I can test for the presence of a key in an NSDictionary in two ways:
BOOL containsKey = [[dictionary allKeys] containsObject:foo]; BOOL containsKey = ([dictionary objectForKey:foo] != nil);
which method is faster, and why?
A hash lookup should be faster in general than going over all the dictionary keys, creating an array from them (memory allocation is relatively expensive) and then searching the array (which can't even be a binary search since the array is not sorted).
For the sake of science, though, I made two executables that just execute each style 1 million times and timed them.
With allKeys:
real 0m4.185s user 0m3.890s sys 0m0.252s
With objectForKey:
real 0m0.396s user 0m0.189s sys 0m0.029s
Obviously, various factors can influence this — size of the dictionary, caching the allKeys return value, etc. I wouldn't expect there to be a case in which the array search is faster than the dictionary lookup, though.
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