lammps/doc/if.txt

"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

if command :h3

[Syntax:]

if value1 operator value2 then T1 T2 ... else E1 E2 ... :pre

value1 = 1st value
operator = "<" or "<=" or ">" or ">=" or "==" or "!="
value2 = 2nd value
then = required word
T1,T2,...,TN = one or more commands to execute if condition is met
else = optional argument
E1,E2,...,EN = one or more commands to execute if condition is not met (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

[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.

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.

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.

Note that by using the line continuation character "&", the if command
can be spread across many lines, though it is still a single
command:

if $a < $b then &
  "print Minimum value = $a"
  "run 1000" &
else &      
  "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 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.

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
condition is met, then continues iterating thru the outer loop.

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
next	    a
jump	    in.script loopa :pre

[Restrictions:] none

[Related commands:]

"variable"_variable.html, "print"_print.html

[Default:] none