Unexpected behavior of %+02d in Sprintf() and friends

[Eric Raymond]

Under Go 1.10.1, feeding an 0 value to a %+02d specifier sometimes yields  "+0", not "+00". The attached tiny Go program may reproduce this behavior.  I say "may" because I first observed it in a series of unit tests of date format conversions - in different format strings %+02d expanded differently.  I haven't found a pattern to this, or I'd report it. On my system this program, at least, has repeatable behavior.

If this behavior were consistent, I'm not sure it would be a bug. It's possible that the sign is supposed to be counted as part of the number width; if so, it's an interesting question whether this is the right thing when explicit sign is forced by +.  The documentation is unclear.

The apparent inconsistency worries me.  There may be some state in the form,at-interpretation code that is not always tracked correctly.

In accordance with the Contribution Guidelines, I'm tossing  the question out here for a sanity check before throwing it on the bugtracker.  Have there been any similar reports?

[peterGo]

Eric,

"Width is specified by an optional decimal number immediately preceding the verb. If absent, the width is whatever is necessary to represent the value. "

https://golang.org/pkg/fmt/

Width is two.

Peter

[peterGo]

Eric,

For example,

package main

import (
    "fmt"
    "runtime"
)

func main() {
    // Correctly prints "+0 00\n" as +00 00
    fmt.Printf("%s: %+03d %02d\n", runtime.Version(), 0, 0)
}

Output:

go1.10.3: +00 00

Playground: https://play.golang.org/p/Pt2-YJfQvEo

Widths three and two.

Peter

[Borman, Paul]

I believe +0 is correct.  That is 2 bytes which is what you requested with %+02d and the + takes up the first byte and 0 fits in the second byte.

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<gobug1.go>

[Eric Raymond]

On Tuesday, August 28, 2018 at 4:49:02 PM UTC-4, peterGo wrote:

"Width is specified by an optional decimal number immediately preceding the verb. If absent, the width is whatever is necessary to represent the value. "

https://golang.org/pkg/fmt/

Width is two.

Thanks for the clarification.  May I recommend appending to that sentence "(including the leading sign, if any, even if the sign is forced by a + modifier)"?

I still think i saw apparently inconsistent behavior.  I'm still writing unit tests involving date conversions, so I'm going to try to reproduce this and turn in an actionable report.

[Borman, Paul]

Is it possible you used the format string “+%02d” vs “%+02d”?  The first will give you the +00 you expected while the second is +0, as discussed.

[Eric Raymond]

On Wednesday, August 29, 2018 at 7:57:23 AM UTC-4, Borman, Paul wrote:

Is it possible you used the format string “+%02d” vs “%+02d”?  The first will give you the +00 you expected while the second is +0, as discussed.

Alas, no.  Here's the line from my test program:

fmt.Printf("%s: %+02d %02d\n", runtime.Version(), 0, 0)

In the original context where I saw the behavior, I was generating [+-]hhmm strings corresponding to timezone offsets. The sign in the output could not be fixed in the format; it had to be derived from the offset.