2007-05-01 07:37:43 +08:00
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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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<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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2010-11-19 01:28:36 +08:00
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<PRE>if boolean then t1 t2 ... elif boolean f1 f2 ... elif boolean f1 f2 ... else e1 e2 ...
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2007-05-01 07:37:43 +08:00
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</PRE>
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2010-11-19 01:28:36 +08:00
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<UL><LI>boolean = a Boolean expression evaluated as TRUE or FALSE (see below)
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<LI>then = required word
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<LI>t1,t2,...,tN = one or more LAMMPS commands to execute if condition is met, each enclosed in quotes
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2010-11-19 01:28:36 +08:00
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<LI>elif = optional word, can appear multiple times
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<LI>f1,f2,...,fN = one or more LAMMPS commands to execute if elif condition is met, each enclosed in quotes (optional arguments)
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2010-01-20 04:04:41 +08:00
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<LI>else = optional argument
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<LI>e1,e2,...,eN = one or more LAMMPS commands to execute if no condition is met, each enclosed in quotes (optional arguments)
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2007-05-01 07:37:43 +08:00
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</UL>
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<P><B>Examples:</B>
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</P>
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2010-11-19 01:28:36 +08:00
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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) || ($n < 1000)" then &
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"timestep 0.005" &
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elif $n<10000 &
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"timestep 0.01" &
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else &
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"timestep 0.02" &
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"print 'Max step reached'"
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if "${eng} > ${eng_previous}" then "jump file1" else "jump file2"
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2007-05-01 07:37:43 +08:00
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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 capability within an input
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script. A Boolean expression is evaluted and the result is TRUE or
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FALSE. Note that as in the examples above, the expression can contain
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variables, as defined by the <A HREF = "variable.html">variable</A> command, which
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will be evaluated as part of the expression. Thus a user-defined
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formula that reflects the current state of the simulation can be used
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to issue one or more new commands.
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</P>
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<P>If the result of the Boolean expression is TRUE, then one or more
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commands (t1, t2, ..., tN) are executed. If it is FALSE, then Boolean
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expressions associated with successive elif keywords are evaluated
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until one is found to be true, in which case its commands (f1, f2,
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..., fN) are executed. If no Boolean expression is TRUE, then the
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commands associated witht the else keyword, namely (e1, e2, ..., eN),
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are executed. The elif and else keywords and their associated
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commands are optional. If they aren't specified and the initial
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Boolean expression is FALSE, then no commands are executed.
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</P>
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2010-11-19 01:49:30 +08:00
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<P>The syntax for Boolean expressions is described below.
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</P>
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<P>Each command (t1, f1, e1, etc) can be any valid LAMMPS input script
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command. If the command is more than one word, it must enclosed in
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quotes, so it will be treated as a single argument, as in the examples
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above.
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</P>
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<P>IMPORTANT NOTE: If a command itself requires a quoted argument (e.g. a
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<A HREF = "print.html">print</A> command), then double and single quotes can be used
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and nested in the usual manner, as in the examples above and below.
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See <A HREF = "Section_commands.html#3_2">this section</A> of the manual for more
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details on using quotes in arguments. Only one of level of nesting is
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allowed, but that should be sufficient for most use cases.
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2007-05-01 07:37:43 +08:00
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</P>
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2010-01-20 04:04:41 +08:00
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<P>Note that by using the line continuation character "&", the if command
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can be spread across many lines, though it is still a single command:
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</P>
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<PRE>if "$a < $b" then &
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"print 'Minimum value = $a'" &
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"run 1000" &
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else &
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'print "Minimum value = $b"' &
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"minimize 0.001 0.001 1000 10000"
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</PRE>
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2010-11-19 01:28:36 +08:00
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<P>Note that if one of the commands to execute is an invalid LAMMPS
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command, such as "exit" in the first example above, then executing the
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command will cause LAMMPS to halt.
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2007-05-01 07:37:43 +08:00
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</P>
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2008-10-07 23:02:40 +08:00
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<P>Note that by jumping to a label in the same input script, the if
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command can be used to break out of a loop. See the <A HREF = "variable.html">variable
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delete</A> command for info on how to delete the associated
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loop variable, so that it can be re-used later in the input script.
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2008-10-07 23:02:40 +08:00
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</P>
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<P>Here is an example of a double loop which uses the if and
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<A HREF = "jump.html">jump</A> commands to break out of the inner loop when a
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condition is met, then continues iterating thru the outer loop.
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</P>
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<PRE>label loopa
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variable a loop 5
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label loopb
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variable b loop 5
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print "A,B = $a,$b"
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run 10000
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if '$b > 2' then "print 'Jumping to another script'" "jump in.script break"
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next b
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jump in.script loopb
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label break
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variable b delete
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</PRE>
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<PRE>next a
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jump in.script loopa
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</PRE>
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2010-11-19 01:28:36 +08:00
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<HR>
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<P>The Boolean expressions for the if and elif keywords have a C-like
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syntax. Note that each expression is a single argument within the if
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command. Thus if you want to include spaces in the expression for
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clarity, you must enclose the entire expression in quotes.
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</P>
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2010-11-19 01:53:54 +08:00
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<P>An expression is built out of numbers:
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</P>
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<PRE>0.2, 100, 1.0e20, -15.4, etc
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</PRE>
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<P>and Boolean operators:
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</P>
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2010-11-19 01:52:37 +08:00
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<PRE>A == B, A != B, A < B, A <= B, A > B, A >= B, A && B, A || B
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</PRE>
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<P>Each A and B is a number or a variable reference like $a or ${abc},
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or another Boolean expression.
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</P>
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<P>If a variable is used it must produce a number when evaluated and
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substituted for in the expression, else an error will be generated.
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</P>
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<P>Expressions are evaluated left to right and have the usual C-style
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precedence: the 4 relational operators "<", "<=", ">", and ">=" have
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the highest precedence; those 4 relational operators before the
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remaining two relational operators "==" and "!="; those two relational
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operators before the logical AND operator "&&"; and the AND operator
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"&&" before the logical OR operator "||". Parenthesis can be used to
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group one or more portions of an expression and/or enforce a different
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order of evaluation than what would occur with the default precedence.
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</P>
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<P>The 6 relational operators return either a 1.0 or 0.0 depending on
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whether the relationship between x and y is true or false. The
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logical AND operator will return 1.0 if both its arguments are
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non-zero, else return a 0.0. The logical OR operator will return 1.0
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if either of its arguments is non-zero, else return a 0.0.
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</P>
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<P>The overall Boolean expression produces a TRUE result if the result is
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non-zero. If the result is zero, the expression result is FALSE.
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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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2010-01-20 04:04:41 +08:00
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<P><A HREF = "variable.html">variable</A>, <A HREF = "print.html">print</A>
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2007-05-01 07:37:43 +08:00
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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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