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?

As far as I can see, each iteration is done only once, Debug: i: n where n goes from 0 to 9 suggests exactly that. Or do you mean something else? Here is my session in the terminal: gist.github.com/4357860 It works exactly as expected. — Vladimir Matveev
– Vladimir Matveev, Commented Dec 22, 2012 at 7:13
@VladimirMatveev If you try to run it on your machine, i think youll see what I mean . Do you see the scanf in the loop? It should read a value from the keyboard 10 times. But for every input I give in, the loop block executes twice. So i increments twice. — gprasant
– gprasant, Commented Dec 22, 2012 at 8:20
@gprasant, the link I gave holds the copy of terminal session on my computer with your program. It went exactly as your code suggests: print "Debug: i: $n " line -> awaiting number -> I enter the number -> it is printed -> repeat again. However, the output you have given cannot be produced by you program in principle; it can be produced by the program that peterSO has written in his answer. — Vladimir Matveev
– Vladimir Matveev, Commented Dec 22, 2012 at 15:16

peterSO · Accepted Answer · 2012-12-22 07:35:44Z

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

Im using windows. I tried your suggestion. Im still having the same problem
Im sorry, I midded out the '\n' in fmt.Scanf("%d\n", &num) . I tried the scanfwith \n and it worked

pexeer · Accepted Answer · 2012-12-24 05:10:12Z

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.

it was a problem with the absense '\n' in the scanf. Is this a windows line ending problem? because I ran the same code above on a friend's mac and it worked perfectly
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').

Zhe Hu · Accepted Answer · 2013-03-08 03:46:59Z

Just to point out fmt.Scanln(&num) would probably work the same as fmt.Scanf("%d\n",&num), since fmt.Scanln(&num) also check the type of "num".

In other words, if

var num float32 fmt.Scanln(&num)

you can input floating number from the console.

Francesco Lo Cascio · Accepted Answer · 2021-06-09 12:03:54Z

I have the same problem, i resolved adding "\n" in a scanf format string: fmt.Scanf("%d\n", &num).

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