Golang converting from rune to string

I have the following code, it is supposed to cast a rune into a string and print it. However, I am getting undefined characters when it is printed. I am unable to figure out where the bug is:

package main

import (
    "fmt"
    "strconv"
    "strings"
    "text/scanner"
)

func main() {
    var b scanner.Scanner
    const a = `a`
    b.Init(strings.NewReader(a))
    c := b.Scan()
    fmt.Println(strconv.QuoteRune(c))
}

That's because you used Scanner.Scan() to read a rune but it does something else. Scanner.Scan() can be used to read tokens or runes of special tokens controlled by the Scanner.Mode bitmask, and it returns special constants form the text/scanner package, not the read rune itself.

To read a single rune use Scanner.Next() instead:

c := b.Next()
fmt.Println(c, string(c), strconv.QuoteRune(c))

Output:

97 a 'a'

If you just want to convert a single rune to string, use a simple type conversion. rune is alias for int32, and converting integer numbers to string:

Converting a signed or unsigned integer value to a string type yields a string containing the UTF-8 representation of the integer.

So:

r := rune('a')
fmt.Println(r, string(r))

Outputs:

97 a

Also to loop over the runes of a string value, you can simply use the for ... range construct:

for i, r := range "abc" {
    fmt.Printf("%d - %c (%v)\n", i, r, r)
}

Output:

0 - a (97)
1 - b (98)
2 - c (99)

Or you can simply convert a string value to []rune:

fmt.Println([]rune("abc")) // Output: [97 98 99]

There is also utf8.DecodeRuneInString().

Try the examples on the Go Playground.

Note:

Your original code (using Scanner.Scan()) works like this:

You called Scanner.Init() which sets the Mode (b.Mode) to scanner.GoTokens.

Calling Scanner.Scan() on the input (from "a") returns scanner.Ident because "a" is a valid Go identifier:

c := b.Scan()
if c == scanner.Ident {
    fmt.Println("Identifier:", b.TokenText())
}

// Output: "Identifier: a"

I know I'm a bit late to the party but here's a []rune to string function:

func runesToString(runes []rune) (outString string) {
    // don't need index so _
    for _, v := range runes {
        outString += string(v)
    }
    return
}

yes, there is a named return but I think it's ok in this case as it reduces the number of lines and the function is only short

Since I came to this question searching for rune and string and char, thought this may help newbies like me

// str := "aഐbc"
// testString(str)
func testString(oneString string){

    //string to byte slice - No sweat -just type cast it
    // As string  IS A byte slice
    var twoByteArr []byte = []byte(oneString)

    // string to rune Slices - No sweat 
    // string IS A slice of runes 
    var threeRuneSlice []rune = []rune(oneString)

   // Hmm! String seems to have a dual personality it is both a slice of bytes and
   // a slice of runes - yeah - read on
    
    // A rune slice can be convered to string -
    // No sweat - as string == rune slice
    var thrirdString string = string(threeRuneSlice)
    
    // There is a catch here and that is in printing "characters", using for loop and range 
    
    fmt.Println("Chars in oneString")
    for i,r := range oneString {
        fmt.Printf(" %d  %v  %c ",i,r,r) //you may not get index 0,1,2,3 here  
        // since the range runs specially over strings  https://blog.golang.org/strings
    }
    
    fmt.Println("\nChars in threeRuneSlice")
    for i,r := range threeRuneSlice {
        fmt.Printf(" %d  %v  %c ",i,r,r) // i = 0,1,2,4 , perfect!!
        // as runes are made up of 4 bytes (rune is int32 and byte in unint8
        // and a set of bytes is used to represent a rune which is used to 
       // represent  UTF characters == the REAL CHARECTER 
    }

    fmt.Println("\nValues in oneString ")
    for j := 0; j < len(oneString); j++ {
        fmt.Printf(" %d %v ",j,oneString[j]) // No you cannot get charecters if you iterate through string in this way
        // as you are going over bytes here - not runes
    }
    fmt.Println("\nValues in twoByteArr")
    for j := 0; j < len(twoByteArr); j++ {
        fmt.Printf(" %d=%v ",j,twoByteArr[j]) // == same as above
    }
    
    fmt.Printf("\none - %s, two %s, three %s\n",oneString,twoByteArr,thrirdString)
}

And some more pointless demo https://play.golang.org/p/tagRBVG8k7V adapted from https://groups.google.com/g/golang-nuts/c/84GCvDBhpbg/m/Tt6089MPFQAJ

to show that the 'characters' are encoded with one to up to 4 bytes depending on the unicode code point

Golang converting from rune to string

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