Manipulating several variables within a for loop in Julia-CodePudding

I'm new to Julia. I want to write code which, for each of several vectors, outputs a new vector, the name of which depends on the name of the input vector.

For example, the following code works

a = ones(10) 
b = ones(10) 
for var in [a, b] 
  global log_var = log.(var)
end

except I want the resulting vectors to be named log_a and log_b (rather than have the loop overwrite log_var). I had thought this would be simple, but having read a few tutorials about locals in Julia, I'm still lost! Is there a simple way to go about this?

In case this question is unclear, I'll describe how I would do this in Stata, with which I'm more familiar:

clear 
set obs 10 
gen a = 1 
gen b = 1 
foreach var in a b {
gen log_`var' = log(`var')
}

Thank you!

CodePudding user response：

You really probably don't want to do that. But, if you had to, you could do it pretty easily with metaprogramming. In this case for example:

macro logify(variable)
   quote
       $(esc(Symbol("log_$variable"))) = log.($variable)
   end
end

then

julia> b = rand(5)
5-element Vector{Float64}:
 0.29129581739244315
 0.21098023915449915
 0.8736387630142392
 0.34378216482772417
 0.621583372934101

julia> @logify b;

julia> log_b
5-element Vector{Float64}:
 -1.2334159735391819
 -1.555990803188027
 -0.13508830339365252
 -1.0677470639708686
 -0.4754852291054692

In general, any time you need to depend on the name of a variable rather than its contents, you're going to need metaprogramming.

However, to emphasize, again, this feels like a bad idea.

Rather than defining new top-level variables, you might consider instead using some sort of data structure like a Dict or a NamedTuple or a DataFrame, or even just a multidimensional Array. For example, with NamedTuples:

julia> data = (a = rand(5), b = rand(5));

julia> typeof(data)
NamedTuple{(:a, :b), Tuple{Vector{Float64}, Vector{Float64}}}

julia> data.a
5-element Vector{Float64}:
 0.7146929585896256
 0.5248314042991269
 0.040560190890127856
 0.9714549101298824
 0.9477790450084252

julia> data.b
5-element Vector{Float64}:
 0.6856764745285641
 0.3066093923258396
 0.5655243277481422
 0.13478854894985115
 0.8495720250298817

julia> logdata = NamedTuple{keys(data)}(log.(data[x]) for x in keys(data));

julia> logdata.a
5-element Vector{Float64}:
 -0.335902257064951
 -0.6446782026336225
 -3.204968213346185
 -0.02896042387181646
 -0.05363387877891503

julia> logdata.b
5-element Vector{Float64}:
 -0.3773493739743169
 -1.182180679204628
 -0.5700019644606769
 -2.0040480325554944
 -0.1630225562612911

CodePudding user response：

if you are looking for something similar to what you do in stata, you can use DataFrames.jl,

julia> using DataFrames
julia> df = DataFrame(a=ones(10), b=ones(10))
julia> for col in ["a", "b"]
           df[:, "log_"*col] = log.(df[:, col])
       end
julia> df