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<A NAME=WHERE>

<TITLE>WHERE</TITLE></A>
<b><a href=r37_idx.html>INDEX</a></b><p><p>



<B>WHERE</B> _ _ _  _ _ _  _ _ _  _ _ _ <B>operator</B><P>
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The <em>where</em> operator provides an infix notation for one-time 
substitutions for kernels in expressions. 
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syntax: </H3>
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&lt;expression&gt; <em>where</em> &lt;kernel&gt; 
 <em>=</em>&lt;expression&gt; 
 {,&lt;kernel&gt; <em>=</em>&lt;expression&gt;}* 
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&lt;expression&gt; can be any REDUCE scalar expression, &lt;kernel&gt; must 
be a 
<A HREF=r37_0002.html>kernel</A>. Alternatively a 
<A HREF=r37_0060.html>rule</A> or a <em>rule list</em> 
can be a member of the right-hand part of a <em>where</em> expression. 
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 <P> <H3> 
examples: </H3>
<P><PRE><TT>
x**2 + 17*x*y + 4*y**2 where x=1,y=2; 


  51 


for i := 1:5 collect x**i*q where q= for j := 1:i product j;
 


        2    3     4      5
  {X,2*X ,6*X ,24*X ,120*X } 


x**2 + y + z where z=y**3,y=3; 

   2    3
  X  + Y  + 3

</TT></PRE><P>Substitution inside a <em>where</em> expression has no effect upon
 the values 
of the kernels outside the expression. The <em>where</em> operator has the 
lowest precedence of all the infix operators, which are lower than prefix 
operators, so that the substitutions apply to the entire expression 
preceding the <em>where</em> operator. However, <em>where</em> is applied 
before command keywords such as <em>then</em>, <em>repeat</em>, or <em>do</em>. 

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A 
<A HREF=r37_0060.html>rule</A> or a <em>rule set</em> in the right-hand part of 
the 
<em>where</em> expression act as if the rules were activated by 
<A HREF=r37_0199.html>let</A> 
immediately before the evaluation of the expression and deactivated 
by 
<A HREF=r37_0190.html>clearrules</A> immediately afterwards. 
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<em>where</em>gives you a natural notation for auxiliary variables in 
expressions. As the second example shows, the substitute expression can be 
a command to be evaluated. The substitute assignments are made in 
parallel, rather than sequentially, as the last example shows. The 
expression resulting from the first round of substitutions is not 
reexamined to see if any further such substitutions can be made. 
<em>where</em> can also be used to define auxiliary variables in 

<A HREF=r37_0055.html>procedure</A> definitions. 
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