Julia: Assign and Get Functions - assign

Question

I would like to be able to assign values to variables in Julia through a simple string loop.
For instance:
x1, x2, x3, x4 = 14, 12, 34, 19
letters = "abcd"
for i in 1:4
val = get("x$i")
var = letters[i]
assign(var, val)
end
a
14
Note, I choose the function "assign" and "get" because I am used to them in R.
The reason I want to do this is because I have a function which takes multiple arguments which are all evaluated identically and I figured there was no point duplicating code. Something like:
function f(a,b,c,d)
if a<0 a==-9999 end
if b<0 b==-9999 end
if c<0 c==-9999 end
if d<0 d==-9999 end
end
Of course the functions that I am applying to each is really much longer than a simple recode command.
I guess I could do something like this:
function f(a=1,b=2,c=3,d=4, p=[a,b,c,d])
for (i in 1:4)
if p[i]<0
p[i]=-9999
end
end
sum(p)
end
However, this creates other problems with "set index" which is something I will need to read up more on. Thanks for your consideration!
Francis

Answer 1

After your second suggestion for what to do, I figure you might actually want something like this.
function f(args...)
acc = zero(args[1])
for a in args
if a < 0
acc -= 9999
else
acc += a
end
end
acc
end
Usage is just like

Answer 2

This does the trick:
x1, x2, x3, x4 = 14, 12, 34, 19
letters = "abcd"
for i in 1:4
assignment = parse("$(letters[i]) = x$(i)")
eval(assignment)
end
You can read about the parse function, which turns strings into expressions, here:
http://julia.readthedocs.org/en/latest/stdlib/base/#Base.parse

Answer 3

To do this kind of weird dynamic variable trixing in Julia, you have to use macros and metaprogramming. The gist of it is a very powerful interface to code generation. I think it would be possible to write a macro that does the same as the above code on the form
x1, x2, x3, x4 = 14, 12, 34, 19
#assign("abcd", 1:4)
and translates that "compile time" to
x1, x2, x3, x4 = 14, 12, 34, 19
begin
a = x1
b = x2
c = x3
d = x4
end
It will take me some time to write and debug such a macro. Can you describe your use case, so I can see if there are some constraints that are actually not important, or more functionality you'd like? The 1:4 for example seems redundant as I can just use for (i, letter) in enumerate("abcd") to get the same loop.