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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_intro.html" > 1. Introduction< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_start.html" > 2. Getting Started< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_commands.html" > 3. Commands< / a > < / li >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_accelerate.html" > 5. Accelerating LAMMPS performance< / a > < / li >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_tools.html" > 9. Additional tools< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_modify.html" > 10. Modifying & extending LAMMPS< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_python.html" > 11. Python interface to LAMMPS< / a > < / li >
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< li > if command< / li >
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< a href = "http://lammps.sandia.gov" > Website< / a >
< a href = "Section_commands.html#comm" > Commands< / a >
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< div class = "section" id = "if-command" >
< span id = "index-0" > < / span > < h1 > if command< a class = "headerlink" href = "#if-command" title = "Permalink to this headline" > ¶< / a > < / h1 >
< div class = "section" id = "syntax" >
< h2 > Syntax< a class = "headerlink" href = "#syntax" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > if boolean then t1 t2 ... elif boolean f1 f2 ... elif boolean f1 f2 ... else e1 e2 ...
< / pre > < / div >
< / div >
< ul class = "simple" >
< li > boolean = a Boolean expression evaluated as TRUE or FALSE (see below)< / li >
< li > then = required word< / li >
< li > t1,t2,...,tN = one or more LAMMPS commands to execute if condition is met, each enclosed in quotes< / li >
< li > elif = optional word, can appear multiple times< / li >
< li > f1,f2,...,fN = one or more LAMMPS commands to execute if elif condition is met, each enclosed in quotes (optional arguments)< / li >
< li > else = optional argument< / li >
< li > e1,e2,...,eN = one or more LAMMPS commands to execute if no condition is met, each enclosed in quotes (optional arguments)< / li >
< / ul >
< / div >
< div class = "section" id = "examples" >
< h2 > Examples< a class = "headerlink" href = "#examples" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > if " ${steps} > 1000" then quit
if " ${myString} == a10" then quit
if " $x < = $y" then " print X is smaller = $x" else " print Y is smaller = $y"
if " (${eng} > 0.0) || ($n < 1000)" then &
" timestep 0.005" &
elif $n< 10000 &
" timestep 0.01" &
else &
" timestep 0.02" &
" print ' Max step reached' "
if " ${eng} > ${eng_previous}" then " jump file1" else " jump file2"
< / pre > < / div >
< / div >
< / div >
< div class = "section" id = "description" >
< h2 > Description< a class = "headerlink" href = "#description" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > This command provides an if-then-else capability within an input
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script. A Boolean expression is evaluted and the result is TRUE or
FALSE. Note that as in the examples above, the expression can contain
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variables, as defined by the < a class = "reference internal" href = "variable.html" > < em > variable< / em > < / a > command, which
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will be evaluated as part of the expression. Thus a user-defined
formula that reflects the current state of the simulation can be used
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to issue one or more new commands.< / p >
< 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
expressions associated with successive elif keywords are evaluated
until one is found to be true, in which case its commands (f1, f2,
..., fN) are executed. If no Boolean expression is TRUE, then the
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commands associated with 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
Boolean expression is FALSE, then no commands are executed.< / p >
< p > The syntax for Boolean expressions is described below.< / p >
< p > Each command (t1, f1, e1, etc) can be any valid LAMMPS input script
command, except an < a class = "reference internal" href = "include.html" > < em > include< / em > < / a > command, which is not
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allowed. 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.< / p >
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< div class = "admonition note" >
< p class = "first admonition-title" > Note< / p >
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< p class = "last" > If a command itself requires a quoted argument (e.g. a
< a class = "reference internal" href = "print.html" > < em > print< / em > < / 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 class = "reference internal" href = "Section_commands.html#cmd-2" > < span > Section_commands 2< / span > < / a > of the manual for
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more details on using quotes in arguments. Only one of level of
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nesting is allowed, but that should be sufficient for most use cases.< / p >
< / div >
< p > Note that by using the line continuation character “ & ” , the if command
can be spread across many lines, though it is still a single command:< / p >
< div class = "highlight-python" > < div class = "highlight" > < pre > if " $a < $b" then &
" print ' Minimum value = $a' " &
" run 1000" &
else &
' print " Minimum value = $b" ' &
" minimize 0.001 0.001 1000 10000"
< / pre > < / div >
< / div >
< p > Note that if one of the commands to execute is < a class = "reference internal" href = "quit.html" > < em > quit< / em > < / a > , as in
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the first example above, then executing the command will cause LAMMPS
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to halt.< / 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 class = "reference internal" href = "variable.html" > < em > variable delete< / em > < / a > command for info on how to delete the associated
loop variable, so that it can be re-used later in the input script.< / p >
< p > Here is an example of a loop which checks every 1000 steps if the
