The IF statement is useful for MAIN programers, especially for loop control.
IF ( value operator value ) a main command sentence
Comparison operators are:
Values can be integer or real numbers and strings. You can only compare values of the same type.
Brackets are obligatory.
When the expression specified ( ... ) is evaluated as true, then the subsequent command sentence is interpreted, otherwise the command interpreter skips to the next input line.
To apply loops a loop counter variable should be defined and the IF statement can serve either to repeat the loop or exit from the command file:
> set variable iatom = iatom + 1 > if ( iatom .gt. natom ) return > set center atom iatom > show image center > rewind file