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For example, suppose I wanted to define a function, f that is $f(x)=x^2$ except at $x=1$, where $f(x)=5$

Two ways I can define this are

f[1]=5;
f[x_]:=x

or

g[x_]:= Piecewise[{{5,x==1}},x^2]

What are the benefits/pitfalls of one approach vs the other? Or are they the same?

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    $\begingroup$ You didn't mention using If[] statement or Which[] They are different. The first method uses two patterns. There are several benefits/pitfalls, but that requires much explaining. $\endgroup$
    – Somos
    Nov 12, 2020 at 0:23
  • $\begingroup$ @Somos Sorry for not mentioning. $\endgroup$
    – user106860
    Nov 12, 2020 at 0:59
  • $\begingroup$ Another way: f[x_]:=Which[x==1,5,True,x^2] $\endgroup$
    – AsukaMinato
    Nov 12, 2020 at 4:19

2 Answers 2

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They differ in how they handle the function's derivative.

Clear["Global`*"]

f1[1] = 5;
f1[x_] := x^2

f1'[x]

(* 2 x *)

f1'[1]

(* 2 *)

f2[x_] := Piecewise[{{5, x == 1}}, x^2]

f2'[x]

(* Piecewise[{{0, x == 1}}, 2*x] *)

f2'[1]

(* 0 *)

f3[x_] := Piecewise[{{x^2, x < 1 || x > 1}}, 5]

f3'[x]

(* Piecewise[{{2*x, x < 1 || x > 1}}, Indeterminate] *)

f3'[1]

(* Indeterminate *)
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  • $\begingroup$ Oof. Thanks. Wouldn't the second technically be better if we cared about derivatives then? $\endgroup$
    – user106860
    Nov 12, 2020 at 0:57
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    $\begingroup$ I would think that the third is better. $\endgroup$
    – Bob Hanlon
    Nov 12, 2020 at 1:11
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When you write

f[1] = 5;
f[x_] := x

you are specifying how expressions will be rewritten during evaluation. f[1] is just a pattern: any expression that matches that pattern will be rewritten as 5. The 'Blank' pattern represents anything else, so:

f[y]
(* y *)

The possibility that y might take the value 1 at some later stage of the computation isn't taken into account.

One the other hand:

g[x_] := Piecewise[{{5, x == 1}}, x^2]
g[y]
(* Piecewise[{{5, y == 1}}, y^2] *)

This treats y properly as a variable.

Note that symbolic mathematics in Mathamatica can "reason" about Piecewise expressions, but cannot do so with collections of rewriting rules, so f is not a suitable function to feed to symbolic methods.

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  • $\begingroup$ Thanks for the response. Quick clarifying question on one sentence in your answer: you say f[1] is just a pattern, any expression that matches that pattern will be rewritten as 5". Isn't f[1] the only expression that matches that pattern? $\endgroup$
    – user106860
    Nov 12, 2020 at 1:31
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    $\begingroup$ @user106860 Yes. Of course, it could be a subexpression of some more complicated expression. $\endgroup$
    – John Doty
    Nov 12, 2020 at 1:36

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