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<TITLE>LambdaMOO Programmer's Manual - Iteration</TITLE>
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<P><HR><P>
<H3><A NAME="SEC32" HREF="ProgrammersManual_toc.html#TOC32">Statements for Looping</A></H3>
<P>
MOO provides three different kinds of looping statements, allowing you to have
a set of statements executed (1) once for each element of a given list, (2)
once for each integer or object number in a given range, and (3) over and over
until a given condition stops being true.
</P>
<P>
To perform some statements once for each element of a given list, use this
syntax:
</P>
<PRE>
for <VAR>variable</VAR> in (<VAR>expression</VAR>)
<VAR>statements</VAR>
endfor
</PRE>
<P>
The expression is evaluated and should return a list; if it does not,
<CODE>E_TYPE</CODE> is raised. The <VAR>statements</VAR> are then executed once for
each element of that list in turn; each time, the given <VAR>variable</VAR> is
assigned the value of the element in question. For example, consider
the following statements:
</P>
<PRE>
odds = {1, 3, 5, 7, 9};
evens = {};
for n in (odds)
evens = {@evens, n + 1};
endfor
</PRE>
<P>
The value of the variable <SAMP>`evens'</SAMP> after executing these statements
is the list
</P>
<PRE>
{2, 4, 6, 8, 10}
</PRE>
<P>
To perform a set of statements once for each integer or object number in a given
range, use this syntax:
</P>
<PRE>
for <VAR>variable</VAR> in [<VAR>expression-1</VAR>..<VAR>expression-2</VAR>]
<VAR>statements</VAR>
endfor
</PRE>
<P>
The two expressions are evaluated in turn and should either both return integers
or both return object numbers; <CODE>E_TYPE</CODE> is raised otherwise. The
<VAR>statements</VAR> are then executed once for each integer (or object number, as
appropriate) greater than or equal to the value of <VAR>expression-1</VAR> and less
than or equal to the result of <VAR>expression-2</VAR>, in increasing order. Each
time, the given variable is assigned the integer or object number in question.
For example, consider the following statements:
</P>
<PRE>
evens = {};
for n in [1..5]
evens = {@evens, 2 * n};
endfor
</PRE>
<P>
The value of the variable <SAMP>`evens'</SAMP> after executing these statements
is just as in the previous example: the list
</P>
<PRE>
{2, 4, 6, 8, 10}
</PRE>
<P>
The following loop over object numbers prints out the number and name of every
valid object in the database:
</P>
<PRE>
for o in [#0..max_object()]
if (valid(o))
notify(player, tostr(o, ": ", o.name));
endif
endfor
</PRE>
<P>
The final kind of loop in MOO executes a set of statements repeatedly as long
as a given condition remains true:
</P>
<PRE>
while (<VAR>expression</VAR>)
<VAR>statements</VAR>
endwhile
</PRE>
<P>
The expression is evaluated and, if it returns a true value, the
<VAR>statements</VAR> are executed; then, execution of the <SAMP>`while'</SAMP> statement
begins all over again with the evaluation of the expression. That is,
execution alternates between evaluating the expression and executing the
statements until the expression returns a false value. The following
example code has precisely the same effect as the loop just shown above:
</P>
<PRE>
evens = {};
n = 1;
while (n <= 5)
evens = {@evens, 2 * n};
n = n + 1;
endwhile
</PRE>
<BLOCKQUOTE>
<P>
<EM>Fine point:</EM> It is also possible to give a `name' to a <SAMP>`while'</SAMP>
loop, using this syntax:
<PRE>
while <VAR>name</VAR> (<VAR>expression</VAR>)
<VAR>statements</VAR>
endwhile
</PRE>
<P>
which has precisely the same effect as
</P>
<PRE>
while (<VAR>name</VAR> = <VAR>expression</VAR>)
<VAR>statements</VAR>
endwhile
</PRE>
<P>
This naming facility is only really useful in conjunction with the <SAMP>`break'</SAMP>
and <SAMP>`continue'</SAMP> statements, described in the next section.
</BLOCKQUOTE>
<P>
With each kind of loop, it is possible that the statements in the body of the
loop will never be executed at all. For iteration over lists, this happens
when the list returned by the expression is empty. For iteration on integers,
it happens when <VAR>expression-1</VAR> returns a larger integer than
<VAR>expression-2</VAR>. Finally, for the <SAMP>`while'</SAMP> loop, it happens if the
expression returns a false value the very first time it is evaluated.
</P>
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