Mathematica Tutorial

Controlling Numerical Evaluation

NHoldAll	prevent any arguments of a function from being affected by N
NHoldFirst	prevent the first argument from being affected
NHoldRest	prevent all but the first argument from being affected

Attributes for controlling numerical evaluation.

Usually N goes inside functions and gets applied to each of their arguments.

This tells Mathematica not to apply N to the first argument of f.

Now the first argument of f is left in its exact form.