Create NSMutableArray with NSDictionary elements slow - ios

Question

This chunk of code is a method that creates an array for use by multiple other classes. Input is an array from a CoreData fetch, of type NSDictionaryResultType.
3 of the fields are strings that I need to break into arrays, thus the componentsSeparatedByString.
The resulting array, _dataProductionArray, works great --- BUT --- this chunk of code takes a FULL 5 SECONDS to process for about 32,000 records.
Any help pointing out glaring mistakes that are causing this slow performance would be greatly appreciated!!
NSMutableArray *dataArray = [NSMutableArray array];
int j = 0;
int maxNumMonths = 0;
for (id obj in _dictionaries) {
if ([_dictionaries[j] [#"month"] length] >0 ) {
// get production values
NSArray *aItems = [_dictionaries[j] [#"prodA"] componentsSeparatedByString:#","];
NSArray *bItems = [_dictionaries[j] [#"prodB"] componentsSeparatedByString:#","];
NSArray *monthItems = [_dictionaries[j] [#"month"] componentsSeparatedByString:#","];
NSMutableArray *productionAArray = [NSMutableArray array];
NSMutableArray *productionBArray = [NSMutableArray array];
int monthLoop = 1;
for (NSNumber *month in monthItems) {
if (monthLoop <= MONTHS_OF_PRODUCTION) {
if ([month intValue] == monthLoop) {
[productionAArray addObject:[aItems objectAtIndex:monthLoop-1]];
[productionBArray addObject:[bItems objectAtIndex:monthLoop-1]];
productionCount ++;
if (monthLoop > maxNumMonths)
maxNumMonths = monthLoop;
}
}
monthLoop++;
}
NSDictionary *arrayItem = #{#"name":_dictionaries[j] [#"name"],
#"type":[NSString stringWithFormat:#"%#",_dictionaries[j] [#"type"]],
#"height":[NSString stringWithFormat:#"%#",_dictionaries[j] [#"height"]],
#"aArray":productionAArray,
#"bArray":productionBArray,
};
[dataArray addObject:arrayItem];
}
j++;
}
_dataProductionArray = [NSArray arrayWithArray:dataArray];

Answer 1

I can see a few optimizations you could do in the loop, but I'm not sure how much these would help (especially if the compiler is doing them anyway). The root problem is that 32k is a lot of iterations.
Do you need all 32k results at once? You could get a dramatic improvement in user experience by doing this work lazily, as the UI demands the transformed record.
This approach would be to make dataProductionArray a mutable dictionary, indexed by an NSNumber index. Then, instead of ...
// replace this
self.dataProductionArray[128];
// with this
[self dataProductionAtIndex:#128];
That new getter method calls the code you wrote lazily, like this ...
- (id)dataProductionAtIndex:(NSNumber *)index {
// replace dataProductionArray with dataProductionDictionary
id result = self.dataProductionDictionary[index];
if (!result) {
result = [self getDataAt:index];
self.dataProductionDictionary[index] = result;
}
return result;
}
Then getDataAt: is a simple refactor of the code you posted, except instead of looping 32k elements, it does the work for just one index that gets passed in....
- (id)getDataAt:(NSNumber *)index {
int j = [index intValue];
// no loop, just skip to iteration j
NSArray *aItems = [_dictionaries[j] [#"prodA"] componentsSeparatedByString:#","];
NSArray *bItems = [_dictionaries[j] [#"prodB"] componentsSeparatedByString:#","];
// and so on, then at the end, don't save arrayItem, just return it
NSDictionary *arrayItem = #{#"name":_dictionaries[j] [#"name"],
#"type":[NSString stringWithFormat:#"%#",_dictionaries[j] [#"type"]],
#"height":[NSString stringWithFormat:#"%#",_dictionaries[j] [#"height"]],
#"aArray":productionAArray,
#"bArray":productionBArray,
};
return arrayItem;
}
PS - A mutable dictionary is a good data structure for lazy evaluation. The next level of sophistication is NSCache, which acts like a mutable dictionary and also manages memory (class ref here).

Answer 2

Your for loop is daft. Just write
for (NSDictionary* dict in _dictionaries)...
and use dict instead of _dictionaries [j]. One method call saved each time.
stringWithFormat: creates a new string each time. Can't you just add the item itself instead of turning it into a string?
Instead of extracting all the items into productionAArray and productionBArray, create an NSIndexSet, fill it in the loop -- or better yet using a block -- and create the arrays in one go.