Entering the format string as a variable throws error

[Kevin-Mattheus-Moerman]

The following works:

@scanf("1.23456", "%f",Float64)

But the following throws an error:

f = "%f"
@scanf("1.23456", f,Float64)

ERROR: ArgumentError: mismatch between # of format specifiers and provided args: 0 != 2
Stacktrace:
 [1] throwa(msg::String)
   @ Scanf ~/.julia/packages/Scanf/5ncBL/src/Scanf.jl:833
 [2] argmismatch(a::Int64, b::Int64)
   @ Scanf ~/.julia/packages/Scanf/5ncBL/src/Scanf.jl:830
 [3] scanner(::BufferedStreams.BufferedInputStream{Base.TTY}, ::Scanf.Format{Base.CodeUnits{UInt8, String}, Tuple{Scanf.LiteralSpec{SubString{String}}}}, ::String, ::Vararg{Any})
   @ Scanf ~/.julia/packages/Scanf/5ncBL/src/Scanf.jl:647
 [4] scanf(::BufferedStreams.BufferedInputStream{Base.TTY}, ::Scanf.Format{Base.CodeUnits{UInt8, String}, Tuple{Scanf.LiteralSpec{SubString{String}}}}, ::String, ::Vararg{Any})
   @ Scanf ~/.julia/packages/Scanf/5ncBL/src/Scanf.jl:126
 [5] scanf(::Base.TTY, ::Scanf.Format{Base.CodeUnits{UInt8, String}, Tuple{Scanf.LiteralSpec{SubString{String}}}}, ::String, ::Vararg{Any})
   @ Scanf ~/.julia/packages/Scanf/5ncBL/src/Scanf.jl:127
 [6] top-level scope
   @ REPL[89]:1

The context is that I'd like the format to be variable, in some cases f="%f, %f", %f" for instance. Or f="%d, %f", %f, %f, %f" .

How can I programmatically enter the format string?

In addition it would be great to replicate the second part that says Float64 n-times, e.g. if f="%f"^n. Any pointers how to do that? Thanks.

[KlausC]

You are right, @scanf only supports string constants as formats, but you can use one of:

julia> f = Scanf.Format("%f") # also variables possible
Scanf.format"%f"

julia> scanf("12.34", f, 1.5)
(1, 12.34)

julia> f = Scanf.format"%f" # only constants
Scanf.format"%f"

julia> scanf("12.34", f, 1.5)
(1, 12.34)

[KlausC]

For your second question:

julia> n = 5
5

julia> f = Scanf.Format("%f"^n)
Scanf.format"%f%f%f%f%f"

julia> scanf("1 2 3 4 5 6 7 8", f, zeros(5)...)
(5, 1.0, 2.0, 3.0, 4.0, 5.0)

but be aware, that calling Scanf.Format has to interpret the format string each time it is called. So that should not be done inside a hot loop.

[Kevin-Mattheus-Moerman]

@KlausC thanks for getting back to me, and for sharing this solution. 🎄