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### if command

Syntax:

    if value1 operator value2 then t1 t2 ... else e1 e2 ...

- value1 = 1st value
- operator = "<" or "<=" or ">" or ">=" or "==" or "!="
- value2 = 2nd value
- then = required word
- t1,t2,...,tN = one or more LAMMPS commands to execute if condition is met, each enclosed in quotes
- else = optional argument
- e1,e2,...,eN = one or more LAMMPS commands to execute if condition is not met, each enclosed in quotes (optional arguments)

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"

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 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"

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 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 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

    next	    a
    jump	    in.script loopa

Restrictions: none

Related commands:

variable, print

Default: none