diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 71 |
1 files changed, 50 insertions, 21 deletions
@@ -7107,39 +7107,64 @@ yields (1 2 3 4 5). In TXR Lisp, this usage can be simulated using .TP Syntax: - (reduce-left <binary-function> <list> <init-value> <key-function>) - (reduce-right <binary-function> <list> <init-value> <key-function>) + (reduce-left <binary-function> <list> : <init-value> <key-function>) + (reduce-right <binary-function> <list> : <init-value> <key-function>) .TP Description: -The reduce-left and reduce-right functions reduce lists of operands -specified in <list> to a single value by the repeated application of -<binary-function>. +The reduce-left and reduce-right functions reduce lists of operands specified +by <list> and <init-value> to a single value by the repeated application of +<binary-function>. -First, both functions initialize an internal accumulator with <init-value>. +An effective list of operands is formed by combining <list> and +<init-value>. If <key-function> is specified and not nil, then +the items of <list> are mapped to a new values through <key-function>. +If an <init-value> is supplied and not nil, then in the +case of reduce-left, the effective list of operands is formed by prepending +<init-value> to <lits>. In the case of reduce-right, the effective +operand list is produced by appending <init-value> to <list>. -Under reduce-left, the list is processed left to right. If elements -remain to be processed, the <binary-function> is repeatedly called with two -arguments: the accumulator and the next element from the list. After each call, -the return value of the function replaces the accumulator. When no more items -remain, the accumulator is returned. +The production of the effective list can be expressed like this, +though this is not to be understood as the actual impelmentation: -Under reduce-right, the list is processed right to left. If elements -remain to be processed, the <binary-function> is repeatedly called with two -arguments: the next element from the list and the accumulator. After each call, -the return value of the function replaces the accumulator. When no more items -remain, the accumulator is returned. + ;; reduce-left + (let ((eff-list (append (if init-value (list init-value)) + [mapcar (or key-function identity) list]))) -The <key-function> specifies how each element from the <list> is converted -to an argument to <binary-function>. The value nil is equivalent to -(fun identity), which means that each list element is taken as the value itself. +In the reduce-right case, the arguments to append are reversed. +If the effective list of operands is empty, then <binary-function> is called +with no arguments at all, and its value is returned. This is the only +case in which <binary-function> is called with no arguments; in all +remaining cases, it is called with two arguments. + +If the effective list contains one item, then that item is returned. + +Otherwise, the effective list contains two or more items, and is decimated as +follows. + +Under reduce-left, the leftmost pair of operands is removed +from the list and passed as arguments to <binary-function>, in the same order +that they appear in the list, and the resulting value initializes an +accumulator. Then, for each remaining item in the list, <binary-function> is +invoked on two arguments: the current accumulator value, and the next element +from the list. After each call, the accumulator is updated with the return +value of <binary-function>. The final value of the accumulator is returned. + +Under reduce-right, the list is processed right to left. The rightmost +pair of elements in the effetive list is removed, and passed as arguments to +<binary-function>, in the same order that they appear in the list. The +resulting value initializes an accumulator. Then, for each remaining item in +the list, <binary-function> is invoked on two arguments: the +next element from the list, in right to left order, and the current +accumulator value. After each call, the accumulator is updated with the return +value of <binary-function>. The final value of the accumulator is returned. .TP Examples: - ;;; list is empty, so 1 is just returned: + ;;; effective list is (1) so 1 is returned (reduce-left (fun +) () 1 nil) -> 1 ;;; computes (- (- (- 0 1) 2) 3) @@ -7149,7 +7174,11 @@ Examples: (reduce-right (fun -) '(1 2 3) 0 nil) -> 2 ;;; computes (* 1 2 3) - (reduce-left (fun *) '((1) (2) (3)) 1 (fun first)) -> 6 + (reduce-left (fun *) '((1) (2) (3)) nil (fun first)) -> 6 + + ;;; computes 1 because the effective list is empty + ;;; and so * is called with no arguments, which yields 1. + (reduce-left (fun *) nil) .SS Function some, all and none |