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<head><title>conditional execution -- if-then-else statements</title>
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<div><a href="index.html" title="">Macaulay2Doc</a> > <a href="___The_sp__Macaulay2_splanguage.html" title="">The Macaulay2 language</a> > <a href="_conditional_spexecution.html" title="if-then-else statements">conditional execution</a></div>
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<div><h1>conditional execution -- if-then-else statements</h1>
<div>The basic way to control the execution of code is with the <a href="_if.html" title="condition testing">if</a> expression. Such an expression typically has the form <pre>if X then Y else Z</pre>
and is evaluated as follows. First <tt>X</tt> is evaluated. If the result is <tt>true</tt>, then the value of <tt>Y</tt> is provided, and if the result is <tt>false</tt>, then the value of <tt>Z</tt> is provided. An error is signalled if the value of <tt>X</tt> is anything but <tt>true</tt> or <tt>false</tt>.<table class="examples"><tr><td><pre>i1 : (-4 .. 4) /
(i -> if i < 0 then "neg"
else if i == 0 then "zer"
else "pos")
o1 = (neg, neg, neg, neg, zer, pos, pos, pos, pos)
o1 : Sequence</pre>
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The else clause may be omitted from an <tt>if</tt> expression. In that case, if value of the predicate <tt>X</tt> is false, then <a href="_null.html" title="the unique member of the empty class">null</a> is provided as the value of the <tt>if</tt> expression.<table class="examples"><tr><td><pre>i2 : (-4 .. 4) /
(i -> if i < 0 then "neg"
else if i == 0 then "zer")
o2 = (neg, neg, neg, neg, zer, , , , )
o2 : Sequence</pre>
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There are a variety of predicate functions (such as <tt><</tt>, used above) that yield <tt>true</tt> or <tt>false</tt> and can be used as the predicate in an <tt>if</tt> expression. For a list, see <a href="___Boolean.html" title="the class of Boolean values">Boolean</a>. Boolean results may be combined with <a href="_not.html" title="negation">not</a>, <a href="_and.html" title="conjunction">and</a>, and <a href="_or.html" title="disjunction">or</a>.</div>
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