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<title>ploticus: conditional expressions</title>
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<td><br><h2>Conditional expressions</h2></td>
<td align=right>
<small>
<a href="../doc/welcome.html"><img src="../doc/ploticus.gif" border=0></a><br>
Version 2.41 Mar2009
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<title>Manual page for Conditional_expressions(PL)</title>
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<b>Note:</b> this page describes conditional expressions as used in ploticus <b>#if statements</b>.<tt> </tt>
<a href="select.html">
select attributes
</a>
are slightly different and are described
<a href="select.html">
here.<tt> </tt>
</a>
<br><br><br>
<p>
Conditional expressions use a simple syntax made up of operators such as <tt>=</tt> and <tt>!=</tt>,
operands, and logical connectors such as <tt>&&</tt> and <tt>||</tt>.
Operands may be literal numeric, alphanumeric, or text string values,
@variables, @data-fields, or
<a href="functions.html">
functions
</a>
. In
<a href="scripts.html">
scripts
</a>
, literal string operands may be enclosed in double quotes (")
(but no quotes may be used in ploticus <b>select:</b> attributes)
.<tt> </tt>
There is no concept of precedence in these expressions (other than left to right);
parentheses cannot be used except within strings.<tt> </tt>
<p>
<br><br><br>
<h2>Examples</h2>
<p>
- In a
<a href="scripts.html">
script
</a>
, see if the value of the variable @SUM is greater than 100:
<pre>
#if @SUM > 100
...
#endif
</pre>
<p>
- Another example:
<pre>
#if @SUM > 100 && @STATUS = s
</pre>
<p>
- Check to see if @description has any contents:
<pre>
#if $len(@description) > 0
</pre>
<p>
- Check to see if @val is a valid number:
<pre>
#if $isnumber(@val) = 1
</pre>
<p>
- See if @C does not countain a double quote:
<pre>
#if @C != "\""
</pre>
<p>
Here's a conditional expression for finding lastname
beginning with <tt>Gr</tt>, <tt>Gu</tt> or <tt>Gy</tt>:
<pre>
@lastname inlike Gr*,Gu*,Gy*
</pre>
<br><br><br>
<h2>Operands</h2>
Operands may be
<ul>
<li>
@variables
<li>
@field numbers, e.g. @1 = first data field
<li>
Literal values (numbers, alphanumerics, text strings,
<a href="commalist.html">
commalists
</a>
)
<li>
<a href="functions.html">
Functions
</a>
</ul>
<p>
In
<a href="scripts.html">
scripts
</a>
, literal alphanumeric, text string, or
<a href="commalist.html">
commalist
</a>
operands may be enclosed in double quotes (");
no variable evaluation occurs inside quoted strings.<tt> </tt>
<br><br><br>
<h2>Comparison operators</h2>
<dl>
<dt><dd><p>
<pre>
<b>= </b> Equal to. This is case sensitive for strings.
<b>!=</b> Not equal to.
<b>> </b> Greater than.
<b>>=</b> Greater than or equal to.
<b>< </b> Less than.
<b><=</b> Less than or equal to.
</pre>
<dt><dd><p>
If at least one operand is non-numeric, comparison
operators such as > will do an ascii value comparison.<tt> </tt>
</dl>
<p>
<b>Wild card matching</b>: Wild card matching may be done
using <b>like</b>. Wild card characters are <b>*</b> which matches
any number of any character, and <b>?</b> which matches a single
instance of any character. This matching is case-insensitive.<tt> </tt>
<dl>
<dt><dd><p>
<pre>
<b>like</b> Wild card match. Example: hello like h* (true)
<b>!like</b> Not a wild card match.
</pre>
</dl>
<p>
<b>Commalist operators</b>: these take a
<a href="commalist.html">
commalist
</a>
on the right side.<tt> </tt>
By default, lists are delimited using commas. If this is not convenient,
the list delimitation character may be changed using
<a href="control">
#control listsep
</a>
.<tt> </tt>
<dl>
<dt><dd><p>
<pre>
<b>in</b> Member of list. Example: z in x,y,z (true)
<b>!in</b> Not a member of list. (Alt: <b>ni</b>)
<b>inlike</b> Same as <b>in</b> but wild card matching is used.
(Wild cards may be used in list members.)
<b>!inlike</b> Same as <b>!in</b> but wild card matching is used.
</pre>
<br><br><br>
</dl>
<h2>Logical connectors</h2>
Individual conditional expressions may be connected together using
logical AND (<b>&&</b>) or OR (<b>||</b>).<tt> </tt>
An entire expression may be negated by putting <b>not:</b> at the beginning
of the expression.<tt> </tt>
<p>
Because parentheses may not be used to establish precedence
mixing AND and OR in the same expression requires care.<tt> </tt>
When the expression is parsed, it is first
split into OR terms, then each of the OR terms is split into AND terms.<tt> </tt>
For example: <tt>A = B and C = D or E = E of F = G</tt> would evaluate to true,
because it is interpreted as if it were written
<tt>(a = B and C = D) or (e = e) or (f = g)</tt>.<tt> </tt>
It may be best to avoid mixing AND and OR in the same expression
and use multiple expressions instead.<tt> </tt>
<br><br><br>
<h2>Limitations and bugs</h2>
<p>
Because parentheses may not be used to establish precedence,
the mixing of AND and OR in the same expression is problematic (see above).<tt> </tt>
<br>
<br>
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