Searching a NSArray for the nearest number(s)

Question

Is there an easy way to search an NSArray of numbers to find the nearest (or exact if it exists) matches to a user-input number?

Say I have an array like this: 7, 23, 4, 11, 18, 2, and the user enters 5.

The program returns the three nearest values in descending order of closeness: 4, 7, 2, and most importantly gives the NSArray index of the three objects: 2, 0, 5.

Answer 1

Update: see below for a better solution than my first one.

Here's a solution using NSDictionary wrappers for each number and its index, with sorting using a comparator block. It probably doesn't scale very well, but it gets the job done.

static NSString *const kValueKey = @"value";
static NSString *const kIndexKey = @"index";

+ (void)searchArray:(NSArray *)array forClosestValuesTo:(int)value resultValues:(NSArray **)values resultIndexes:(NSArray **)indexes
{
    NSMutableArray *searchObjs = [NSMutableArray arrayWithCapacity:[array count]];

    [array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        [searchObjs addObject:[NSDictionary dictionaryWithObjectsAndKeys:obj, kValueKey, [NSNumber numberWithUnsignedInt:idx], kIndexKey, nil]];
    }];

    [searchObjs sortUsingComparator:^NSComparisonResult(id obj1, id obj2) {
        NSUInteger d1 = ABS([[obj1 objectForKey:kValueKey] intValue] - value);
        NSUInteger d2 = ABS([[obj2 objectForKey:kValueKey] intValue] - value);
        if (d1 == d2) { return NSOrderedSame; }
        if (d1 <  d2) { return NSOrderedAscending; }
        return NSOrderedDescending;
    }];

    NSArray *results = [searchObjs subarrayWithRange:NSMakeRange(0, 3)];

    if (values) {
        *values = [results valueForKey:kValueKey];
    }

    if (indexes) {
        *indexes = [results valueForKey:kIndexKey];
    }
}

Update: here's an updated solution that sorts a C array of indexes, eliminating the need for NSDictionary wrappers

static NSString *const kValueKey = @"value";
static NSString *const kArrayKey = @"array";

int
CSCompareIndexes(void *data, const void *value1, const void *value2)
{
    NSDictionary *dict = (NSDictionary *)data;

    NSArray *array = [dict objectForKey:kArrayKey];
    int valueToFind = [[dict objectForKey:kValueKey] intValue];

    int index1 = *(int *)value1;
    int index2 = *(int *)value2;

    NSNumber *num1 = [array objectAtIndex:index1];
    NSNumber *num2 = [array objectAtIndex:index2];

    return ABS([num1 intValue] - valueToFind) - ABS([num2 intValue] - valueToFind);
}

void
CSSearchNumberArray(NSArray *array, int valueToFind, NSArray **resultValues, NSArray **resultIndexes)
{
    NSInteger numValues = [array count];

    NSUInteger *indexes = malloc(sizeof(NSUInteger) * numValues);
    assert(indexes);

    int i;
    for (i = 0; i < numValues; i++) {
        indexes[i] = i;
    }

    NSDictionary *data = [NSDictionary dictionaryWithObjectsAndKeys:array, kArrayKey, [NSNumber numberWithInt:valueToFind], kValueKey, nil];
    qsort_r(indexes, numValues, sizeof(NSUInteger), (void *)data, CSCompareIndexes);

    NSMutableArray *tmpValues  = [NSMutableArray arrayWithCapacity:3],
                   *tmpIndexes = [NSMutableArray arrayWithCapacity:3];

    for (i = 0; i < 3; i++) {
        [tmpValues addObject:[array objectAtIndex:indexes[i]]];
        [tmpIndexes addObject:[NSNumber numberWithInt:indexes[i]]];
    }

    if (resultValues) {
        *resultValues = [NSArray arrayWithArray:tmpValues];
    }

    if (resultIndexes) {
        *resultIndexes = [NSArray arrayWithArray:tmpIndexes];
    }

    free(indexes);
}

int main (int argc, char *argv[])
{
    NSAutoreleasePool *pool = [NSAutoreleasePool new];

    NSMutableArray *test = [NSMutableArray array];

    int i;
    for (i = 0; i < 10; i++) {
        [test addObject:[NSNumber numberWithInt:(arc4random() % 100)]];
    }

    NSLog(@"Searching: %@", test);

    NSArray *values, *indexes;
    CSSearchNumberArray(test, 50, &values, &indexes);

    NSLog(@"Values: %@", values);
    NSLog(@"Indexes: %@", indexes);

    [pool drain];
    return 0;
}