Variable assignment to a function

Question

I am trying to do a seemingly simple thing:

f[x_, y_] := x + 3 y /. {x -> -x y}

the output for $x=1$ and $y=2$ is 7, so it doesn't make a substitution. How can I make this work without manually substituting the variables?

score 4 · Accepted Answer · edited Apr 13 '17 at 12:56

4

You want to use Set rather than SetDelayed

Clear[f]

f[x_, y_] := x + 3 y /. {x -> -x y}

DownValues[f]

(*  {HoldPattern[f[x_, y_]] :> (x + 3 y /. {x -> -x y})}  *)

Trace will show you the evaluation sequence

f[1, 2] // Trace

Clear[f]

f[x_, y_] = x + 3 y /. {x -> -x y}

(*  3 y - x y  *)

DownValues[f]

(*  {HoldPattern[f[x_, y_]] :> 3 y - x y}  *)

f[1, 2] // Trace

Beware that if x or y are defined before the Set this will fail. For a solution see:

edited Apr 13 '17 at 12:56

Community

answered Dec 14 '16 at 17:20

Bob Hanlon

I added a caveat and link to your answer (as well as my vote). I hope you do not mind. – Mr.Wizard Dec 23 '16 at 19:18
@Mr.Wizard - Thanks. – Bob Hanlon Dec 23 '16 at 19:21

1 Answers1