I recently posted a technique for flattening a list of nested sublists. It appeared at the end of a long Code Exchange article that had begun as a discussion of list flattening but then devoted a large amount of its discussion to an alternative topic. I thought that the newly posted technique was useful enough that it should perhaps be “flattened” and posted at the “top level” of Code Exchange.
In the prior post, I reviewed a few previously described list-flattening solutions. One consists of a text-item-delimiters technique that is applicable to the case where all list and sublist items are text strings. Another technique uses recursive handler calls that is broadly applicable but runs the risk of a stack overflow error if a deeply nested list requires too many recursive handler calls. A third method uses ASObjC’s NSArray and its unionOfArrays operator that is applicable to the very specific case where all list items are themselves lists, and nesting is only one level deep.
The current technique will flatten a list of nested sublists of items of any class and nesting depth, and it does not entail the risk of a stack overflow error. It leverages the fact that AppleScript’s list concatenation operator effectively raises a sublist by one nesting depth. For example, when the sublist {1, 2, {3, 4, {5, 6}}} is concatenated to another list with the command: set newList to newList & {1, 2, {3, 4, {5, 6}}}, the new list becomes {…, 1, 2, {3, 4, {5, 6}}, …}, with the sublist’s top-level items now flattened, and lower-level items raised by one nesting depth. By repeatedly applying list concatenation to the list’s items, the list becomes progressively flattened until it reaches the point where all of its items are at the top level, and no nested sublists remain. As mentioned before, lists are processed as script properties in the repeat loops to enhance execution speed.
Handler:
on flattenList(theList)
script util
property flattenedList : theList
property interimFlattenedList : missing value
end script
repeat while util's flattenedList's lists's length > 0
set util's interimFlattenedList to {}
repeat with currItem in util's flattenedList
tell currItem's contents
if its class = list then
set util's interimFlattenedList to util's interimFlattenedList & it
else
set end of util's interimFlattenedList to it
end if
end tell
end repeat
set util's flattenedList to util's interimFlattenedList
end repeat
return util's flattenedList as list -- "as list" assures that the return value is dereferenced
end flattenList
Examples:
set theList to {1, 2, {{3, {4, 5}, 6}, 7}, 8}
flattenList(theList)
--> {1, 2, 3, 4, 5, 6, 7, 8}
set deeplyNestedList to {1, {2, {3, {4, {5, {6, {7, {8, {9, {10, {11, {12, {13, {14, {15, {16, {17, {18, {19, {20, {21, {22, {23, {24, {25, {26, {27, {28, {29, {30, {31, {32, {33, {34, {35, {36, {37, {38, {39, {40, {41, {42, {43, {44, {45, {46, {47, {48, {49, {50, {51, {52, {53, {54, {55, {56, {57, {58, {59, {60, {61, {62, {63, {64, {65, {66, {67, {68, {69, {70, {71, {72, {73, {74, {75, {76, {77, {78, {79, {80, {81, {82, {83, {84, {85, {86, {87, {88, {89, {90, {91, {92, {93, {94, {95, {96, {97, {98, {99, {100, {101, {102, {103, {104, {105, {106, {107, {108, {109, {110, {111, {112, {113, {114, {115, {116, {117, {118, {119, {120, {121, {122, {123, {124, {125, {126, {127, {128, {129, {130, {131, {132, {133, {134, {135, {136, {137, {138, {139, {140, {141, {142, {143, {144, {145, {146, {147, {148, {149, {150, {151, {152, {153, {154, {155, {156, {157, {158, {159, {160, {161, {162, {163, {164, {165, {166, {167, {168, {169, {170, {171, {172, {173, {174, {175, {176, {177, {178, {179, {180, {181, {182, {183, {184, {185, {186, {187, {188, {189, {190, {191, {192, {193, {194, {195, {196, {197, {198, {199, {200, {201, {202, {203, {204, {205, {206, {207, {208, {209, {210, {211, {212, {213, {214, {215, {216, {217, {218, {219, {220, {221, {222, {223, {224, {225, {226, {227, {228, {229, {230, {231, {232, {233, {234, {235, {236, {237, {238, {239, {240, {241, {242, {243, {244, {245, {246, {247, {248, {249, {250, {251, {252, {253, {254, {255, {256}, 257}}, 258}}, 259}}, 260}}, 261}}, 262}}, 263}}, 264}}, 265}}, 266}}, 267}}, 268}}, 269}}, 270}}, 271}}, 272}}, 273}}, 274}}, 275}}, 276}}, 277}}, 278}}, 279}}, 280}}, 281}}, 282}}, 283}}, 284}}, 285}}, 286}}, 287}}, 288}}, 289}}, 290}}, 291}}, 292}}, 293}}, 294}}, 295}}, 296}}, 297}}, 298}}, 299}}, 300}}, 301}}, 302}}, 303}}, 304}}, 305}}, 306}}, 307}}, 308}}, 309}}, 310}}, 311}}, 312}}, 313}}, 314}}, 315}}, 316}}, 317}}, 318}}, 319}}, 320}}, 321}}, 322}}, 323}}, 324}}, 325}}, 326}}, 327}}, 328}}, 329}}, 330}}, 331}}, 332}}, 333}}, 334}}, 335}}, 336}}, 337}}, 338}}, 339}}, 340}}, 341}}, 342}}, 343}}, 344}}, 345}}, 346}}, 347}}, 348}}, 349}}, 350}}, 351}}, 352}}, 353}}, 354}}, 355}}, 356}}, 357}}, 358}}, 359}}, 360}}, 361}}, 362}}, 363}}, 364}}, 365}}, 366}}, 367}}, 368}}, 369}}, 370}}, 371}}, 372}}, 373}}, 374}}, 375}}, 376}}, 377}}, 378}}, 379}}, 380}}, 381}}, 382}}, 383}}, 384}
flattenList(deeplyNestedList)
--> {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384}
Of note, the second example involving a very deeply nested list caused a stack overflow error with two different recursive-handler-call solutions I tried, but runs without problems with the current handler.
And you’d have to try pretty hard to get lists nested that deep in the first place!