How to search array for most recent date time - iphone

Question

I am trying to build an application that includes the on-call technicians name. I have an simple NSArray that contains 4 objects in the following format;
20130910;0800;John Doe
20130910;1400;Sally Smith
20130910;2000;Jim Jones
20130911;0800;Jane Johnson
The format above is date in yyyyMMdd, time in 2400 hour time, and the technicians name.
I have two stings *timeString and *dateString that have the local device's time and date in the same format as above.
I would like to search through the array looking for the most recent past due date/time to assign the technicians name to a new string.
Using the example above, if it is 1600 (4PM) on Sept 10 I am looking to get Sally Smith returned because she started her on-call at 1400 (2PM).

Answer 1

That should be pretty simple. Sort your array in alpha order, which will also be date/time/name order.
Then when you have a new date/time, do a binary search of your array, comparing your new date/time string to the date/time strings from the array. The binary search will give you the correct item in log2 comparisons at most, by my calculations.
Binary search: Have a max an min search range. Set it to the beginning/end of the array to start. Divide the array index by 2, and compare your string with the item at that index. If your string is > the string at that index, set min search range to the current index. If the string is < the string at that array index, set the max search range to the new index. If it equals, you have a match.
Repeat the above steps until you've found your item. (I'm too tired to spell out the exit conditions exactly. I'll leave that as an exercise for you.)

Answer 2

Depending on the size of list of technicians a loop would work. The code below iterates through the list, splits each item into three parts (date, time, technician), calculates the interval since now and determine which is the most recent/active agent (interval should be the largest negative).
To get something meaningful I changed the dates in the array.
NSArray *agents = [NSArray arrayWithObjects:
#"20130920;0800;John Doe",
#"20130920;1400;Sally Smith",
#"20130920;2000;Jim Jones",
#"20130921;0800;Jane Johnson",nil];
// Setup date formatter
NSDateFormatter* onCallFormatter = [[[NSDateFormatter alloc] init] autorelease];
[onCallFormatter setDateFormat:#"yyyyMMddHHmm"];
[onCallFormatter setTimeZone:[NSTimeZone timeZoneWithName:#"GMT"]];
NSTimeInterval mostRecent = -9999999999999;
NSInteger agentIndex;
int i;
for ( i=0; i < [agents count]; i++ ) {
// Split string into elements
NSArray *elements = [[agents objectAtIndex:i] componentsSeparatedByString:#";"];
// Convert date/time into NSDate
NSDate *onCallDateTime = [onCallFormatter dateFromString:[NSString stringWithFormat:#"%#%#", elements[0], elements[1]]];
// Calculate the time interval against current date/time
NSTimeInterval onCallInterval = [onCallDateTime timeIntervalSinceNow];
// The agent on call would be the one with the largest negative interval
// onCallInterval should be < 0 (postive would be in the future)
if ( mostRecent < onCallInterval && onCallInterval < 0) {
mostRecent = onCallInterval;
agentIndex = i;
}
NSLog( #"%# on call since %# - %# - %f hrs ", elements[2], elements[0], elements[1], onCallInterval/(60*60) );
}
NSLog( #"On call = %#", [agents objectAtIndex:agentIndex] );