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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 71 |
1 files changed, 71 insertions, 0 deletions
@@ -28797,6 +28797,77 @@ according to the modulo 3 congruence: -> #H(() (0 (0 3 6 9)) (1 (1 4 7 10)) (2 (2 5 8))) .cble +.coNP Function @ group-reduce +.synb +.mets (group-reduce < hash < classify-fun < binary-fun < seq +.mets \ \ >> [ init-value <> [ filter-fun ]]) +.syne +.desc +The +.code group-reduce +updates hash table +.meta hash +by grouping and reducing sequence +.metn seq . + +The function regards the hash table as being populated with +keys denoting accumulator values. Missing accumulators which +need to be created in the hash table are initialized with +.meta init-value +which defaults to +.codn nil . + +The function iterates over +.meta seq +and treats each element according to the following steps: +.RS +.IP 1. +Each element is mapped to a hash key through +.metn classify-fun . +.IP 2. +The value associated with the hash key (the accumulator for that +key) is retrieved. If it doesn't exist, +.meta init-value +is used. +.IP 3. +The function +.meta binary-fun +is invoked with two arguments: the accumulator from step 2, and the +original element from +.metn seq . +.IP 4. +The resulting value is stored back into the hash table under the +same key. +.RE + +.IP +If +.code group-reduce +is invoked on an empty hash table, its net result closely resembles a +.code group-by +operation followed by separately performing a +.code reduce-left +on each value in the hash. + +.TP* Examples: + +Frequency histogram: + +.cblk + [group-reduce (hash) identity (do inc @1) + "fourscoreandsevenyearsago" 0] + --> #H(() (#\ea 3) (#\ec 1) (#\ed 1) (#\ee 4) (#\ef 1) + (#\eg 1) (#\en 2) (#\eo 3) (#\er 3) (#\es 3) + (#\eu 1) (#\ev 1) (#\ey 1)) +.cble + +Separate the integers 1-10 into even and odd, and sum these groups: + +.cblk + [group-reduce (hash) evenp + (range 1 10) 0] + -> #H(() (t 30) (nil 25)) +.cble + .coNP Functions @ make-similar-hash and @ copy-hash .synb .mets (make-similar-hash << hash ) |