increasing-number-problem.clj

(ns increasing-number-problem)

(defn increasing?
  "Returns if the digits of a number are increasing."
  [n]
  (->> (str n) ; Stringify the number
       (map int) ; Turn into list of character codes
       (reduce (fn [last current] ; Go over each pair of char codes
                 (if (> current last)
                   current
                   (reduced false)))) ; Exit early because a test failed
       (boolean))) ; Cast to bool for consistency of output

(defn all-increasing-digits
  "Returns all the increasing numbers from start to end (inclusive)."
  [start end]
  (->> (range start (inc end))
       (filter increasing?))) ; Keep only the numbers that are increasing

(all-increasing-digits 0 1000)
=>
(0
 1
 2
 3
 4
 5
 6
 7
 8
 9
 12
 13
 14
 15
 16
 17
 18
 19
 23
 24
 25
 26
 27
 28
 29
 34
 35
 36
 37
 38
 39
 45
 46
 47
 48
 49
 56
 57
 58
 59
 67
 68
 69
 78
 79
 89
 123
 124
 125
 126
 127
 128
 129
 134
 135
 136
 137
 138
 139
 145
 146
 147
 148
 149
 156
 157
 158
 159
 167
 168
 169
 178
 179
 189
 234
 235
 236
 237
 238
 239
 245
 246
 247
 248
 249
 256
 257
 258
 259
 267
 268
 269
 278
 279
 289
 345
 346
 347
 348
 349
 356
 357
 358
 359
 367
 368
 369
 378
 379
 389
 456
 457
 458
 459
 467
 468
 469
 478
 479
 489
 567
 568
 569
 578
 579
 589
 678
 679
 689
 789)