Golang Increcementing numbers in strings (using runes)

Question

I have a string mixed with characters and numerals, but i want to increment the last character which happens to be a number, here is what i have, it works, but once i reach 10 rune goes to black since 10 decimal is zero, is there a better way to do this?

package main

import (
    "fmt"

)

func main() {
str := "version-1.1.0-8"
rStr := []rune(str)


last := rStr[len(rStr)-1]
rStr[len(rStr)-1] = last + 1


}

So this works for str := "version-1.1.0-8" = version-1.1.0-9 str := version-1.1.0-9 = version-1.1.0-

I understand why it is happening, but I dont know how to fix it

Answer 1

Your intention is to increment the number represented by the last rune, so you should do that: parse out that number, increment it as a number, and "re-encode" it into string.

You can't operate on a single rune, as once the number reaches 10, it can only be represented using 2 runes. Another issue is if the last number is 19, incrementing it needs to alter the previous rune (and not adding a new rune).

Parsing the numbers and re-encoding though is much easier than one might think.

You can take advantage of the fmt package's fmt.Sscanf() and fmt.Sprintf() functions. Parsing and re-encoding is just a single function call.

Let's wrap this functionality into a function:

const format = "version-%d.%d.%d-%d"

func incLast(s string) (string, error) {
    var a, b, c, d int
    if _, err := fmt.Sscanf(s, format, &a, &b, &c, &d); err != nil {
        return "", err
    }

    d++
    return fmt.Sprintf(format, a, b, c, d), nil
}

Testing it:

s := "version-1.1.0-8"

for i := 0; i < 13; i++ {
    var err error
    if s, err = incLast(s); err != nil {
        panic(err)
    }
    fmt.Println(s)
}

Output (try it on the Go Playground):

version-1.1.0-9
version-1.1.0-10
version-1.1.0-11
version-1.1.0-12
version-1.1.0-13
version-1.1.0-14
version-1.1.0-15
version-1.1.0-16
version-1.1.0-17
version-1.1.0-18
version-1.1.0-19
version-1.1.0-20
version-1.1.0-21

Another option would be to just parse and re-encode the last part, and not the complete version text. This is how it would look like:

func incLast2(s string) (string, error) {
    i := strings.LastIndexByte(s, '-')
    if i < 0 {
        return "", fmt.Errorf("invalid input")
    }

    d, err := strconv.Atoi(s[i+1:])
    if err != nil {
        return "", err
    }

    d++
    return s[:i+1] + strconv.Itoa(d), nil
}

Testing and output is the same. Try this one on the Go Playground.