C :: How To Pass Symbolically Name Of Struc Member To Function

I have a struct with several members, for instance:

insert
Code:
typedef struct {
float u;
float v;
float w;
} mystruct;
main ()
mystruct test;
...

I have a piece of code that should operate on test.u that looks like this:

switch (j) {
case 0:
f = test.u + ...;
break;
case 1:
f = X - test.u;
break;
case 2:
f = test.u + 2;
break;
}

Now I would like to use a similar piece of code but for the other members, test.v and test.w. In principle I could copy the piece of code above and just replace test.u by test.v or test.w. Since it is the same code for all members, I would like to avoid this (copying many times the same piece of code) and replace it by a call to function. The question is, how to pass the name of the struct member I am considering to the function ? How can I tell the function, operates on member .u or .v ? Would it be possible to have a generic piece of code

insert
Code:
function (member y,...)
switch (j) {
case 0:
f = test.y + ...;
break;
case 1:
f = X - test.y;
break;
case 2:
f = test.y + 2;
break;
}

where the function could be called with function(u) or function(w) and would replace automatically .y by .u or .w ?