Iterators: Difference between revisions

=={{header|J}}==

J's operations are designed to be applied to the data structure as a whole, and this explicitly includes mapping between representations. Also, all data in J is array-like, and type is data. And this necessarily includes linked lists (though we can introduce arbitrarily complex mechanisms to obfuscate a linked list structure).

Still, one approach here might be:

<lang J>dow=: ;:'monday tuesday wednesday thursday friday saturday sunday'

col=: (,<)/;:'red orange yellow green blue purple'</lang>

This gives us:

<lang J>

dow

┌──────┬───────┬─────────┬────────┬──────┬────────┬──────┐

│monday│tuesday│wednesday│thursday│friday│saturday│sunday│

└──────┴───────┴─────────┴────────┴──────┴────────┴──────┘

col

┌───┬─────────────────────────────────────────┐

│red│┌──────┬────────────────────────────────┐│

│ ││orange│┌──────┬───────────────────────┐││

│ ││ ││yellow│┌─────┬───────────────┐│││

│ ││ ││ ││green│┌────┬────────┐││││

│ ││ ││ ││ ││blue│┌──────┐│││││

│ ││ ││ ││ ││ ││purple││││││

│ ││ ││ ││ ││ │└──────┘│││││

│ ││ ││ ││ │└────┴────────┘││││

│ ││ ││ │└─────┴───────────────┘│││

│ ││ │└──────┴───────────────────────┘││

│ │└──────┴────────────────────────────────┘│

└───┴─────────────────────────────────────────┘</lang>

Here, language's array indexing would see the linked list representation as a two element array. To index arbitrary elements from the linked list, we might map back from the linked list representation to a flat array representation, perhaps using <S:0 (which is a no-op on our array of days of week).

<lang J> echo ;:inv <S:0 dow

monday tuesday wednesday thursday friday saturday sunday

echo ;:inv <S:0 col

red orange yellow green blue purple

echo ;:inv 0 3 4 {<S:0 dow

monday thursday friday

echo ;:inv 0 3 4 {<S:0 col

red green blue

echo ;:inv _1 _3 _4 {<S:0 dow

sunday friday thursday

echo ;:inv _1 _3 _4 {<S:0 col

purple green yellow</lang>

The downside of this approach is that the programmer must understand the data.

However, given the intrinsic value of comprehension when programming, this is frequently a worthwhile price.