As well as selecting the type of the elements, a loop can specify extra parameters to filter and order the elements it iterates over. These parameters are specified by the where, orderby and unique keywords described below. The keywords may be used alone or together, but when used together they must come in this order, and will be applied in this order. Also note that where, orderby and unique clauses are run once for each element, before executing the actual loop.

The where keyword allows you to add a condition to select only certain elements. A simple form is:

foreach .objType; where :propName = 'value'
{ /* Operations for the elements */ }

Note the semicolon after the object type, to separate it from the where keyword. See the If clause for the full details of the syntax of conditions.

The orderby keyword allows you to specify the order in which to iterate over the selected elements. With no sort criteria specified, elements will be sorted by type, name, and oid. You can give multiple sort criteria per loop, e.g. sort first by name and then by type:

orderby id, type

Each criterion is a series of generator commands whose output will form the strings to be compared. The criteria are separated by commas as seen above.

You can add num after a criterion to specify numeric comparison:

orderby :count; num, :key; num

You can add desc to the end of a criterion to specify descending order, i.e. reverse the sort. The default order can be made explicit with asc:

orderby :count; num asc, :name; desc

The unique keyword allows you to specify one or more uniqueness criteria for loops to avoid handling the same kind of element multiple times. Each criterion is a series of generator commands whose output will form the strings which must be unique. For example, assume that you have a graph of Widgets. Each Widget has a property specifying which Library it is defined in, and that library must be included. If you want to output the include statements for the whole graph, without repeatedly including each library, you can do the following:

foreach .Widget; unique :Library
{ '#include <' :Library '>' newline }

Multiple criteria can be separated by commas, and an element must pass as unique on each criterion to be accepted. To allow all unique combinations to pass, simply place both within a single criterion.

If just unique is specified with no criteria, elements must be unique (i.e. not the same object etc., the same as comparing by oid). Foreach guarantees this kind of uniqueness anyway, but with do you could for instance be iterating over connected objects, and there could be two different relationships to the same object.