
<A NAME=PART>

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



<B>PART</B> _ _ _  _ _ _  _ _ _  _ _ _ <B>operator</B><P>
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The operator <em>part</em> permits the extraction of various parts or 
operators of expressions and 
<A HREF=r37_0053.html>list</A><em>s</em>. 
 <P> <H3> 
syntax: </H3>
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<em>part</em>(&lt;expression,integer&gt;{,&lt;integer&gt;}*) 
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&lt;expression&gt; can be any valid REDUCE expression or a list, 
integer may be an expression that evaluates to a positive or negative 
integer or 0. A positive integer &lt;n&gt; picks up the n th term, 
counting from the first term toward the end. A negative integer n 
picks up the n th term, counting from the back toward the front. The 
integer 0 picks up the operator (which is <em>LIST</em> when the expression 
is a 
<A HREF=r37_0053.html>list</A>). 
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examples: </H3>
<P><PRE><TT>
part((x + y)**5,4); 

      2  3
  10*X *Y  


part((x + y)**5,4,2); 

   2
  X  


part((x + y)**5,4,2,1); 

  X 


part((x + y)**5,0); 

  PLUS 


part((x + y)**5,-5); 

        4
  5*X *Y  


part((x + y)**5,4) := sin(x); 

   5      4         3  2                 4    5
  X  + 5*X *Y + 10*X *Y  + SIN(X) + 5*X*Y  + Y  


alist := {x,y,{aa,bb,cc},x**2*sqrt(y)}; 


                                        2
  			 ALIST := {X,Y,{AA,BB,CC},SQRT(Y)*X } 


part(alist,3,2); 

  BB 


part(alist,4,0); 

  TIMES

</TT></PRE><P>Additional integer arguments after the first one examine the 
terms recursively, as shown above. In the third line, the fourth term 
is picked from the original polynomial, 10x^2y^3, 
then the second term from that, x^2, and finally the first 
component, x. If an integer's absolute value is too large for 
the appropriate expression, a message is given. 
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<em>part</em>works on the form of the expression as printed, or as it would 
have been printed at that point of the calculation, bearing in mind the 
current switch settings. It is important to realize that the switch settings 
change the operation of <em>part</em>. 
<A HREF=r37_0319.html>pri</A> must be on when 
<em>part</em> is used. 
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When <em>part</em> is used on a polynomial expression that has minus signs, the 

<em>+</em> is always returned as the top-level operator. The minus is found 
as a unary operator attached to the negative term. 
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<em>part</em>can also be used to change the relevant part of the expression or 
list as shown in the sixth example line. The <em>part</em> operator returns the 

changed expression, though original expression is not changed. You can 
also use <em>part</em> to change the operator. 
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