Extract elements from a formatted list at given position(s).

%let res=%clist_index(clist, index, mark=%str(%"), sep=%quote(,));

clist : a formatted (e.g., parentheses-enclosed, comma-separated quote-enhanced) list;
index : a list of numeric indexes providing with the positions of items to extract from list; must that the values of items in index should be < length of the list and >0;
mark, sep : (option) characters/strings used respectively as a "quote" and a separator in the input list; default: mark=str(%"), and" sep=%quote(,), i.e. clist is a comma-separated list of quote-enhanced items; see %clist_unquote for further details.

res : output list defined as the sequence of elements extract from the input list list so that:

the i-th element in res is equal to the j-th element of list where the position j is given by the i-th element of index.

%let index = 3 5 2 100 4 1;
%let list=("a","bb","ccc","dddd","bb","fffff");
%let res=%list_index(&list, &index);

returns: res=("ccc","bb","bb","dddd","a") since the index 100 is ignored.

Run macro %_example_clist_index for more examples.

Indexes larger than the length of the input list clist are ignored, while whenever one index is <0, and error is generated.
For wrongly typed indexes (e.g. non numeric index), an error is also generated.
In general case of error, the output res returned is empty (i.e. res=).
The macro will not return exactly what you want if the symbol $ appears somewhere in the list. If you need to use $, you can reset the global macro variable G_PING_UNLIKELY_CHAR (see _setup_ file) to another dumb (unlikely) character of your own.