How do I use the strconv.Atoi() method in Go?

Question

I am trying to get a user input in this small program. I have tried doing this several ways with the strconv.Atoi() method (my input is obviously a string, and I'm trying to convert it to an integer). Here's my first attempt:

package main
    import (
        "fmt"
        "strconv"
    )
    
    func main() {
        //fmt.Println(strconv.Itoa)
        fmt.Println("Say something, in numbers.")
        var inputstr string
        fmt.Scanln("%s", &inputstr)
        input := strconv.Atoi(inputstr)
        output := (input * 2)
        outputstr := strconv.Itoa(output)
        fmt.Println(outputstr)
    }

and got the following error when it came to compiling:

(line 19) multiple-value strconv.Atoi() in single-value context

I then looked into Godocs and tried to figure this out for myself, and then realized that an error value is returned as well. So, I changed the

input := strconv.Atoi(inputstr)

to

input, _ := strconv.Atoi(inputstr)

Now this compiles just fine, without any errors. However, when I run the program, here's what I get:

Say something, in numbers.

0

and then it exits... What am I doing wrong? I believe this is a question about to Atoi() method, but if it's concerning the Scanln() then please correct me.

Solution

The problem turns out to be the Scanln. Scanln is returning an error type not a pointer because of the %s. This then leaves inputstr blank, which when given to Atoi is returning an error: strconv.ParseInt: parsing "": invalid syntax.

Using Scanf as follows with no change to the Atoi:

func main() {
    //fmt.Println(strconv.Itoa)
    fmt.Println("Say something, in numbers.")
    var inputstr string

    //fmt.Scanln("%s", &inputstr)
    _, err := fmt.Scanf("%s", &inputstr)
    if err != nil {
        fmt.Println(err)
    }
    input, e := strconv.Atoi(inputstr)
    if e != nil {
        fmt.Println(e)
    }
    output := (input * 2)
    outputstr := strconv.Itoa(output)
    fmt.Println(outputstr)
}

Probably the simplest solution is to remove the "%s" from the Scanln.

Comments

[Chris Morris]

So basically, the "%s" is just a format character that converts the input to a string?

[miltonb]

Actually, the %s is used as a format character for Sscan, but in your code its getting two errors one from the Scanf("%s",..) not liking the %s and the next from Atoi not liking an empty string.