cap work with zero (nil) values: http://golang.org/doc/go_spec.html#Length_and_capacity

See The Go Programming Language Specification

http://golang.org/ref/spec#Array_types

http://golang.org/ref/spec#Constants

It says:"The length is part of the array's type; it must evaluate to a non- negative constant representable by a value of type int. "

Constants by no means vary.