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fmt: spaces instead of zeroes in padding when precision is 0 #34003

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inliquid opened this issue Sep 1, 2019 · 3 comments
Closed

fmt: spaces instead of zeroes in padding when precision is 0 #34003

inliquid opened this issue Sep 1, 2019 · 3 comments
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@inliquid
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inliquid commented Sep 1, 2019

I read fmt docs several times, but didn't find any reasoning why the following would ever happen.

So here is normal working behavior:

func main() {
	i := 1
	fmt.Printf("%03d", i)
}

Output (ok):

001

Now, we add one additional dot . making precision equal to zero, and we'll see spaces instead of zeroes as it should be with 0 flag:

func main() {
	i := 1
	fmt.Printf("%03.d", i)
}

Output (wrong):

  1

System details

go version go1.12.9 windows/amd64
GOARCH="amd64"
GOBIN=""
GOCACHE="C:\Users\***\AppData\Local\go-build"
GOEXE=".exe"
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="windows"
GOOS="windows"
GOPATH="C:\Users\***\go"
GOPROXY=""
GORACE=""
GOROOT="C:\Go"
GOTMPDIR=""
GOTOOLDIR="C:\Go\pkg\tool\windows_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="NUL"
GOROOT/bin/go version: go version go1.12.9 windows/amd64
GOROOT/bin/go tool compile -V: compile version go1.12.9
@KarthikNayak
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Did a little deep diving and found this in the source code.

// Two ways to ask for extra leading zero digits: %.3d or %03d.
// If both are specified the f.zero flag is ignored and
// padding with spaces is used instead.

This seems a little obsolete since the code below is modified, but the gist from the code is that when you use a precision value, only space padding is done. Happy to submit a patch if this isn't intended behavior

@robpike
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robpike commented Sep 1, 2019

It's working as intended and as explained in this comment. It is intentional, but a historical artifact: C's printf works the same way.

@inliquid
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inliquid commented Sep 1, 2019

I see, thank you for clarifying.

@inliquid inliquid closed this as completed Sep 1, 2019
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@KarthikNayak @robpike @inliquid @gopherbot