File r37/lisp/csl/html/r37_0052.html artifact d43321d33b part of check-in d58ccc1261

<A NAME=IF>

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



<B>IF</B> _ _ _  _ _ _  _ _ _  _ _ _ <B>command</B><P>
<P>
 
The <em>if</em> command is a conditional statement that executes a statement 
if a condition is true, and optionally another statement if it is not. 
 <P> <H3> 
syntax: </H3>
<P>
<P>
<em>if</em>&lt;condition&gt; <em>then</em> &lt;statement&gt; 
  _ _ _ option(<em>else</em> &lt;statement&gt;) 
<P>
<P>
<P>
&lt;condition&gt; must be a logical or comparison operator that evaluates to 
a 
<A HREF=r37_0109.html>boolean value</A>. 
&lt;statement&gt; must be a single REDUCE statement or a 

<A HREF=r37_0038.html>group</A> (<em>&lt;&lt;</em>...<em>&gt;&gt;</em>) or 

<A HREF=r37_0041.html>block</A> (<em>begin</em>...<em>end</em>) statement. 
<P>
<P>
 <P> <H3> 
examples: </H3>
<P><PRE><TT>
if x = 5 then a := b+c else a := d+f;
			 


  D + F 


x := 9; 

  X := 9 


if numberp x and x&lt;20 then y := sqrt(x) else write &quot;illegal&quot;;
			 


  3  


clear x; 

if numberp x and x&lt;20 then y := sqrt(x) else write &quot;illegal&quot;;
			 


  illegal 


x := 12; 

  X := 12 


a := if x &lt; 5 then 100 else 150;
			 


  A := 150 


b := u**(if x &lt; 10 then 2);
			 

  B := 1 


bb := u**(if x &gt; 10 then 2);
			 

         2
  BB := U

</TT></PRE><P>An <em>if</em> statement may be used inside an assignment statemen
t and sets 
its value depending on the conditions, or used anywhere else an 
expression would be valid, as shown in the last example. If there is no 
 <em>else</em> clause, the value is 0 if a number is expected, and nothing 
otherwise. 
<P>
<P>
The <em>else</em> clause may be left out if no action is to be taken if the 
condition is false. 
<P>
<P>
The condition may be a compound conditional statement using 
<A HREF=r37_0039.html>and</A> or 

<A HREF=r37_0054.html>or</A>. If a non-conditional statement, such as a constant
, is used by 
accident, it is assumed to have value true. 
 <P>
<P>
Be sure to use 
<A HREF=r37_0038.html>group</A> or 
<A HREF=r37_0041.html>block</A> statements after 
 <em>then</em> or <em>else</em>. 
<P>
<P>
The <em>if</em> operator is right associative. The following constructions are 
examples: 
<P>
<P>
 _ _ _ (1) 
 <P> <H3> 
syntax: </H3>
<P>
<P>
<em>if</em>&lt;condition&gt; <em>then</em> <em>if</em> &lt;condition&gt; <em>the
n</em> 
 &lt;action&gt; <em>else</em> &lt;action&gt; 
<P>
<P>
<P>
which is equivalent to 
 <P> <H3> 
syntax: </H3>
<P>
<P>
<em>if</em>&lt;condition&gt; <em>then</em> (<em>if</em> &lt;condition&gt; 
 <em>then</em> &lt;action&gt; <em>else</em> &lt;action&gt;); 
<P>
<P>
<P>
 _ _ _ (2) 
 <P> <H3> 
syntax: </H3>
<P>
<P>
<em>if</em>&lt;condition&gt; <em>then</em> &lt;action&gt; <em>else if</em> 
 &lt;condition&gt; <em>then</em> &lt;action&gt; <em>else</em> &lt;action&gt; 
<P>
<P>
<P>
which is equivalent to 
 <P> <H3> 
syntax: </H3>
<P>
<P>
<em>if</em>&lt;condition&gt; <em>then</em> &lt;action&gt; <em>else</em> 
<P>
<P>
(<em>if</em> &lt;condition&gt; <em>then</em> &lt;action&gt; 
 <em>else</em> &lt;action&gt;). 
<P>
<P>
<P>
<P>
<P>
<P>