<HTML>
<HEAD>
<!-- This HTML file has been created by texi2html 1.51
from ProgrammersManual.texinfo on 4 March 1997 -->
<TITLE>LambdaMOO Programmer's Manual - Variables</TITLE>
</HEAD>
<BODY>
Go to the <A HREF="ProgrammersManual_1.html">first</A>, <A HREF="ProgrammersManual_12.html">previous</A>, <A HREF="ProgrammersManual_14.html">next</A>, <A HREF="ProgrammersManual_77.html">last</A> section, <A HREF="ProgrammersManual_toc.html">table of contents</A>.
<P><HR><P>
<H3><A NAME="SEC13" HREF="ProgrammersManual_toc.html#TOC13">Naming Values Within a Verb</A></H3>
<P>
As discussed earlier, it is possible to store values in properties on
objects; the properties will keep those values forever, or until another
value is explicitly put there. Quite often, though, it is useful to
have a place to put a value for just a little while. MOO provides local
variables for this purpose.
</P>
<P>
Variables are named places to hold values; you can get and set the value
in a given variable as many times as you like. Variables are temporary,
though; they only last while a particular verb is running; after it
finishes, all of the variables given values there cease to exist and the
values are forgotten.
</P>
<P>
Variables are also "local" to a particular verb; every verb has its own
set of them. Thus, the variables set in one verb are not visible to the
code of other verbs.
</P>
<P>
The name for a variable is made up entirely of letters, digits, and the
underscore character (<SAMP>`_'</SAMP>) and does not begin with a digit. The
following are all valid variable names:
</P>
<PRE>
foo
_foo
this2that
M68000
two_words
This_is_a_very_long_multiword_variable_name
</PRE>
<P>
Note that, along with almost everything else in MOO, the case of the
letters in variable names is insignificant. For example, these are all
names for the same variable:
</P>
<PRE>
fubar
Fubar
FUBAR
fUbAr
</PRE>
<P>
A variable name is itself an expression; its value is the value of the named
variable. When a verb begins, almost no variables have values yet; if you try
to use the value of a variable that doesn't have one, the error value
<CODE>E_VARNF</CODE> is raised. (MOO is unlike many other programming languages in
which one must `declare' each variable before using it; MOO has no such
declarations.) The following variables always have values:
</P>
<PRE>
INT FLOAT OBJ
STR LIST ERR
player this caller
verb args argstr
dobj dobjstr prepstr
iobj iobjstr NUM
</PRE>
<P>
The values of some of these variables always start out the same:
</P>
<DL COMPACT>
<DT><CODE>INT</CODE>
<DD>
an integer, the type code for integers (see the description of the function
<CODE>typeof()</CODE>, below)
<DT><CODE>NUM</CODE>
<DD>
the same as <CODE>INT</CODE> (for historical reasons)
<DT><CODE>FLOAT</CODE>
<DD>
an integer, the type code for floating-point numbers
<DT><CODE>LIST</CODE>
<DD>
an integer, the type code for lists
<DT><CODE>STR</CODE>
<DD>
an integer, the type code for strings
<DT><CODE>OBJ</CODE>
<DD>
an integer, the type code for objects
<DT><CODE>ERR</CODE>
<DD>
an integer, the type code for error values
</DL>
<P>
For others, the general meaning of the value is consistent, though the
value itself is different for different situations:
</P>
<DL COMPACT>
<DT><CODE>player</CODE>
<DD>
an object, the player who typed the command that started the task that
involved running this piece of code.
<DT><CODE>this</CODE>
<DD>
an object, the object on which the currently-running verb was found.
<DT><CODE>caller</CODE>
<DD>
an object, the object on which the verb that called the
currently-running verb was found. For the first verb called for a given
command, <SAMP>`caller'</SAMP> has the same value as <SAMP>`player'</SAMP>.
<DT><CODE>verb</CODE>
<DD>
a string, the name by which the currently-running verb was identified.
<DT><CODE>args</CODE>
<DD>
a list, the arguments given to this verb. For the first verb called for
a given command, this is a list of strings, the words on the command
line.
</DL>
<P>
The rest of the so-called "built-in" variables are only really
meaningful for the first verb called for a given command. Their
semantics is given in the discussion of command parsing, above.
</P>
<P>
To change what value is stored in a variable, use an <STRONG>assignment</STRONG>
expression:
</P>
<PRE>
<VAR>variable</VAR> = <VAR>expression</VAR>
</PRE>
<P>
For example, to change the variable named <SAMP>`x'</SAMP> to have the value 17,
you would write <SAMP>`x = 17'</SAMP> as an expression. An assignment
expression does two things:
</P>
<UL>
<LI>
it changes the value of of the named variable, and
<LI>
it returns the new value of that variable.
</UL>
<P>
Thus, the expression
</P>
<PRE>
13 + (x = 17)
</PRE>
<P>
changes the value of <SAMP>`x'</SAMP> to be 17 and returns 30.
</P>
<P><HR><P>
Go to the <A HREF="ProgrammersManual_1.html">first</A>, <A HREF="ProgrammersManual_12.html">previous</A>, <A HREF="ProgrammersManual_14.html">next</A>, <A HREF="ProgrammersManual_77.html">last</A> section, <A HREF="ProgrammersManual_toc.html">table of contents</A>.
</BODY>
</HTML>