NSArray -> Find closest value - fastest way - objective-c

Question

Lets say I've got an ordered NSArray of NSNumbers:
2, 4, 8, 15, 16, 20 // for simplicity let's treat it as array of int's instead of NSNumbers
Now I need to find closest index to let's say value == 19.
searchValue = 19;
minIndex = 0;
maxIndex = array.count - 1;
currentIndex = (int)floorf(maxIndex / 2.f);
while (maxIndex - minIndex == 1) {
if (array[currentIndex] < searchValue) { // go right
minIndex = currentIndex;
} else if (array[currentIndex] > searchValue) { // go left
maxIndex = currentIndex;
} else { // exact value, rather low probability of happening
return currentIndex;
}
currentIndex = (int)floorf((maxIndex - minIndex) / 2.f);
}
// let's check values around, who has smaller difference
int leftDifference = (currentIndex - 1 >= 0) ? abs(array[currentIndex - 1] - searchValue) : INT_MAX;
int rightDifference = (currentIndex + 1 < array.count) ? abs(array[currentIndex + 1] - searchValue) : INT_MAX;
int centralDifference = abs(array[currentIndex] - searchValue);
if (leftDifference < rightDifference && leftDifference < centralDifference) {
return currentIndex - 1;
} else if () {
return currentIndex + 1;
} else {
return currentIndex;
}
This is the fastest way I can imagine, maybe someone has different idea? How can I improve the algorithm?
I've took a look into egSOF question, but it search for value not index and does it by browsing all values. In case of index, we don't have to browse full array.

Answer 1

Lets assume you have an array of NSNumbers:
NSArray *array = #[#(2), #(4), #(8), #(15), #(16), #(20)];
And you are looking for myValue as below:
NSNumber *myValue = #(17);
Use indexOfObject:inSortedRange:options:usingComparator method to find the nearest index of array to you value. Binary search has O(log n) performance so is pretty fast.
NSInteger searchIndex = MIN([array indexOfObject: myValue inSortedRange:NSMakeRange(0, array.count)
options:NSBinarySearchingFirstEqual | NSBinarySearchingInsertionIndex
usingComparator:^NSComparisonResult(NSNumber *obj1, NSNumber *obj2) {
return [obj1 compare:obj2];
}], [array count] - 1);
Then check if exists a number closest to yours myValue on the searchIndex - 1 index:
if (searchIndex > 0) {
CGFloat leftHandDiff = ABS(((NSNumber *)array[searchIndex - 1]).floatValue - myValue.floatValue);
CGFloat rightHandDiff = ABS(((NSNumber *)array[searchIndex]).floatValue - myValue.floatValue);
if (leftHandDiff == rightHandDiff) {
//here you can add behaviour when your value is in the middle of range
NSLog(#"given value is in the middle");
} else if (leftHandDiff < rightHandDiff) {
searchIndex--;
}
}
NSLog(#"The nearest value to %f is %d in array at index %d", myValue.floatValue, array[searchIndex], searchIndex);
and voila! Now you now the closest value to myValue.
Remember that your array has to be sorted ascending to make this trick.