fold()

fold([<obj>/]<attr>, <list>[, <base case>[, <delimiter>]])

This function “folds” a list through the user-defined function, set in the specified <obj>/<attribute>.

If no <base case> is provided, fold() passes the first element of <list> as %0, and the second element of <list> as %1, to the user-defined function. The user-defined function is then called again, with the result of the first evaluation being %0, and the next (third) element of the list as %1. This is repeated until all the elements of the list have been used. The result of the last call of <obj>/<attr> is returned.

If a base case is provided, it is passed as %0, and the first element of list is passed as %1, to the user-defined function. The process for the no-base-case fold() is then used.

If <list> is empty, <attr> is never called: fold() returns the <base case> when one was given, and nothing when one wasn’t. Folding an empty list is the base case: it is the answer when there is nothing to combine into it, not merely a seed for a first call.

The number of times <attr> has been called is passed as %2, starting from 0.

Note that it’s not possible to pass a <delimiter> to fold without also giving a <base case>; see the examples for a way around this.

Fold examples

Examples:

&REP_NUM test=%0[repeat(%1,%1)]
say fold(test/rep_num,1 2 3 4 5)
You say, "122333444455555"
say fold(test/rep_num,1 2 3 4 5,List:)
You say, "List:122333444455555"

> &ADD_NUMS test=add(%0,%1)
> say fold(test/add_nums,1 2 3 4 5)
You say, "15"

If your list uses a delimiter, you need to give a <base case>. This can be a problem for dynamically generated lists. One solution is to use a register and pop the first element off the list. For example:

&GEN_LIST test=lnum(1,rand(5,10),|)
&ADD_NUMS test=add(%0,%1)
say letq(fl, u(gen_list), fold(test/add_nums, rest(%q<fl>,|), first(%q<fl>,|), |))
You say, "36"

::: seealso

anonymous attributes :::