Question

How would I search my dictionary for the closest value to the query? Say I search for 0.000475, I know it’s not available but I want to return it’s closest value 0.000500.

I suppose I would use keysOfEntriesPassingTest to achieve this on a dictionary, clueless on how to go about it though.

<key>1/8000</key><real>0.000125</real> 
<key>1/6400</key><real>0.000156</real> 
<key>1/5000</key><real>0.000200</real> 
<key>1/4000</key><real>0.000250</real> 
<key>1/3200</key><real>0.000313</real> 
<key>1/2500</key><real>0.000400</real> 
<key>1/2000</key><real>0.000500</real> 
<key>1/1600</key><real>0.000625</real> 

Answer 1

Since keysOfEntriesPassingTest returns a set of values, it would be better suited to a task like finding all keys that are within a given epsilon range from your query value, but not finding the single closest key like in this case.

So here’s one way to do it, although it certainly is a bit inefficient:

NSDictionary* dict = [NSDictionary dictionaryWithObjects:[NSArray arrayWithObjects:[NSNumber numberWithFloat:1.0],
                                                          [NSNumber numberWithFloat:2.0],
                                                          [NSNumber numberWithFloat:3.0], nil]
                                                 forKeys:[NSArray arrayWithObjects:@"foo", @"bar", @"baz", nil]];    
float query = ...;
NSArray* matches = [dict keysSortedByValueUsingComparator:^(id obj1, id obj2) {
    NSNumber* x = [NSNumber numberWithFloat:fabs([obj1 floatValue] - query)];
    NSNumber* y = [NSNumber numberWithFloat:fabs([obj2 floatValue] - query)];
    return [x compare:y];
}];

[matches objectAtIndex:0] will then hold the key of the value with the best match, index 1 the key to the second-best match, etc.

edit: yap, if you don’t need the other matches, it’s almost certainly a better idea to just call enumerateKeysAndObjectsUsingBlock: in conjunction with a variable outside the passed block that holds the key/value of the closest match found so far and updates it if the current value is closer to the query value.