[julia-users] feval(f,x1=NaN) - A function to evaluate one or more expressions - A good or a bad idea?

[Dominik Holenstein]

I am new to Julia programming and not an expert either. 

While working on a project which involves the translation of Matlab code to 
Julia I had the idea to write a
 feval(f, x1=NaN)

function in Julia to evaluate a single expression or an array of two or 
more expressions by using just one function. 


This is the code of the feval(f, x1=NaN) function:

function feval(f,x1 = NaN)
   if isnan(x1) == false
      global x = x1
   end
   if typeof(f) == Array{Expr,1}
      n = size(f,1)
      F = zeros(n,1)
      for i=1:n
         F[i] = eval(f[i])
      end
   elseif typeof(f) == Expr
      F = 0
      F = eval(f)
   else
      println("f is a ", typeof(f))
      error("f is nor an expression nor a single array of expressions.")
   end
   return F
end


You can use this function with a single expression or with an array of 
expressions (x = 3.5) : 

julia> f1 = :(3*x - 2*x + 5)
:(+(-(*(3,x),*(2,x)),5))

julia> f2 = :(4*x - 3*x/2)
:(-(*(4,x),/(*(3,x),2)))

julia> f = [f1;f2]
2-element Array{Expr,1}:
:(+(-(*(3,x),*(2,x)),5))
:(-(*(4,x),/(*(3,x),2)))

julia> x = 3.5
3.5


The results:
julia> res_f1 = feval(f1)
8.5

julia> res_f2 = feval(f2)
8.75


julia> res_f = feval(f)
2x1 Array{Float64,2}:
8.5
8.75


Further, you can deliver an optional input value for x with the function 
call:

julia> res_f1 = feval(f1, 5.25)
10.25


Note that this changes the global variable x:
julia> x
5.25



In general: Is this appraoch a good or bad idea? Or are there better 
options to consider (types, macros etc.)?

Many thanks for your support.

Regards,
Dominik