fmt.Sscanf strange behaviour

[Victor Giordano]

Hi all! I'm dealing with some multi valued column in postgres, and i have stomp into a bizzare behavoir in `fmt.Sscanf`

Please take a look at the following snippet, i also leave my question within the code and this link to the playground so you can actually try it and see with you own eyes.

```
package main

import "fmt"

func main() {
        var a int
        var b string
        input := "(1,beto)" // doesn't like the parenthesis

        _, err := fmt.Sscanf(input, "(%d,%s)", &a, &b)
        if err != nil {
                fmt.Println(err)
        } else {
                fmt.Println(a, b)
        }

        input = "1,beto" // this works
        _, err = fmt.Sscanf(input, "%d,%s", &a, &b)
        if err != nil {
                fmt.Println(err)
        } else {
                fmt.Println(a, b)
        }

        input = "beto,1" // doesn't like an string at first place, ¿¿WHAT i do?!?!
        _, err = fmt.Sscanf(input, "%s,%d", &b, &a)
        if err != nil {
                fmt.Println(err)
        } else {
                fmt.Println(a, b)
        }
}

```

I saw this ticket i'm wondering about how realiable is the Sscanf function indeed, as i saw that you guys having problems too.

Thanks for reading me!

V

[Ian Lance Taylor]

Using %s in fmt.Sscanf will read all characters up to a space or
newline. This is documented at https://pkg.go.dev/fmt which says
"Input processed by verbs is implicitly space-delimited: the
implementation of every verb except %c starts by discarding leading
spaces from the remaining input, and the %s verb (and %v reading into
a string) stops consuming input at the first space or newline
character."

Basically fmt.Sscanf is not the right tool for the problem you are
trying to solve.

Ian

[Victor Giordano]

Thanks Ian, I will take your advice and use another approach, I regret this cuz it would be nice to have something already working. 

Without any intent to offense or say something hard, as a client of the library it kind looks quite buggy, the parenthesis are still an issue disregarding that the values are actually space-delimited. The error messages clearly are an invitation to see the source for deeper understanding rather than given any clues.

Greetings

V

--

V

[Ian Lance Taylor]

On Tue, Jan 25, 2022 at 12:45 PM Victor Giordano <vituc...@gmail.com> wrote:
>
> Thanks Ian, I will take your advice and use another approach, I regret this cuz it would be nice to have something already working.
>
> Without any intent to offense or say something hard, as a client of the library it kind looks quite buggy, the parenthesis are still an issue disregarding that the values are actually space-delimited. The error messages clearly are an invitation to see the source for deeper understanding rather than given any clues.

Your newer example is still working as documented. The %s swallows
everything up to the space, including the right parenthesis. Then
there is nothing to match the right parenthesis in the format string.

This behavior dates back to the C function sscanf that the Go code is
based on. The %s code can be useful for space-separated fields, but
is not very useful for anything else.

Ian

[Victor Giordano]

Thanks for the insight again. It is appreciated.

Till next time.

V

--

V

[Roland Müller]

The tool you are looking for is most probably the regex module:

https://pkg.go.dev/regexp

BR
Roland

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[Brian Candler]

On Wednesday, 26 January 2022 at 04:51:04 UTC rol...@gmail.com wrote:

The tool you are looking for is most probably the regex module:

https://pkg.go.dev/regexp

"Then you have two problems" :-)  But it could be reasonable here if the format is fixed.

If this is a true multi-valued column in postgres, such as an array type, then I would be trying to get the postgres API to return this to me in a structured way, e.g. see here.

If this is just a string field which happens to contain the text "(foo,bar)" then this is a custom serialization of your data, and you'll have to parse this custom format yourself.  I'd be thinking "what should happen if the parentheses are missing? How does this format represent a value which contains a comma or contains a parenthesis?"