Given an input device (basically a keyboard) that reports keyup and keydown, how may I most efficiently store and retrieve information about which keys are currently depressed? My first thought was a dynamic array, but maintaining and searching a dynamic array seems like a serious drain on resources.

Many thanks in advance for any help


  • Why would that be a major drain on resources?
    – John
    Feb 17, 2013 at 14:56
  • You can use bit flags easily enough for this and it would be very fast to determine what buttons are depressed.
    – Akira71
    Feb 17, 2013 at 14:57
  • sorry, bit flags?
    – jamesson
    Feb 17, 2013 at 14:59
  • john: well, an array of undefined length + searching it every time to see if a particular button is depressed seems like a good way to waste resources and slow down the program. Maybe I'm not understanding something correctly (always a possibility :)).
    – jamesson
    Feb 17, 2013 at 15:02
  • @jamesson Sorry, here is an interesting explanation for C++ but they exists for any major language. cplusplus.com/forum/general/1590 . I will try to write up a nice explanation as an answer if I can in a bit (or anyone else) but I think this is a great solution for you. I have used bit flags for enumerating input types on embedded devices a fair bit as they are fast and memory efficient.
    – Akira71
    Feb 17, 2013 at 15:04

1 Answer 1


A fixed-length array will work just fine, since the number of keys can't change. So, make an array, call it depressed, of length N, where N is the number of keys on your input device. When key X is depressed, set depressed[X] to True. When key X is released, set depressed[X] to False. To report the True/False state of any given key, you'll need a mapping from user-/API-level names, to indices into your array. e.g. MOTOR_REVERSE_TOGGLE -> position 6 in your array.

As for efficiency, an array should be adequate for space, unless you've got very tight memory restrictions on whatever device you're programming. In that case, you'd use a bitfield to store the data. Basically, it's the same as your array, except instead of words being set to True or False (which would end up a some magic numbers, say for example 11111111 and 00000000), you set individual bits in a word to 1 or 0.

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