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I have a very large function f[x_,y_,z_] and it's too cumbersome to type it directly. So I wish to break the definition and use (/.) multiple times, as follows (indicatively):

f[x_, y_,z_] := (int1 + int2 + int3)/.rule1/.rule2;
rule1 = {int1 -> iint1, int2 -> iint2, int3 -> iint3};
rule2 = {iint1 -> 1*x, iint2 -> 2*y, iint3 -> 3*z};

But this does not work. When I try to evaluate f[1,2,3], it does not perform the transformation rule2 and gives me f[1,2,3]=x + 2y + 3z instead of the evaluated number value 1*1 + 2*2 + 3*3 = 14.

I think this is because rule2's definition on line 3 does not come into the scope of my function declaration on line 1, so the x I write in rule2 is not the same x_ I'm trying to relate to.

How do I solve this? Thank you.

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  • $\begingroup$ This is a good reason to use a scoping construct like Block[] or Module[]. You can then do something like f[x_, y_, z_] := Module[{int1, int2, int3, rule1, rule2}, int1 = (* stuff *); (*stuff *) (int1 + int2 + int3) /. rule1 /. rule2] $\endgroup$ – J. M. is in limbo Oct 10 '18 at 10:55
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(1) define rule1 and rule2 before f, and (2) wrap the rhs of the definition with Evaluate to suppress the HoldAll attribute of SetDelayed:

f[x_, y_, z_] := Evaluate[(int1 + int2 + int3) /. rule1 /. rule2];
f[1, 2, 3]

14

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  • $\begingroup$ Works perfect! Thanks. $\endgroup$ – QMrules Oct 10 '18 at 11:09
  • $\begingroup$ @QMrules, my pleasure. Welcome to mma.se. $\endgroup$ – kglr Oct 10 '18 at 11:09

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