Question

Stuck on KVCs in Obj-C again.

I am wanting to use KVC to find the minimum value across multiple keys.

Consider the following array:

NSArray *data = [[NSArray alloc] initWithObjects:
                 [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithFloat:2.0], @"a", [NSNumber numberWithFloat:5.0], @"b", [NSNumber numberWithFloat:4.0], @"c", nil],
                 [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithFloat:3.0], @"a", [NSNumber numberWithFloat:1.0], @"b", [NSNumber numberWithFloat:1.5], @"c", nil],
                 [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithFloat:9.0], @"a", [NSNumber numberWithFloat:7.0], @"b", [NSNumber numberWithFloat:6.0], @"c", nil],
                 nil];

I can find the minimum value for ‘a’, ‘b’, or ‘c’ easily with:

float minKeyA = [[data valueForKeyPath:@"@min.a"] floatValue];  // value: 2.0
float minKeyB = [[data valueForKeyPath:@"@min.b"] floatValue];  // value: 1.0
float minKeyC = [[data valueForKeyPath:@"@min.c"] floatValue];  // value: 1.5

What I am wanting to achieve is to take a NSSet or NSArray of keys and find the minimum value across the pool of those keys.

NSSet *keySet1 = [NSSet setWithObjects:@"a", @"b", nil]; 
// use to find absolute minimum value across keys 'a' and 'b' --> desired value 1.0
NSSet *keySet2 = [NSSet setWithObjects:@"a", @"c", nil];
// use to find absolute minimum value across keys 'a' and 'c' --> desired value 1.5
NSSet *keySet3 = [NSSet setWithObjects:@"a", @"b", @"c", nil];
// use to find absolute minimum value across keys 'a', 'b', and 'c' --> desired value 1.0

Appreciate any pointers 🙂

Answer 1

A naive solution would be to first find the minimum value for each key and then find the minimum among those minimum values in a second step.

NSMutableSet *localMinima = [NSMutableSet setWithCapacity:[keySet1 count]];
for (NSString *key in keySet1) {
    NSString *keyPath = [NSString stringWithFormat:@"@min.%@", key];
    NSNumber *localMin = [data valueForKeyPath:keyPath];
    [localMinima addObject:localMin];
}
NSNumber *globalMin = [localMinima valueForKeyPath:@"@min.self"];