Find the largest number from NSArray that is smaller than the given number

问题

Suppose I have a sorted array of integers int[], and I want to search the closest smaller value to some input number.

for example if the array contains 1, 23, 57, 59, 120 and the input is 109, the output should be 59. I have tried this but it is not working out as needed.

NSTimeInterval currentTime = self.player.currentTime;
NSInteger playerTime=currentTime;
NSUInteger index = [_timesArray indexOfObject:@(playerTime)
                                inSortedRange:NSMakeRange(0, _timesArray.count-1)
                                      options:NSBinarySearchingFirstEqual | NSBinarySearchingInsertionIndex
                              usingComparator:^(id a, id b) {
                                  return [a compare:b];
                              }];

回答1:

Your code does work as expected with the example data you gave:

NSArray *_timesArray = @[@1, @23, @57, @59, @120];
NSTimeInterval currentTime = 109;
NSInteger playerTime=currentTime;
NSUInteger index = [_timesArray indexOfObject:@(playerTime)
                                inSortedRange:NSMakeRange(0, _timesArray.count-1)
                                      options:NSBinarySearchingFirstEqual | NSBinarySearchingInsertionIndex
                              usingComparator:^(id a, id b) {
                                  return [a compare:b];
                              }];
NSLog(@"Index: %lu", (unsigned long)index);

Output:

Index: 4

Check your inputs. Likely they are not what you think they are.

回答2:

As I said before: "you have sorted array and what you need is to step downward from biggest number to smallest... algorithm is simple here!"

    NSNumber* givenNumber = @(105);
    for (NSNumber* item in [@[@(1), @(23), @(57), @(59), @(120)] reverseObjectEnumerator]){
        // go downward
        if (item.integerValue < givenNumber.integerValue)
        {
            NSLog(@"%@ < %@",item,givenNumber);
            // save this number somewhere
            break;
        }
    }

回答3:

It can be solved by binary_search_like algorithm which runs in O(lg n) , but I don't know objective-c so I'll give code in c++.

I assume the given element is not in the array.

int binary_search_like(int s[], int key, int low, int high)
{
    int middle; 
    if (low > high) return (s[high]); 

    middle = (low + high) / 2;

    if (s[middle] > key)
        return(binary_search_like(s, key, low, middle - 1));
    else
        return(binary_search_like(s, key, middle + 1, high));
}

int Search(int s[], int key, int size){

    if (key > s[size - 1]) return s[size - 1];
    // Check if key < smallest element , give error !

    return binary_search_like(s, key, 0, size);

 }

来源：https://stackoverflow.com/questions/25238072/find-the-largest-number-from-nsarray-that-is-smaller-than-the-given-number

标签

objective-c

sorting

nsarray