How to join a slice of strings into a single string?

The title of your question is:

How to join a slice of strings into a single string?

but in fact, reg is not a slice, but a length-three array. [...]string is just syntactic sugar for (in this case) [3]string.

To get an actual slice, you should write:

reg := []string {"a","b","c"}

(Try it out: https://play.golang.org/p/vqU5VtDilJ.)

Incidentally, if you ever really do need to join an array of strings into a single string, you can get a slice from the array by adding [:], like so:

fmt.Println(strings.Join(reg[:], ","))

(Try it out: https://play.golang.org/p/zy8KyC8OTuJ.)

Use a slice, not an arrray. Just create it using

reg := []string {"a","b","c"}

An alternative would have been to convert your array to a slice when joining :

fmt.Println(strings.Join(reg[:],","))

Read the Go blog about the differences between slices and arrays.

This is still relevant in 2018.

To String

import strings
stringFiles := strings.Join(fileSlice[:], ",")

Back to Slice again

import strings
fileSlice := strings.Split(stringFiles, ",")