Processing a file byte by byte in go

Question

Similar to this question, what is the idiomatic way to process a file one byte at a time in go?

Put another way, is there a better way to write the following?

file, err := ioutil.ReadFile(filename)
file_string = string(file)
for i, c := range file_string {
    // -snip-
}

Answer 1

You're reading the entire file as a string (not bytes) at once, then processing it rune-by-rune (not byte-by-byte). To literally read the file one byte at a time (which is very likely not what you want), you must do exactly that:

f, err := os.Open("path")
if err != nil {
    panic(err)
}

b := make([]byte, 1)

for {
    err := f.Read(b)
    if err != nil && !errors.Is(err, io.EOF) {
        fmt.Println(err)
        break
    }

    // process the one byte b[0]

    if err != nil {
        // end of file
        break
    }
}

However, what you more likely want is to read the file efficiently, and process it one byte at a time. For this, you should use a buffered reader:

f, err := os.Open("path")
if err != nil {
    panic(err)
}

br := bufio.NewReader(f)

// infinite loop
for {

    b,err := br.ReadByte()

    if err != nil && !errors.Is(err, io.EOF) {
        fmt.Println(err)
        break
    }

    // process the one byte b

    if err != nil {
        // end of file
        break
    }
}