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Nesting a hash within itself


Lists and hashes have always been one of my favorite tools in IDL due to their versatile nature. A cool trick is that a hash can be stored recursively within itself. For example:


h = hash()
h['myself'] = h


With only level, this may not be particularly useful. However, this trick can be powerful for creating loops of information. Take days of the week, for instance. For fun, I'll use a DICTIONARY instead of a traditional hash, which has the convenience of the dot syntax for access.


Monday = dictionary('today', 'Monday')
Tuesday = dictionary('today', 'Tuesday')
Wednesday = dictionary('today', 'Wednesday')
Thursday = dictionary('today', 'Thursday')
Friday = dictionary('today', 'Friday')
Saturday = dictionary('today', 'Saturday')
Sunday = dictionary('today', 'Sunday')


Now define each day's tomorrow:



Monday.tomorrow = Tuesday
Tuesday.tomorrow = Wednesday
Wednesday.tomorrow = Thursday
Thursday.tomorrow = Friday
Friday.tomorrow = Saturday
Saturday.tomorrow = Sunday
Sunday.tomorrow = Monday



We can now loop through these days indefinitely. If we take Monday and request "tomorrow" seven times, we get Monday back again:


Next_Monday = Monday.tomorrow.tomorrow.tomorrow.tomorrow.tomorrow.tomorrow.tomorrow


We can store additional information in each of these hashes/dictionaries, such as a schedule for each day. 


Monday.Morning = 'Math 251, 9:00-10:00, Room 304'
Tuesday.Morning = 'Writing 301, 10:00-11:30, Room 211'


If today = Monday, then we can get today's morning schedule calling today.Morning. At the end of the day on Monday, we can say today = today.tomorrow, and tomorrow is the new today. Likewise, if we want today's morning schedule, we call today.Morning

Loops, routines, and cycles, are very common in the world as well as in scientific data. This method of information storage can be used for displaying radar data loops, with each frame stored in a hash that contains a "previous" and "next" frame. Additionally, it could be used for modeling states of pendulum or circular motion. It can be used for storing geographic information, such as time zones containing UTC offsets. There are probably many additional use cases.

As always, use caution with recursion. It is always unfortunate to find yourself killing code because it is stuck in an infinite loop. That said, be aware that JSON_SERIALIZE will not work in the examples above. Fortunately, given this use-case, there is no significant need to serialize the hashes' information into a string.