forked from lijiext/lammps
88 lines
2.8 KiB
HTML
88 lines
2.8 KiB
HTML
<HTML>
|
|
<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
|
|
</CENTER>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<HR>
|
|
|
|
<H3>if command
|
|
</H3>
|
|
<P><B>Syntax:</B>
|
|
</P>
|
|
<PRE>if value1 operator value2 then command1 else command2
|
|
</PRE>
|
|
<UL><LI>value1 = 1st value
|
|
<LI>operator = "<" or "<=" or ">" or ">=" or "==" or "!="
|
|
<LI>value2 = 2nd value
|
|
<LI>then = required word
|
|
<LI>command1 = command to execute if condition is met
|
|
<LI>else = optional word
|
|
<LI>command2 = command to execute if condition is not met (optional argument)
|
|
</UL>
|
|
<P><B>Examples:</B>
|
|
</P>
|
|
<PRE>if ${steps} > 1000 then exit
|
|
if $x <= $y then "print X is smaller = $x" else "print Y is smaller = $y"
|
|
if ${eng} > 0.0 then "timestep 0.005"
|
|
if ${eng} > ${eng_previous} then "jump file1" else "jump file2"
|
|
</PRE>
|
|
<P><B>Description:</B>
|
|
</P>
|
|
<P>This command provides an in-then-else test capability within an input
|
|
script. Two values are numerically compared to each other and the
|
|
result is TRUE or FALSE. Note that as in the examples above, either
|
|
of the values can be variables, as defined by the
|
|
<A HREF = "variable.html">variable</A> command, so that when they are evaluated when
|
|
substituted for in the if command, a user-defined computation will be
|
|
performed which can depend on the current state of the simulation.
|
|
</P>
|
|
<P>If the result of the if test is TRUE, then command1 is executed. This
|
|
can be any valid LAMMPS input script command. If the command is more
|
|
than 1 word, it should be enclosed in double quotes, so that it will
|
|
be treated as a single argument, as in the examples above.
|
|
</P>
|
|
<P>The if command can contain an optional "else" clause. If it does and
|
|
the result of the if test is FALSE, then command2 is executed.
|
|
</P>
|
|
<P>Note that by jumping to a label in the same input script, the if
|
|
command can be used to break out of a loop. See the <A HREF = "variable.html">variable
|
|
delete</A> for info on how to delete the associated loop
|
|
variable, so that it can be re-used later in the input script.
|
|
</P>
|
|
<P>Note that if either command1 or command2 is a bogus LAMMPS command,
|
|
such as "exit" in the first example, then executing the command will
|
|
cause LAMMPS to halt.
|
|
</P>
|
|
<P>Here is an example of a double loop which uses the if and
|
|
<A HREF = "jump.html">jump</A> commands to break out of the inner loop when a
|
|
condition is met, then continues iterating thru the outer loop.
|
|
</P>
|
|
<PRE>label loopa
|
|
variable a loop 5
|
|
label loopb
|
|
variable b loop 5
|
|
print "A,B = $a,$b"
|
|
run 10000
|
|
if $b > 2 then "jump in.script break"
|
|
next b
|
|
jump in.script loopb
|
|
label break
|
|
variable b delete
|
|
</PRE>
|
|
<PRE>next a
|
|
jump in.script loopa
|
|
</PRE>
|
|
<P><B>Restrictions:</B> none
|
|
</P>
|
|
<P><B>Related commands:</B>
|
|
</P>
|
|
<P><A HREF = "variable.html">variable</A>
|
|
</P>
|
|
<P><B>Default:</B> none
|
|
</P>
|
|
</HTML>
|