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361 nconc
nconc could have been defined by:
nconc Keyword For loop
nconc expr {into var}
(defun nconc (x y)
;for simplicity, this definition
(cond «null x) y)
;only works for 2 arguments.
(t (rplacd (last x) y) ;hook y onto x
x») ;and return the modified x.
I
Causes the values of expr on each iteration to be nconced together, for example:
(loop for i from 1 to 3
nconc (list i (* i i»)
=> (1 1 2 4 3 9)
When the epilogue of the loop is reached, var has been set to the accumulated
result and can be used by the epilogue code.
It is safe to reference the values in var during the loop, but they should not be
modified until the epilogue code for the loop is reached.
The forms nconc and nconcing are synonymous.
Examples:
(de fun indexing (small-list)
(loop for x from 0
for item in small-list
nconc (list x item») => INDEXING
(indexing '(a b cd» => (0 A 1 B 2 C 3 D)
Is equivalent to
(defun indexing (small-list)
(loop for x from 0
for item in small-list
nconcing (list x item») => INDEXING
(indexing '(a b cd» => (0 A 1 B 2 C 3 D)
Not only can there be mUltiple accumulations in a loop, but a single accumulation
can come from multiple places within the same loop form, if the types of the
collections are compatible. nconc, collect, and append are compatible.
See the section "loop Clauses", page 310.