How do I use fmt.Scanf in Go

Question

I seem to be having a queer problem while getting user input within a for loop in go. Here is my code

package main

import "fmt"

func main() {
    var num int
    for i := 0; i < 10; i++ {
        fmt.Printf("Debug: i : %d ", i)
        fmt.Scanf("%d", &num)
        fmt.Println(num)
    }
}

What happens when I run this code is this :

Debug: i : 0
Enter next number
1
1
Debug: i : 1
Enter next number
1
Debug: i : 2
Enter next number
2
2
Debug: i : 3
Enter next number
2
Debug: i : 4
Enter next number
3
3
Debug: i : 5
Enter next number
3
Debug: i : 6
Enter next number
4
4
Debug: i : 7
Enter next number
4
Debug: i : 8
Enter next number
5
5
Debug: i : 9
Enter next number
5

What I notice is that each iteration of the loop happens twice, Could this be because Go is using parallelism by default and causing both processors to run the code within a for loop?

Answer 1

What OS are you using? Windows?

Try this:

package main

import "fmt"

func main() {
    var num int
    for i := 0; i < 10; i++ {
        fmt.Printf("Debug: i : %d\n", i)
        fmt.Println("Enter next number")
        n, err := fmt.Scanf("%d\n", &num)
        if err != nil {
            fmt.Println(n, err)
        }
        fmt.Println(num)
    }
}

Output:

Debug: i : 0
Enter next number
1
1
Debug: i : 1
Enter next number
2
2
Debug: i : 2
Enter next number
3
3
Debug: i : 3
Enter next number
4
4
Debug: i : 4
Enter next number
5
5
Debug: i : 5
Enter next number
6
6
Debug: i : 6
Enter next number
7
7
Debug: i : 7
Enter next number
8
8
Debug: i : 8
Enter next number
9
9
Debug: i : 9
Enter next number
10
10

Answer 2

The above answer is a good suggestion. the code

    if err != nil {
        fmt.Println(n, err)
    }

will output the reason of this problem.

  10 unexpected newline

So I change the code to this, and it works.

package main

import "fmt"

func main() {
    var num int
    for i := 0; i < 10; i++ {
        fmt.Printf("Debug: i : %d ", i)
        fmt.Scanf("%d\n", &num) // add "\n"
        fmt.Println(num)
    }
}

this is because of the different line endings. the windows uses carriage return and line feed(\r\n) as a line ending. the Unix uses the line feed(\n).

you can use notepad2 to create a file (a.txt) with \r line feed. and do this:

  go run s.go < input.txt

this will work correctly.