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doc/if.txt
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@ -10,64 +10,81 @@ if command :h3
[Syntax:]
if value1 operator value2 then t1 t2 ... else e1 e2 ... :pre
if boolean then t1 t2 ... elif boolean f1 f2 ... elif boolean f1 f2 ... else e1 e2 ... :pre
value1 = 1st value
operator = "<" or "<=" or ">" or ">=" or "==" or "!="
value2 = 2nd value
boolean = a Boolean expression evaluated as TRUE or FALSE (see below)
then = required word
t1,t2,...,tN = one or more LAMMPS commands to execute if condition is met, each enclosed in quotes
elif = optional word, can appear multiple times
f1,f2,...,fN = one or more LAMMPS commands to execute if elif condition is met, each enclosed in quotes (optional arguments)
else = optional argument
e1,e2,...,eN = one or more LAMMPS commands to execute if condition is not met, each enclosed in quotes (optional arguments) :ul
e1,e2,...,eN = one or more LAMMPS commands to execute if no condition is met, each enclosed in quotes (optional arguments) :ul
[Examples:]
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\} > $\{eng_previous\} then "jump file1" else "jump file2" :pre
if "$\{steps\} > 1000" then exit
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
[Description:]
This command provides an in-then-else 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 two values can be variables, as defined by the
"variable"_variable.html command, so that when the if command is
executed, the variable(s) will be evaluated, which could calculate a
user-defined formula that reflects the current state of the
simulation.
script. A Boolean expression is evaluted and the result is TRUE or
FALSE. Note that as in the examples above, the expression can contain
variables, as defined by the "variable"_variable.html command, which
will be evaluated as part of the expression. Thus a user-defined
formula that reflects the current state of the simulation can be used
to issue one or more new commands.
If the result of the if test is TRUE, then one or more commands (t1,
t2, ..., tN) are executed. If the result of the if test is FALSE and
no optional "else" argument is included, then the if command does
nothing. If the result of the if test is FALSE and the optional
"else" argument is included, then one or more commands (e1,
e2, ..., eN) are executed.
If the result of the Boolean expression is TRUE, then one or more
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
commands associated witht the else keyword, namely (e1, e2, ..., eN),
are executed. The elif and else keywords and their associated
commands are optional. If they aren't specified and the initial
Boolean expression is FALSE, then no commands are executed.
Each then or else command (t1, e1, etc) can be any valid LAMMPS input
script command. Each command should be enclosed in quotes, so it will
be treated as a single argument, as in the examples above.
The syntax for Boolean expressions is described below.
Each command (t1, f1, e1, etc) can be any valid LAMMPS input script
command. If the command is more than one word, it must enclosed in
quotes, so it will be treated as a single argument, as in the examples
above.
IMPORTANT NOTE: If a command itself requires a quoted argument (e.g. a
"print"_print.html command), then double and single quotes can be used
and nested in the usual manner, as in the examples above and below.
See "this section"_Section_commands.html#3_2 of the manual for more
details on using quotes in arguments. Only one of level of nesting is
allowed, but that should be sufficient for most use cases.
Note that by using the line continuation character "&", the if command
can be spread across many lines, though it is still a single
command:
can be spread across many lines, though it is still a single command:
if $a < $b then &
"print Minimum value = $a" &
if "$a < $b" then &
"print 'Minimum value = $a'" &
"run 1000" &
else &
"print Minimum value = $b" &
'print "Minimum value = $b"' &
"minimize 0.001 0.001 1000 10000" :pre
Note that if any executed comand is a bogus LAMMPS command, such as
"exit" in the first example above, then executing the command will
cause LAMMPS to halt.
Note that if one of the commands to execute is an invalid LAMMPS
command, such as "exit" in the first example above, then executing the
command will cause LAMMPS to halt.
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 "variable
delete"_variable.html for info on how to delete the associated loop
variable, so that it can be re-used later in the input script.
delete"_variable.html command for info on how to delete the associated
loop variable, so that it can be re-used later in the input script.
Here is an example of a double loop which uses the if and
"jump"_jump.html commands to break out of the inner loop when a
@ -79,7 +96,7 @@ variable a loop 5
variable b loop 5
print "A,B = $a,$b"
run 10000
if $b > 2 then "jump in.script break"
if '$b > 2' then "print 'Jumping to another script'" "jump in.script break"
next b
jump in.script loopb
label break
@ -87,6 +104,52 @@ variable b delete :pre
next a
jump in.script loopa :pre
:line
The Boolean expressions for the if and elif keywords have a C-like
syntax. Note that each expression is a single argument within the if
command. Thus if you want to include spaces in the expression for
clarity, you must enclose the entire expression in quotes.
An expression is built out of numbers
0.2, 100, 1.0e20, -15.4, etc :pre
and Boolean operators:
A == B
A != B
A < B
A <= B
A > B
A >= B
A && B
A || B :pre
Each A and B is a number or a variable reference like $a or $\{abc\},
or another Boolean expression.
If a variable is used it must produce a number when evaluated and
substituted for in the expression.
Expressions are evaluated left to right and have the usual C-style
precedence: the 4 relational operators "<", "<=", ">", and ">=" have
the highest precedence; those 4 relational operators before the
remaining two relational operators "==" and "!="; those two relational
operators before the logical AND operator "&&"; and the AND operator
"&&" before the logical OR operator "||". Parenthesis can be used to
group one or more portions of an expression and/or enforce a different
order of evaluation than what would occur with the default precedence.
The 6 relational operators return either a 1.0 or 0.0 depending on
whether the relationship between x and y is true or false. The
logical AND operator will return 1.0 if both its arguments are
non-zero, else return a 0.0. The logical OR operator will return 1.0
if either of its arguments is non-zero, else return a 0.0.
The overall Boolean expression produces a TRUE result if the result is
non-zero. If the result is zero, the expression result is FALSE.
[Restrictions:] none
[Related commands:]