How to write tests against user input in Go - input

Question

How would I test against user input from fmt.Scan/Scanf/Scanln?
For example how could I test that the function input will accept "4 5\n" and "1 2 3 4\n" from STDIN and return n == 5 and array == [1, 2, 3, 4].
package main
import (
"fmt"
)
// input gets an array from the user.
func input() (m int, array []int) {
fmt.Print("Enter the size of the array, n, and the difference, m: ")
var n int
_, err := fmt.Scanf("%d %d", &n, &m)
if err != nil {
panic(err)
}
fmt.Print("Enter the array as a space seperated string: ")
array = make([]int, n)
for i := 0; i < n; i++ {
_, _ = fmt.Scan(&array[i])
}
return m, array
}
func main() {
m, array := input()
fmt.Println(m, array)
}

Answer 1

Here's a very rough draft to illustrate the principle.
program.go
package main
import (
"fmt"
"os"
)
// input gets an array from the user.
func input(in *os.File) (m int, array []int) {
if in == nil {
in = os.Stdin
}
fmt.Print("Enter the size of the array, n, and the difference, m: ")
var n int
_, err := fmt.Fscanf(in, "%d %d", &n, &m)
if err != nil {
panic(err)
}
fmt.Print("Enter the array as a space seperated string: ")
array = make([]int, n)
for i := 0; i < n; i++ {
_, _ = fmt.Fscan(in, &array[i])
}
return m, array
}
func main() {
m, array := input(nil)
fmt.Println(m, array)
}
program_test.go
package main
import (
"fmt"
"io"
"io/ioutil"
"os"
"testing"
)
func TestInput(t *testing.T) {
var (
n, m int
array []int
)
in, err := ioutil.TempFile("", "")
if err != nil {
t.Fatal(err)
}
defer in.Close()
_, err = io.WriteString(in, "4 5\n"+"1 2 3 4\n")
if err != nil {
t.Fatal(err)
}
_, err = in.Seek(0, os.SEEK_SET)
if err != nil {
t.Fatal(err)
}
n, array = input(in)
if n != 5 || fmt.Sprintf("%v", array) != fmt.Sprintf("%v", []int{1, 2, 3, 4}) {
t.Error("unexpected results:", n, m, array)
}
}
Output:
$ go test
ok command-line-arguments 0.010s

Answer 2

You can't. At least not so easily, such that, it would be worth the effort.