Algorithms – Fastest Way to Determine if a Value is in a List

The code in the question (a linear search), as you rightly point out, is going to get slow for large float arrays. Technically it's O(n) where n is the number of float values in your array.

In general, the best that you can do to find a value in an ordered array is a recursive tree search of some kind (e.g. binary search), in which case you can achieve an O(log n) lookup time in the number of elements in your array. O(log n) is much better than O(n) for large values of n.

My suggested approach would therefore be a simple binary search of the array, i.e.:

1. Set min/max integer indexes to cover your whole float array
2. test the value in the middle of the range at index mid=(min+max/2) against the search value x
3. if x is lower than this value, set max to mid, else set min to mid
4. repeat (2-4) until you have found the correct value

This is an O(log n) algorithm which should be fast enough for nearly all situations. Intuitively, it works by halving the range to be searched at each step until you find the correct value.

It's really hard to beast the simple binary search, so if you have already implemented this correctly then you may be pretty close to optimal already. However, if you know the distributions of the data and/or have a limited range of lookup values (x), there are still some other more advanced tricks you can try:

• Bucketing - create buckets (e.g. for each interval between two integers), each of which contains a smaller sorted list of the float values between the two bounding integers plus two values immediately below and immediately above each range. You can then start your search at (trunc(x)+0.5). This should give you a good speedup if you choose appropriately sized buckets (it's effectively increasing the branching factor of the tree.....). If integers don't work for you, then you can try buckets of some other fixed-point precision (e.g. multiples of 1/16).
• Bit-mapping - if the range of possible lookup values is small enough, you could try creating a big lookup table indexed by the bitwise value of x. This will be O(1) but you may need a lot of memory which will be very unfriendly on your cache... so use with caution. This is especially nasty because you are looking up float values, so you may well need several GBs to account for all of the less significant bits......
• Rounding and hashing - hash tables are probably not the best data structure for this problem, but if you can survive losing a bit of accuracy they could work - simply round off the lowest bits of your lookup values and use a hashmap to directly look up the correct value. You will have to experiment on the right trade-off between hashmap size and precision, and also ensure that all possible hash values are populated so this can be a bit tricky......
• Tree-balancing - your ideal tree should have a 50% chance of going left or right. So if you create a tree based on the distribution of lookup values (x), then you can optimise the tree to produce answers with the minimal amount of tests. This is likely to be a good solution if a lot of values in your float array are very close together, since it will enable you to avoid searching these branches too often.
• Crit-bit trees - these are still trees (so still O(log n)...) but some cases: you would however need to convert your floats into some fixed-point format in order to make the comparisons work

However, unless you are in a very special situation I'd probably recommend sticking with the simple binary search. Reasons:

• it's much easier to implement
• it's very fast for most common cases
• the extra overhead of the more complex approaches (e.g. higher memory usage / cache pressure) often outweighs the minor theoretical gains
• it will be more robust to future changes in the data distributions....

This calls for a straightforward binary search - only instead of returning failure when the bounds converge without a hit, you return the next lowest index. (Unless the total number of values is so small that it easily fits into memory - then a complete cache is the way to go.)