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system temperature has reached a certain value, and if so, breaks out
of the loop to finish the run. Note that any variable could be
checked, so long as it is current on the timestep when the run
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completes. As explained on the < a class = "reference internal" href = "variable.html" > < em > variable< / em > < / a > doc page,
this can be insured by includig the variable in thermodynamic output.< / p >
< div class = "highlight-python" > < div class = "highlight" > < pre > variable myTemp equal temp
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label loop
variable a loop 1000
run 1000
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if " ${myTemp} < 300.0" then " jump SELF break"
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next a
jump SELF loop
label break
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print " ALL DONE"
< / pre > < / div >
< / div >
< p > Here is an example of a double loop which uses the if and
< a class = "reference internal" href = "jump.html" > < em > jump< / em > < / a > commands to break out of the inner loop when a
condition is met, then continues iterating thru the outer loop.< / p >
< div class = "highlight-python" > < div class = "highlight" > < 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 " jump SELF break"
next b
jump in.script loopb
label break
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variable b delete
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next a
jump SELF loopa
< / pre > < / div >
< / div >
< hr class = "docutils" / >
< 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
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.< / p >
< p > An expression is built out of numbers (which start with a digit or
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period or minus sign) or strings (which start with a letter and can
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contain alphanumeric characters or underscores):< / p >
< div class = "highlight-python" > < div class = "highlight" > < pre > < span class = "mf" > 0.2< / span > < span class = "p" > ,< / span > < span class = "mi" > 100< / span > < span class = "p" > ,< / span > < span class = "mf" > 1.0e20< / span > < span class = "p" > ,< / span > < span class = "o" > -< / span > < span class = "mf" > 15.4< / span > < span class = "p" > ,< / span > < span class = "n" > etc< / span >
< span class = "n" > InP< / span > < span class = "p" > ,< / span > < span class = "n" > myString< / span > < span class = "p" > ,< / span > < span class = "n" > a123< / span > < span class = "p" > ,< / span > < span class = "n" > ab_23_cd< / span > < span class = "p" > ,< / span > < span class = "n" > etc< / span >
< / pre > < / div >
< / div >
< p > and Boolean operators:< / p >
< div class = "highlight-python" > < div class = "highlight" > < pre > A == B, A != B, A < B, A < = B, A > B, A > = B, A & & B, A || B, !A
< / pre > < / div >
< / div >
< p > Each A and B is a number or string or a variable reference like $a or
${abc}, or A or B can be another Boolean expression.< / p >
< p > If a variable is used it can produce a number when evaluated, like an
< a class = "reference internal" href = "variable.html" > < em > equal-style variable< / em > < / a > . Or it can produce a string,
like an < a class = "reference internal" href = "variable.html" > < em > index-style variable< / em > < / a > . For an individual
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Boolean operator, A and B must both be numbers or must both be
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strings. You cannot compare a number to a string.< / p >
< p > Expressions are evaluated left to right and have the usual C-style
precedence: the unary logical NOT operator ” !” has the highest
precedence, the 4 relational operators “ < ” , “ < =” , “ > ” , and “ > =” are
next; the two remaining relational operators “ ==” and ” !=” are next;
then the logical AND operator “ & & ” ; and finally the logical OR
operator “ ||” has the lowest precedence. 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.< / p >
< p > When the 6 relational operators (first 6 in list above) compare 2
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numbers, they return either a 1.0 or 0.0 depending on whether the
relationship between A and B is TRUE or FALSE. When the 6 relational
operators compare 2 strings, they also return a 1.0 or 0.0 for TRUE or
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FALSE, but the comparison is done by the C function strcmp().< / p >
< p > When the 3 logical operators (last 3 in list above) compare 2 numbers,
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they also return either a 1.0 or 0.0 depending on whether the
relationship between A and B is TRUE or FALSE (or just A). The
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logical AND operator will return 1.0 if both its arguments are
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non-zero, else it returns 0.0. The logical OR operator will return
1.0 if either of its arguments is non-zero, else it returns 0.0. The
logical NOT operator returns 1.0 if its argument is 0.0, else it
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returns 0.0. The 3 logical operators can only be used to operate on
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numbers, not on strings.< / p >
< p > The overall Boolean expression produces a TRUE result if the result is
non-zero. If the result is zero, the expression result is FALSE.< / p >
< / div >
< hr class = "docutils" / >
< div class = "section" id = "restrictions" >
< h2 > Restrictions< a class = "headerlink" href = "#restrictions" title = "Permalink to this headline" > ¶< / a > < / h2 >
< blockquote >
< div > none< / div > < / blockquote >
< / div >
< div class = "section" id = "related-commands" >
< h2 > Related commands< a class = "headerlink" href = "#related-commands" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > < a class = "reference internal" href = "variable.html" > < em > variable< / em > < / a > , < a class = "reference internal" href = "print.html" > < em > print< / em > < / a > < / p >
< p > < strong > Default:< / strong > none< / p >
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