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git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@2157 f3b2605a-c512-4ea7-a41b-209d697bcdaa

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sjplimp
2008-10-06 23:07:25 +00:00
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doc/variable.html
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@ -40,7 +40,7 @@
vx[], vy[], vz[], fx[], fy[], fz[]
compute references = c_ID, c_ID[2], c_ID[N], c_ID[N][2], c_ID[], c_ID[][2]
fix references = f_ID, f_ID[2], f_ID[N], f_ID[N][2], f_ID[], f_ID[][2]
other variables = v_abc, v_abc[N], v_abc[]
variable references = v_abc, v_abc[N], v_abc[]
</PRE>
</UL>
@ -97,15 +97,14 @@ script (with 2 exceptions, read further). This is to allow an input
script to be processed multiple times without resetting the variables;
see the <A HREF = "jump.html">jump</A> or <A HREF = "include.html">include</A> commands. It also
means that using the <A HREF = "Section_start.html#2_6">command-line switch</A> -var
will override a corresponding variable setting in the input script.
will override a corresponding index variable setting in the input
script.
</P>
<P>There are two exceptions to this rule. First, variables of style
<I>equal</I> and <I>atom</I> ARE redefined each time the command is encountered.
This allows them to be reset, when their formulas contain a
substitution for another variable, e.g. $x. This can be useful in a
loop. This also means an <I>equal</I>-style variable will re-define a
command-line switch -var setting, so an <I>index</I>-style variable should
be used for such settings instead, as in bench/in.lj.
loop.
</P>
<P>Second, as described below, if a variable is iterated on to the end of
its list of strings via the <A HREF = "next.html">next</A> command, it is removed
@ -298,13 +297,13 @@ compute. N is a global atom ID (positive integer).
</TD></TR></TABLE></DIV>
<P>Fix references access one or more quantities calculated by a
<A HREF = "fix.html">fix</A>. The ID in the reference should be replaced by
the actual ID of the fix defined elsewhere in the input script.
See the doc pages for individual computes to see which ones calculate
global versus per-atom quantities. If the compute reference contains
empty brackets, then per-atom values calculated by the compute are
accessed. Otherwise a single value (global or per-atom) calculated by
the compute is accessed.
<A HREF = "fix.html">fix</A>. The ID in the reference should be replaced by the
actual ID of the fix defined elsewhere in the input script. See the
doc pages for individual computes to see which ones calculate global
versus per-atom quantities. If the fix reference contains empty
brackets, then per-atom values calculated by the fix are accessed.
Otherwise a single value (global or per-atom) calculated by the
fix is accessed.
</P>
<P>Note that some fixes only generate quantities on certain timesteps.
If a variable attempts to access the fix on non-allowed timesteps, an
@ -338,27 +337,30 @@ print $a
</PRE>
<P>then LAMMPS will run for a while when the print statement is invoked!
</P>
<P>Another way to reference a variable in a formula is using the $x form
instead of v_x. There is a subtle difference between the two
references that has to do with when the evaluation of the included
variable is done.
<HR>
<P>It is useful to understand the distinction between referencing a
variable in a formula using the $x form instead of v_x. There is a
subtle difference between the two references that has to do with when
the evaluation of the included variable is done.
</P>
<P>Using a $x, the value of the include variable is substituted for
immediately when the line is read from the input script, just as it
would be in other input script command. This could be the desired
behavior if a static value is desired. Or it could be the desired
behavior for an equal-style variable if the variable command appears
in a loop (see the <A HREF = "jump.html">jump</A> and <A HREF = "next.html">next</A> commands),
since the substitution will be performed anew each time thru the loop
as the command is re-read. Note that if the variable formula is
enclosed in double quotes, this prevents variable substitution and
thus an error will be generated when the variable formula is
evaluated.
<P>Referencing the variable as $x, the value of the include variable is
substituted for immediately when the line is read from the input
script, just as it would be in other input script command.
</P>
<P>Using a v_x, the value of the included variable will not be accessed
until the variable formula is evaluated. Thus the value may change
each time the evaluation is performed. This may also be desired
behavior.
<P>This could be the desired behavior if a static value is desired. Or
it could be the desired behavior for an equal-style variable if the
variable command appears in a loop (see the <A HREF = "jump.html">jump</A> and
<A HREF = "next.html">next</A> commands), since the substitution will be performed
anew each time thru the loop as the command is re-read. Note that if
the variable formula is enclosed in double quotes, this prevents
variable substitution and thus an error will be generated when the
variable formula is evaluated.
</P>
<P>Referencing the variable as v_x, the value of the included variable
will not be accessed until the variable formula is evaluated. Thus
the value may change each time the evaluation is performed. This may
also be desired behavior.
</P>
<P>As an example, if the current simulation box volume is 1000.0, then
these lines: