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set versus setdelay
 To: mathgroup at smc.vnet.net
 Subject: [mg75208] set versus setdelay
 From: siewsk at bp.com
 Date: Fri, 20 Apr 2007 00:43:07 0400 (EDT)
Informative Article
===================
SET versus SETDELAY
Newbie are always told to use SETDELAY instead of SET when they create
a mathematica function.
The objective of this article is to explain the differnce between SET
and SETDELAY using ordinary language (what linguistics call vernacular
language)
The main differences between those two are:
SET means CALCULATE FIRST THEN CHANGE
SETDELAY means CHANGE FIRST THEN CALCULATE
To explain, consider the following two functions f ang g
f[x_] = Expand[(x+7)^2] (* Using SET *)
g[x_]:= Expand[(x+7)^2] (* Using SETDELAY *)
The two functions are identical except f uses SET while g uses
SETDELAY. In normal usage there is no difference in the two functions.
However they produce their results using different steps.
================
SET
================
Consider f[4]
(Step 1. CALCULATE FIRST )
f[x] = Expand[(x+7)^2]
f[x] = x^2 + 14 x + 49
(Step 2. THEN CHANGE )
f[x>4] = x^2 + 14 x + 49
f[4] < 4^2 + 14 * 4 + 49
f[4] < 121
================
SETDELAY
================
Consider g[4]
(Step 1. CHANGE FIRST )
g[x] := Expand[(x+7)^2]
g[x>4] := Expand[(x+7)^2]
g[4] < Expand[(4+7)^2]
(Step 2. THEN CALCULATE )
g[4] < Expand[(11)^2]
g[4] < Expand[121]
g[4] < 121
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