假设我有一个 NSNumbers
的有序 NSArray
:
2, 4, 8, 15, 16, 20 // for simplicity let's treat it as array of int's instead of NSNumbers
现在我需要找到最接近的索引让我们说 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;
}
这是我能想到的最快的方式,也许有人有不同的想法?如何改进算法?
我查看了例如 SOF question ,但它搜索值而不是索引并通过浏览所有值来实现。在索引的情况下,我们不必浏览完整数组。
最佳答案
假设你有一个 NSNumbers 数组:
NSArray *array = @[@(2), @(4), @(8), @(15), @(16), @(20)];
您正在寻找如下所示的 myValue
:
NSNumber *myValue = @(17);
使用 indexOfObject:inSortedRange:options:usingComparator
方法找到最接近您值的数组索引。二分搜索具有 O(log n) 性能,因此非常快。
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);
然后检查 searchIndex - 1
索引中是否存在与您的 myValue
最接近的数字:
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);
瞧!现在您的值最接近 myValue
。
记住您的数组
必须按升序排序才能实现此技巧。
关于objective-c - NSArray -> 找到最接近的值 - 最快的方法,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/25864880/