if

The if command starts a block of commands that James reads only when its test is true.

Syntax

if ( <expression> ) then
    ...
[ elseif ( <expression> ) then
    ... ]
[ else
    ... ]
endif

expression compares parameters, for example %n.eq.2. The word then is required. if and its elseif, else and endif each stand alone on a line.

What it does

James tests the expression when the command parser reaches the if. If it is true, James reads the commands that follow as if they stood there unconditionally. If it is false, James skips them and tests the next elseif, or takes the else. Only one branch of a construct is ever taken. The commands of a branch that is not taken are not read at all: a para or an include there never takes effect.

When James applies it

By the command parser, when it is read. The branch not taken is skipped. It is legal in any phase: Control, Part or Merge.

Rules James adds

The command parser evaluates an if as it reaches it, in script order, and not before. This is why a para in a false branch does not run, and why %nextsrf and other values a command sets are available to a guard that comes after that command.
A construct holds at most 64 open if levels. James reports the 65th and treats it as not opened.
An if must close in the file that opened it. An if opened in an included file and closed in the file that included it is an error in both files.
An if never sits inside a when, for or while, or holds one, and a control statement shares its line with nothing else.
A guard is evaluated once, so a rand or norm in it advances the generator the same way whichever keyword it sits on.

Diagnostics

E0306: the if is inside, or encloses, a when, for or while scope.
E0309: the keyword shares its line with other input.
E0317: the then is missing.
E0319: the if is not closed by an endif in the same file.
E0320: more than 64 if constructs are open.
E0203: a guard names a parameter that has no value yet.
E0221: a guard is not a well-formed expression, for example one that holds a [ ].

Example

The script tests/corpus/132-if-false-branch-inert/input.tg, which make test runs:

if(1.eq.2)then
para x 1;
include missing.inc
sd 9 sp 0 0 0 1
endif
sd [%nextsrf] sp 0 0 0 1

The test 1.eq.2 is false, so James skips lines 2 to 4. The para, the include of a file that does not exist and the sd 9 all do nothing, and James raises no error for the missing file. The last line takes the surface number %nextsrf, which is still 1, so the IR holds one surface with id 1.

if