Reading from a file and check for file endings

Question

I am trying to do the following tasks in Go;

1. Read from a file
2. Backup the file
3. Overwrite and perform actions based on the files output on a character by character basis (which may include newLine characters or "b" for example)

Unfortunately, I am stuck on step 3 and would like some assistance with "new line" characters (i.e. "/n"). I have tried using filesText, err := ioutil.ReadFile(fileNameAndDirectory) but unfortunately if I try and check for file endings after converting filesText to a string I am unable to detect new line characters (i.e. If files text is "/nhello/n" then the following code snippet will not print the string "match"

filesText, err := ioutil.ReadFile(fileNameAndDirectory) 
if (string(filesText)[:2]) == "/n") {
    fmt.Println("match")
}

).

Is there something that I can do to detect new lines without reading the file line by line manually?

Example: If the file contents is "r/n3$/n;" then I should able to perform 6 predefined actions (one for for each character) as I move from left to right over the files contents.

Answer 1

We can only guess. You haven't defined your problem clearly. Please clarify your specific problem or add additional details to highlight exactly what you need. Provide sample input and output.

---

The Go Programming Language Specification

After a backslash, certain single-character escapes represent special values:

\n   U+000A line feed or newline

---

Here's a guess:

lines.go:

package main

import (
    "fmt"
    "io/ioutil"
    "os"
    "strings"
)

func lines(data []byte) {
    text := string(data)
    for i, j := 0, 0; j >= 0; i += j + 1 {
        var line string
        j = strings.IndexByte(text[i:], '\n')
        if j < 0 {
            line = text[i:]
            if len(line) == 0 {
                continue
            }
        } else {
            line = text[i : i+j+1]
        }
        // process line
        fmt.Printf("%d %q\n", len(line), line)
    }
}

func main() {
    filename := `test.file`
    data, err := ioutil.ReadFile(filename)
    if err != nil {
        fmt.Fprintln(os.Stderr, err)
        return
    }
    fmt.Printf("%d %q\n", len(data), data)
    lines(data)
}

Output:

$ cat test.file
line1
line2
line3
$ go run lines.go
18 "line1\nline2\nline3\n"
6 "line1\n"
6 "line2\n"
6 "line3\n"

---

Comment:

I'll try and clarify it through an example. Let say that I have a file with contents "ara/n;>$g9s", my application will perform an action defined by that input character as it moves through the contents of the file. I.e. If "a" does action 1, "r" does action 2, "/n" does action 3 and so on then the input above will perform the following actions 1,2,1,3... in that order. However, if you turn the byte array to a string then I'm unable to identify "/n" characters since they appear to be removed despite the string having the same formatting as before if you print it out or concat it into a file. – Elliot Smith

Why do you write /n for a newline character! The newline character U+000A, as I've already pointed out, is written as \n.

For example,

package main

import "fmt"

func es(s string) {
    for _, r := range s {
        switch r {
        case 'a':
            fmt.Printf("action 1 for %q\n", r)
        case 'r':
            fmt.Printf("action 2 for %q\n", r)
        case '\n':
            fmt.Printf("action 3 for %q\n", r)
        default:
            fmt.Printf("action ? for %q\n", r)
        }
    }
}

func main() {
    b := []byte("ara\n;>$g9s")
    s := string(b)
    es(s)
}

Playground: https://play.golang.org/p/3J0pxXh3Wkc

Output:

action 1 for 'a'
action 2 for 'r'
action 1 for 'a'
action 3 for '\n'
action ? for ';'
action ? for '>'
action ? for '$'
action ? for 'g'
action ? for '9'
action ? for 's'

---

Revised Question:

Example: If the file contents is "r/n3$/n;" then I should able to perform 6 predefined actions (one for for each character) as I move from left to right over the files contents. Elliot Smith

---

Why do you write /n for a newline character! The newline character U+000A, as I've already pointed out, is written as \n.

For example,

package main

import "fmt"

func es(s string) {
    for _, r := range s {
        switch r {
        case 'a':
            fmt.Printf("action for %q\n", r)
        case 'r':
            fmt.Printf("action for %q\n", r)
        case '\n':
            fmt.Printf("action for %q\n", r)
        default:
            fmt.Printf("action for %q\n", r)
        }
    }
}

func main() {
    file := []byte("r\n3$\n;")
    s := string(file)
    es(s)
}

Playground: https://play.golang.org/p/X1gtrPRmlqq

Output:

action for 'r'
action for '\n'
action for '3'
action for '$'
action for '\n'
action for ';'