[Help-glpk] fixing a variable

[hussin hassen]

Hi everybody,

Is there any one who can help me by telling me how to fix the value of a 
variable at certain set members? For example:
I tried:

var X {i} , X['1'] = -18;

but it didn't work in MathProg.
(note:  i is a set := 1,2,...24)

I tried also, in data section:

fix X('1') = -18; 
but it did not work too.

Thanks,

Hussin