Generating primes List in objective C? - objective-c

Question

I create this objective C class to Genrate prime numbers from n to limit. I have problem I could not get the item in NSMutableArray inside for loop. can some one show me how can I fix it?
#implementation Prime
-(NSMutableArray *)generatePrimes:(int)upperLimit{
NSMutableArray *primes = [[NSMutableArray alloc]init];
bool isPrime;
int j;
[primes addObject:[NSDecimalNumber numberWithInt:2]];
for (int i= 3; i <= upperLimit ; i+=2) {
j = 0;
isPrime = YES;
NSInteger index;
for(id obj in primes)
{
index = [primes indexOfObject:obj];
if((index * index) <= i )
{
if(i % index == 0)
{
isPrime = NO;
break;
}
}
}
if(isPrime)
{
[primes addObject:[NSDecimalNumber numberWithInt:i]];
}
}
return primes;
}
#end

Answer 1

This question is a little vague, but it seems to me what you're trying to do is convert the NSDecimalNumber back into an int. What your code is actually doing is getting the number's index inside the array (ie the first object is 0, the second is 1, etc.) If you are trying to get the original value of i, change these lines:
for(id obj in primes)
{
index = [primes indexOfObject:obj];
to this:
for(NSDecimalNumber num in primes)
{
index = [num integerValue];
I'd also recommend using a different name than index, as that's misleading as to what you're actually doing.

Answer 2

-(NSMutableArray *)generatePrimes:(int)upperLimit
{
NSMutableArray *primes = [[NSMutableArray alloc]init];
bool isPrime;
for (int i=2; i<upperLimit; i++)
{
bool prime = true;
for (int j=2; j*j<=i; j++)
{
if (i % j == 0)
{
prime = false;
break;
}
}
if(prime)
{
[primes addObject:[NSDecimalNumber numberWithInt:i]];
}
}
return primes;
}
or
-(NSMutableArray *)generatePrimes:(int)upperLimit
{
NSMutableArray *primes = [[NSMutableArray alloc]init];
[primes addObject:[NSDecimalNumber numberWithInt:2]];
for(int i=3; i < upperLimit; i++)
{
bool prime=true;
for(int j=0;j<primes.count && (((NSDecimalNumber *)primes[j]).integerValue*((NSDecimalNumber *)primes[j]).integerValue) <= i;j++)
{
if(i % (((NSDecimalNumber *)primes[j]).integerValue) == 0)
{
prime=false;
break;
}
}
if(prime)
{
[primes addObject:[NSDecimalNumber numberWithInt:i]];
}
}
return primes;
}
Hope this helps!