Printf with variable number of digits

General Usage
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1

I am trying to write some code that computes some number up to some precision and prints it with the correct number of digits. I tried the following:

using Printf

function print_number(x, err)
    numofdigits = ceil(Int, -log10(err))
    @printf("%.*f ± %.*f\n", numofdigits, x, numofdigits, err)
end

print_number(3.12345, 0.01)  # Should print 3.12 ± 0.01

but the code produces the error ArgumentError: invalid format string: '%.*f ± %.f', invalid type specifier: '*'.

I wasn’t expecting this, as the @printf documentation explicitly points to the C++ specification of format strings, where the %.*f syntax is supported. Am I missing something, or it is not possible with @printf to do this? And if it is not, what would be a simple workaround?

2

Don’t use @printf, use format + Format:

julia> using Printf

julia> function print_number(x, err)
           numofdigits = ceil(Int, -log10(err))
           Printf.format(Printf.Format("%.$(numofdigits)f ± %.$(numofdigits)f\n"), x, err)
       end
print_number (generic function with 1 method)

julia> print_number(3.12345, 0.01)  # Should print 3.12 ± 0.01
"3.12 ± 0.01\n"
3 Likes
3

Thanks! So I understand that the * character is not supported in Julia. Do you think this should be stated somewhere in the documentation?

4

I’m not familiar with the internals of the Printf module, but it’d appear it doesn’t work, so yes, it may be worth mentioning it in the documentation.

2 Likes
5

Linking similar post and PR.

1 Like
6

Thanks! I looked for existing threads in this forum but wasn’t able to find that one. (I could just have looked in the issues of the Printf package, silly of me…)

7

Most flags that have some dynamic interpretation aren’t supported (we do some length checking to make sure the format object & the number of arguments match for example, to prevent security issues). The general rule of thumb is that if the formatting can be done purely based on a constant format string, it should work. This should definitely be better documented though, instead of just linking to an external ressource.