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<h2>16. Control-flow constructs
<a name="16. Control-flow constructs"></a>
</h2>
<p style="margin-top: 1em"><font color="#000000">The
<b>pic</b> language provides conditionals and looping. For
example,</font></p>
<p style="margin-left:10%; margin-top: 1em"><font color="#000000">pi
= atan2(0,-1); <br>
for i = 0 to 2 * pi by 0.1 do { <br>
"-" at (i/2, 0); <br>
"." at (i/2, sin(i)/2); <br>
":" at (i/2, cos(i)/2); <br>
}</font></p>
<p style="margin-top: 1em"><font color="#000000">which
yields this:</font></p>
<p align="center" style="margin-top: 1em"><font color="#000000"><img src="img/pic45.png" alt="Image img/pic45.png"></font></p>
<p align="center" style="margin-top: 1em"><font color="#000000">Figure
16-1: Plotting with a <b>for</b> loop</font></p>
<p style="margin-top: 1em"><font color="#000000">The syntax
of the <b>for</b> statement is:</font></p>
<p style="margin-left:10%; margin-top: 1em"><font color="#000000"><b>for</b>
<i>variable</i> <b>=</b> <i>expr1</i> <b>to</b> <i>expr2</i>
[<b>by</b> [<b>*</b>]<i>expr3</i>] <b>do</b> <i>X body
X</i></font></p>
<p style="margin-top: 1em"><font color="#000000">The
semantics are as follows: Set <i>variable</i> to
<i>expr1</i>. While the value of <i>variable</i> is less
than or equal to <i>expr2</i>, do <i>body</i> and increment
<i>variable</i> by <i>expr3</i>; if <b>by</b> is not given,
increment <i>variable</i> by 1. If <i>expr3</i> is
prefixed by <b>*</b> then <i>variable</i> is multiplied
instead by <i>expr3</i>. The value of <i>expr3</i> can be
negative for the additive case; <i>variable</i> is then
tested whether it is greater than or equal to <i>expr2</i>.
For the multiplicative case, <i>expr3</i> must be greater
than zero. If the constraints aren’t met, the loop
isn’t executed. <i>X</i> can be any character not
occurring in <i>body</i>; or the two <i>X</i>s may be paired
braces (as in the <b>sh</b> command).</font></p>
<p style="margin-top: 1em"><font color="#000000">The syntax
of the <b>if</b> statement is as follows:</font></p>
<p style="margin-left:10%; margin-top: 1em"><font color="#000000"><b>if</b>
<i>expr</i> <b>then</b> <i>X if-true X</i> [<b>else</b> <i>Y
if-false Y</i>]</font></p>
<p style="margin-top: 1em"><font color="#000000">Its
semantics are as follows: Evaluate <i>expr</i>; if it is
non-zero then do <i>if-true</i>, otherwise do
<i>if-false</i>. <i>X</i> can be any character not occurring
in <i>if-true</i>. <i>Y</i> can be any character not
occurring in <i>if-false</i>.</font></p>
<p style="margin-top: 1em"><font color="#000000">Eithe or
both of the <i>X</i> or <i>Y</i> pairs may instead be
balanced pairs of braces ({ and }) as in the <b>sh</b>
command. In either case, the <i>if-true</i> may contain
balanced pairs of braces. None of these delimiters are seen
inside strings.</font></p>
<p style="margin-top: 1em"><font color="#000000">All the
usual relational operators my be used in conditional
expressions; <b>!</b> (logical negation, not factorial),
<b>&&</b>, <b>||</b>, <b>==</b>, <b>!=</b>,
<b>>=</b>, <b><=</b>, <b><</b>,
<b>></b>.</font></p>
<p style="margin-top: 1em"><font color="#000000">String
comparison is also supported using <b>==</b> and <b>!=</b>.
String comparisons may need to be parenthesized to avoid
syntactic ambiguities.</font></p>
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