x
x
x


Do so in a way which:

 does not involve string manipulation of the input source code
 is plausibly extensible to a runtime-chosen set of bindings rather than just x
 does not make x a global variable


or note that these are impossible.



See also

 For more general examples and language-specific details, see Eval.
 Dynamic variable names is a similar task.


function r = calcit(f, val1, val2)
  x = val1;
  a = eval(f);
  x = val2;
  b = eval(f);
  r = b-a;
end