NSMutable Array High CPU Usage - ios

Question

I recently change c array to NSMutable array because I had a lot of troubles with memory. However it works, but starting from
/* If two starttimes are within interval millisec, make them the same */
for (int i = 0; i < [starttimes count] - 1; i++) {
if ([[starttimes objectAtIndex:i+1] integerValue] - [[starttimes objectAtIndex:i] integerValue] <= interval) {
[starttimes insertObject: [starttimes objectAtIndex:i] atIndex:(i+1)];
}
}
CPU usage is nearly 100%. I do not know what is going on? The logic is fairly simple. All it does is to get two elements from this NSMutable array, then compare and insert. BTW, this chunk of codes never stop. It keeps running 3 seconds, then my app crashes.
(TimeSignature*)time {
/* Get all the starttimes in all tracks, in sorted order */
NSInteger initsize = 1;
if ([tracks count] > 0) {
MidiTrack *track = [tracks objectAtIndex:0];
initsize = [track.notes count];
initsize = initsize * [tracks count]/2;
}
NSMutableArray* starttimes = [[NSMutableArray alloc]initWithCapacity:initsize];
for (int tracknum = 0; tracknum < [tracks count]; tracknum++) {
NSLog(#"tracknum is %d",tracknum);
MidiTrack *track = [tracks objectAtIndex:tracknum];
NSLog(#"WE ARE HERE %ld",(long)initsize);
for (int j = 0; j < [track.notes count]; j++) {
MidiNote *note = [track.notes objectAtIndex:j];
NSLog(#"%#",note);
[starttimes addObject:[NSNumber numberWithInteger:note.startTime]];
}
}
/* Notes within "millisec" milliseconds apart should be combined */
int interval = time.quarter * millisec * 1000 / time.tempo;
/* If two starttimes are within interval millisec, make them the same */
for (int i = 0; i < [starttimes count] - 1; i++) {
if ([[starttimes objectAtIndex:i+1] integerValue] - [[starttimes objectAtIndex:i] integerValue] <= interval) {
[starttimes insertObject: [starttimes objectAtIndex:i] atIndex:(i+1)];
}
}
Thanks so much

Answer 1

General Objective-C advice: read the method names. They tell you want the methods do.
You've misunderstood -insertObject:. What insertObject does is... insert an object. What it does not do: replace an object. You're increasing the size of the array every time you call insertObject. In practice you're creating an infinite loop should the if condition pass even once.
You probably wanted -replaceObjectAtIndex:withObject:. Which, like insertObject:, will do what the name says.