forked from lijiext/lammps
64 lines
2.1 KiB
HTML
64 lines
2.1 KiB
HTML
<HTML>
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<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>
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</CENTER>
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<HR>
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<H3>if command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>if value1 operator value2 then command1 else command2
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</PRE>
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<UL><LI>value1 = 1st value
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<LI>operator = "<" or "<=" or ">" or ">=" or "==" or "!="
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<LI>value2 = 2nd value
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<LI>then = required word
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<LI>command1 = command to execute if condition is met
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<LI>else = optional word
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<LI>command2 = command to execute if condition is not met (optional argument)
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>if ${steps} > 1000 then exit
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if $x <= $y then "print X is smaller = $x" else "print Y is smaller = $y"
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if ${eng} > 0.0 then "timestep 0.005"
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if ${eng} > ${eng_previous} then "jump file1" else "jump file2"
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>This command provides an in-then-else test capability within an input
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script. Two values are numerically compared to each other and the
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result is TRUE or FALSE. Note that as in the examples above, either
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of the values can be variables, as defined by the
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<A HREF = "variable.html">variable</A> command, so that when they are evaluated when
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substituted for in the if command, a user-defined computation will be
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performed which can depend on the current state of the simulation.
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</P>
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<P>If the result of the if test is TRUE, then command1 is executed. This
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can be any valid LAMMPS input script command. If the command is more
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than 1 word, it should be enclosed in double quotes, so that it will
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be treated as a single argument, as in the examples above.
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</P>
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<P>The if command can contain an optional "else" clause. If it does and
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the result of the if test is FALSE, then command2 is executed.
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</P>
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<P>Note that if either command1 or command2 is a bogus LAMMPS command,
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such as "exit" in the first example, then executing the command will
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cause LAMMPS to halt.
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</P>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "variable.html">variable</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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