@sprintf is a macro, not a function. It doesn't evaluate its inputs: it just
rewrites the inputs into something else (usually less readable) that carries
out the actual computation. You can see what it does using the macroexpand
function:
julia> macroexpand(quote @sprintf("%8.1e", 3.1415) end)
:(begin # none, line 1:
Base.Printf.sprint(#63#io->begin # printf.jl, line 783:
begin
#59#out = #63#io
#60###x#3463 = 3.1415
local #61#neg, #57#pt, #58#len, #62#exp
if Base.Printf.isfinite(#60###x#3463)
Base.Printf.ini_dec(#60###x#3463,2)
#61#neg = Base.Printf.NEG[1]
#62#exp = Base.Printf.-(Base.Printf.POINT[1],1)
(Base.Printf.-(Base.Printf.-(1,Base.Printf.|((#62#exp Base.Printf.<= -100),(100
Base.Printf.<= #62#exp))),#61#neg) Base.Printf.> 0) &&
Base.Printf.write(#59#out,' ')
#61#neg && Base.Printf.write(#59#out,'-')
Base.Printf.write(#59#out,Base.Printf.DIGITS[1])
Base.Printf.write(#59#out,'.')
Base.Printf.write(#59#out,Base.Printf.+(Base.Printf.pointer(Base.Printf.DIGITS),1),1)
Base.Printf.write(#59#out,'e')
Base.Printf.print_exp(#59#out,#62#exp)
else
Base.Printf.write(#59#out,begin # printf.jl, line
141:
if Base.Printf.isnan(#60###x#3463)
" NaN"
else
if (#60###x#3463 Base.Printf.< 0)
" -Inf"
else
" Inf"
end
end
end)
end
Base.Printf.nothing
end
end)
end)
-- John
On Apr 11, 2014, at 11:46 PM, Dominique Orban <[email protected]> wrote:
> As a follow-up question, why is the following not allowed?
>
> julia> fmt = "%8.1e";
>
> julia> @sprintf(fmt, 3.1415)
> ERROR: first or second argument must be a format string
>
> I don't see how it's different from
>
> julia> @sprintf("%8.1e", 3.1415)
>
> What's the appropriate syntax?
>
> Thanks.
>
>
> On Friday, April 11, 2014 11:24:50 PM UTC-7, Dominique Orban wrote:
> Thank you! Such a basic operation could feature a bit more prominently in the
> documentation.
>
>
> On Friday, April 11, 2014 11:21:28 PM UTC-7, John Myles White wrote:
> @sprintf
>
> On Apr 11, 2014, at 11:18 PM, Dominique Orban <[email protected]> wrote:
>
>> Sorry if this is a RTFM, but I can't find the answer in the documentation or
>> on the web. I may have missed it. I come from Python where I can build
>> strings with formatted data using a syntax like
>>
>> s = "pi=%7.1e" % acos(-1)
>>
>> How do I accomplish that in Julia? @printf doesn't do the job because it
>> doesn't return anything:
>>
>> julia> s = @printf("%7.1e", 3.14)
>> 3.1e+00
>> julia> s
>>
>>
>>
>>
>> Thanks.
>>
>
