Stop function evaluating input at time of definition

Question

I would like to create a function that acts on whether the input is even or odd, e.g.:

In[644]:= f[a_] = If[EvenQ[a], 2 a, 3 a];
          f[2]

Out[645]= 6

I've determined (I think) that this is because a is an expression and QEven returns false for expressions. How do I hold off evalution of this until input is given? I thought something like Defer would work but then it is never evaluated (see below)!

In[641]:= g[a_] = If[Defer[EvenQ[a]], 2 a, 3 a]

Out[641]= If[EvenQ[a], 2 a, 3 a]

In[643]:= g[3]

Out[643]= If[EvenQ[3], 2 * 3, 3 * 3]

Apologies if this gets asked a lot - I assume this is a common issue, but I was unable to come up with appropriate search terms to describe it (I'm not very familiar with Mathematica terminology yet). Thanks!

Brilliant thanks, that thread looks very useful. It should have been my first port of call! — Hemmer, Oct 25 '13 at 10:39

score 0 · Answer 1 · answered Oct 25 '13 at 10:40

0

fun[x_?EvenQ] := 2 x
fun[x_?OddQ] := 3 x

Non-integer arguments will just return the fun[arg].

fun[2] yields 4 and fun[3] yields 9.

answered Oct 25 '13 at 10:40

ubpdqn

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Stop function evaluating input at time of definition

1 Answers1