Making a filename truncator in go - go

Question

I am new to Go, and I've been trying to make a program that truncates filenames after (and including) the character "-" of all files in its directory. It's not working, and I don't know where I'm going wrong.
func changeFilename() {
file, err := os.Open(".")
if err != nil {
log.Fatalf("failed opening directory: %s", err)
}
defer file.Close()
oldNames, _ := file.Readdirnames(0)
var s string
for _, i := range oldNames {
for _, j := range i {
for j != '-' {
k := strconv.QuoteRune(j)
s += k
j++
}
}
err := os.Rename(i, s)
if err != nil {
log.Fatal("failed to rename: %s", err)
}
}
}
I expected filenames in the test directory to be changed when I ran the executable, but they didn't. The program builds without any error messages.

Answer 1

Fixing your logic to correspond to your description of the problem:
package main
import (
"fmt"
"log"
"os"
"path/filepath"
)
// changeFilename truncates filenames after (and including) the character "-"
// of all files in the directory. File extensions are preserved.
func changeFilename(dir string) {
file, err := os.Open(dir)
if err != nil {
log.Fatalf("failed opening directory: %s", err)
}
defer file.Close()
fis, err := file.Readdir(0)
if err != nil {
log.Fatalf("failed reading directory: %s", err)
}
for _, fi := range fis {
if !fi.Mode().IsRegular() {
continue
}
old := fi.Name()
ext := filepath.Ext(old)
for i, r := range old[:len(old)-len(ext)] {
if r == '-' {
new := old[:i] + ext
err := os.Rename(old, new)
if err != nil {
log.Fatalf("failed to rename: %s", err)
}
fmt.Printf("%q %q\n", old, new)
break
}
}
}
}
func main() {
changeFilename(".")
}
Output:
"trunc-ate.typ" "trunc.typ"
"hyp-hen.ext" "hyp.ext"

Answer 2

Your s variable should be moved into the outermost loop since its lifetime should be tied to a single oldName. The innermost loop has no exit condition once a non - character is encountered. That should be an if statement with a corresponding break if a - is found. As for building up the new filename, you ought to use += string(j) instead of quoting the rune.
func changeFilename() {
file, err := os.Open(".")
if err != nil {
log.Fatalf("failed opening directory: %s", err)
}
defer file.Close()
oldNames, _ := file.Readdirnames(0)
for _, i := range oldNames {
var s string
for _, j := range i {
if j != '-' {
s += string(j)
} else {
break
}
}
err := os.Rename(i, s)
if err != nil {
log.Fatal("failed to rename: %s", err)
}
}
}