printing an empty slice with fmt.Printf()

[Jochen Voss]

Hello,

I can print slices of bytes as hex strings, using code like the following:

x := []byte{0, 1, 2, 3}

fmt.Printf("%02x", x[:l])

This gives the output "00010203" as expected.  But this fails for the empty slice: running

    x := []byte{}

    fmt.Printf("%02x", x[:l])

gives "00" instead of the empty string.  See https://play.golang.org/p/rvLLqydDDE6 for a demonstration.

Is this a bug, or is this expected behaviour?

All the best,

Jochen

[Marcin Romaszewicz]

It's expected behavior.

Your for loop runs once for l=0, since your condition is <=0 because len([]byte{}) is 0.

-- Marcin

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[Dan Kortschak]

I think that's the question. Here's a simpler example,
https://play.golang.org/p/9Kv3PhlM-OF

That is, is 00 an expected %02x representation of a zero-length byte
slice?

The answer to that is yes; the 02 forces leading zeros. The %x verb
essentially renders bit strings as hex, so a zero-length bit string
with mandated two leading zeros is 00.

If you leave out the 02, you get the expected empty string.

https://play.golang.org/p/uVFt3lecKxf

[Jochen Voss]

Dear all,

Thanks for your help.  The solution is indeed to use "%x" instead of "%02x".

My confusion was caused by the following sentence in the docs: "For compound operands such as slices and structs, the format applies to the elements of each operand".  I never spotted the relevant exception mentioned a few lines later: "However, when printing a byte slice with a string-like verb (%s %q %x %X), it is treated ... as a single item."  So the "%02" applies to the whole byte slice, and not to each byte as I had wrongly thought.

All the best,

Jochen