Find nearest date in string array - ios

Question

So, I've got an sorted NSArray that contains NSString object (downloaded from a server), with the format: yyyy-MM-dd.
It's pretty much like this:
NSArray <NSString *> *dates = #[#"2017-06-25",
#"2017-06-26",
#"2017-06-27",
#"2017-06-28",
#"2017-06-30",
#"2017-07-01",
#"2017-07-02",
#"2017-07-03"];
So, today is 2017-06-29, and it's not in the array. How do I get the next nearest one? In this sample is 06-30, but it might be 07-01 if 06-30 doesn't exist...
Update
So people are asking me about what I've attempted to do. So it's like this (not very effective, but work)
Find if today is in the array (if yes, return)
Loop dates:
2.1 Convert dateString to date
2.2 Compare if date is greater than today => return if YES
If not found in step#2, return last object in dates array.
Actual code:
NSDateFormatter *formatter = [NSDateFormatter new];
formatter.dateFormat = #"yyyy-MM-dd";
NSDate *today = [NSDate date];
NSUInteger index = [dates indexOfObject:[formatter stringFromDate:today]];
// Step 1
if (index == NSNotFound) {
// Step 2: Loop converted
NSInteger i = 0;
for (NSString *date in dates) {
// Step2.1: find the next nearest date's index
NSDate *convertedDate = [formmater dateFromString:date];
// Step2.2: Compare
if ([convertedDate intervalSinceDate:today] > 0) {
index = i;
break;
}
i++;
}
// Step 3: Still not found, index = last index
if (index == NSNotFound) index = i-1;
}
return dates[index];
This doesn't look so good because I might reload the dates array pretty much. Can I have a better solution?

Answer 1

Your algorithm is not bad, though your code doesn't appear to implement it (no sort?). If you'd like to improve it consider this:
First there is probably little point in doing a first scan to check for an exact match - that is potentially a linear search (implemented by indexOfObject:) through an unordered array, and if it fails you have to scan again for a close match, just do them at the same time.
Second there is no advantage in sorting, which is at best O(NlogN), as a linear search, O(N), will find you the answer you need.
Here is a sketch:
Convert the date you are searching for from NSString to NSDate, call it, say, target
Set bestMatch, an NSString to nil. Set bestDelta, an NSTimeInterval, to the maximum possible value DBL_MAX.
Iterate over your dates array:
3.1. Convert the string date to an NSDate, say date
3.2. Set delta to the difference between date and target
3.3. If delta is zero you have an exact match, return it
3.4. If delta is better than bestDelta, update bestDelta and bestMatch
After iteration bestMatch is the best match or nil if there wasn't one.
That is a single iteration, O(N), early return on exact match.
HTH

Answer 2

Please find the simplest solution for your problem. Updated solution based on sorting order!
We can use NSPredicate Block to solve.
static NSDateFormatter* formatter = nil;
static NSDate* today = nil;
// return an NSDate for a string given in yyyy-MM-dd
- (NSDate *)dateFromString:(NSString *)string {
if (formatter == nil) {
formatter = [NSDateFormatter new];
formatter.dateFormat = #"yyyy-MM-dd";
}
return [formatter dateFromString:string];
}
// Helps to return today date.
-(NSDate*) getTodayDate {
if (today == nil) {
today = [NSDate date];
}
return today;
}
// Helps to find nearest date from Array using Predicate
-(NSString*)findNearestDate:(NSArray*)dateArray {
today = nil;
NSPredicate *predicate = [NSPredicate predicateWithBlock:^BOOL(NSString *dateString, NSDictionary *bind){
// this is the important part, lets get things in NSDate form so we can use them.
NSDate *dob = [self dateFromString:dateString];
NSComparisonResult result = [[self getTodayDate] compare:dob];
if (result == NSOrderedSame || result == NSOrderedAscending) {
return true;
}
return false;
}];
// Apply the predicate block.
NSArray *futureDates = [dateArray filteredArrayUsingPredicate:predicate];
if ([futureDates count] > 0) {
// Sort the Array.
futureDates = [futureDates sortedArrayUsingSelector: #selector(compare:)];
return [futureDates objectAtIndex:0];
}
return nil;
}
NSArray <NSString *> *dates = #[#"2017-06-25",
#"2017-06-26",
#"2017-06-27",
#"2017-06-28",
#"2017-06-30",
#"2017-07-01",
#"2017-07-02",
#"2017-07-03"];
NSLog(#"Nearest Date: %#", [self findNearestDate:dates]);
Answer: Nearest Date: 2017-06-30

Answer 3

1. Input
So you have an array of NSString like this
// input
NSArray<NSString *> * words = #[#"2017-06-25",
#"2017-06-26",
#"2017-06-27",
#"2017-06-28",
#"2017-06-30",
#"2017-07-01",
#"2017-07-02",
#"2017-07-03"];
2. Converting the array of NSString into an array of NSDate
First of all you need to convert the each input string into an NSDate
NSMutableArray<NSDate *> * dates = [NSMutableArray new];
NSDateFormatter * dateFormatter = [NSDateFormatter new];
dateFormatter.dateFormat = #"yyyy-MM-dd";
for (NSString * word in words) {
[dates addObject:[dateFormatter dateFromString:word]];
}
3. Finding the nearestDate
Now you can find the nearest date
NSDate * nearestDate = nil;
NSTimeInterval deltaForNearesttDate = 0;
NSDate * now = [NSDate new];
for (NSDate * date in dates) {
NSTimeInterval delta = fabs([date timeIntervalSinceDate:now]);
if (nearestDate == nil || (delta < deltaForNearesttDate)) {
deltaForNearesttDate = delta;
nearestDate = date;
}
}
4. Conclusion
The result is into the nearestDate variable so
NSLog(#"%#", nearestDate);
Wed Jun 28 00:00:00 2017