Preprocessing function arguments

New to Julia
1

I have a working code like the following:

function test(a,b)  
        c = a + b
        d = a - b
        e = c + 2 * d 
end

but I now want to sometimes make b a constant s.t.

const b = 1.5
function test(a)
        c = a + b
        d = a - b
        e = c + 2 * d 
end

My experience with C makes me want to do something like

#ifdef USECONST
const b = 1.5
function test(a)
#else
function test(a,b)
#endif
        c = a + b
        d = a - b
        e = c + 2 * d 
end

Is there a Julia syntax that can do this? I’m basically looking for an easy way for me to reuse the function body (c=…, d=…, e=…) while still have the option of using b as either a constant or an input parameter in different applications. Thanks!

2 
function test(a,b = 1.5)  
        c = a + b
        d = a - b
        e = c + 2 * d 
end

I believe that is what you want. With this you can call test(3.3) or call test(3.3, 1.25). If b isn’t provided then it’s value will be 1.5.

3

Wow thank you for your instant answer! Unfortunately, in my case, b is loaded from a file and is in fact a large array that I cannot hard-code it…

5

Then I don’t see the advantage of just defining test(a, b), and calling it with a b you either define in the source or load from a file.

6

Maybe a closure?

function maketest(defaultB)
      return a -> begin
           c = a + defaultB
           d = a - defaultB
           e = c + 2*d
   end
end
test = maketest(1.5)
7

I realize I’m not giving the full picture. I’m trying to embed my Julia code from C calling the function test(a,b). However, b is a struct containing several large arrays, and I don’t know how to do embedding with that. Fortunately, b is unchanged through the entire calculation of test so my solution is to write b into a file, or files as I have three different sets of b’s as options and load it back when start-up (when I do using MyModule). The original function test(a,b) is still useful when I’m in pure Julia mode and want to switch between different sets of b’s or explore a new b.

8

I just realized that the default value of b doesn’t have to be hard-coded! The following code fits my need

struct Par{T}
    x::T
    y::T
    z::T
end

b_global = Par{Float64}(1,1,1)

function test(a, b=b_global)
        @show b
        #some code
end

test(1)
test(1,Par{Float64}(0,0,0))

Thanks everyone!