Reading an integer from standard input

Question

How do I use the fmt.Scanf function in Go to get an integer input from the standard input?

If this can\'t be done using fmt.Scanf, what\'s

Answer 1

Here is my "Fast IO" method for reading positive integers. It could be improved with bitshifts and laying out memory in advance.

package main

import (
    "io/ioutil"
    "bufio"
    "os"
    "strconv"
)


func main() {
    out := bufio.NewWriter(os.Stdout)
    ints := getInts()
    var T int64
    T, ints = ints[0], ints[1:]
    ..
    out.WriteString(strconv.Itoa(my_num) + "\n")
    out.Flush()
    }
}

func getInts() []int64 {
    //assumes POSITIVE INTEGERS. Check v for '-' if you have negative.
    var buf []byte
    buf, _ = ioutil.ReadAll(os.Stdin)
    var ints []int64
    num := int64(0)
    found := false
    for _, v := range buf {
        if '0' <= v && v <= '9' {
            num = 10*num + int64(v - '0') //could use bitshifting here.
            found = true
        } else if found {
            ints = append(ints, num)
            found = false
            num = 0
        }
    }
    if found {
        ints = append(ints, num)
        found = false
        num = 0
    }
    return ints
}

Answer 2

You can use fmt.Scanf with a format specifier. The format specifier for the integer is %d. So you can use standard input like below.

func main() {
    var someVar int
    fmt.Scanf("%d", &someVar)
}

or else you can use fmt.Scan or fmt.Scanln as below.

func main() {
   var someVar int
   fmt.Scanln(&someVar)
}

Answer 3

An alternative that can be a bit more concise is to just use fmt.Scan:

package main

import "fmt"

func main() {
    var i int
    fmt.Scan(&i)
    fmt.Println("read number", i, "from stdin")
}

This uses reflection on the type of the argument to discover how the input should be parsed.

http://golang.org/pkg/fmt/#Scan

Answer 4

Golang fmt.Scan is simpler than Golang fmt.Scanf (which is simpler than Clang scanf)

If fmt.Scan errors i.e. if not nil, log & return

1 Read single variable:

import (
    "fmt"
    "log"
)

var i int
if    _, err := fmt.Scan(&i);    err != nil {
    log.Print("  Scan for i failed, due to ", err)
    return
}

fmt.Println(i)

2 Read multiple variables:

import (
    "fmt"
    "log"
)

var i, j, k int  
if    _, err := fmt.Scan(&i, &j, &k);    err != nil {
    log.Print("  Scan for i, j & k failed, due to ", err)
    return
}

fmt.Println(i, j, k)

Best of luck

Example from: http://www.sortedinf.com/?q=golang-in-1-hour

Answer 5

http://golang.org/pkg/fmt/#Scanf

All the included libraries in Go are well documented.

That being said, I believe

func main() {
    var i int
    _, err := fmt.Scanf("%d", &i)
}

does the trick