Reading and working with an external file in the Go programming language

Question

I have been learning the Go programming language by doing some of the Project Euler problems. I am now on [problem 13] (http://projecteuler.net/problem=13). It contains an external file with 100 lines of 50 digit numbers. My question is: How can this file be read into a Go program and worked with? Does Go have a readlines function? I've read about the io and ioutil packages, and about all I can come up with is reading in the file and printing it; however, I am not sure how to work with the file... Can it be assigned to a variable? Is there a readlines function, etc...

Any help would be appreaciated.

Here is what I have so far:

package main

import "fmt"
import "io/ioutil"

func main() {
        fmt.Println(ioutil.ReadFile("one-hundred_50.txt"))
}

score 3 · Accepted Answer · answered May 24 '12 at 23:28

There are ways to read a file line by line (and there are examples if you search here on SO) but really ioutil.ReadFile is a good start there. Sure you can assign it to a variable. Look at the function signature for ReadFile and see how it returns both a byte slice and an error. Assign both; check that the error is nil. Print the error if it's not nil so you can see what's wrong. Then once you have the bytes in a variable, try spitting it up by lines. Try bytes.Split, or easier, convert it to a string and use strings.Split.

score 1 · Answer 2 · edited May 23 '17 at 10:34

Check out bufio. This answer uses it to read the entire file into memory.

For this Euler problem you can just use ReadString:

package main

import (
  "os"
  "bufio"
  "fmt"
)

func main() {
  r := bufio.NewReader(os.Stdin)
  line, err := r.ReadString('\n')
  for i := 1; err == nil; i++ {
    fmt.Printf("Line %d: %s", i, line)
    line, err = r.ReadString('\n')
  }
}

To use:

go run solution.go < inputfile

score 0 · Answer 3 · answered Jul 26 '13 at 19:12

Since this question was asked and answered the bufio package has been updated (for Go 1.1) and perhaps a nicer solution is now available (not that any of these are bad).

The Scanner type from the bufio package makes this really easy:

func main() {
    f, e := os.Open("one-hundred_50.txt")
    if e != nil {
        // error opening file, handle it
    }
    s := bufio.NewScanner(f)
    for s.Scan() {
        // scanner.Text() contains the current line
    }
    if e = s.Err(); e != nil {
        // error while scanning; no error at EOF
    }
}

Reading and working with an external file in the Go programming language

3 Answers3