Invalid operations to binary expression ('id' and 'id') - objective-c

Question

I have a array fetched from the core data and trying to do some simple calculation but getting errors of Invalid operations to binary expression
pop_cum[i]= (pop_ln_array[i-1] + pop_ln_array[i]);
//getting error at this point of "Invalid operations to binary expression('id' and 'id')
I know we have to change the type of the array values into int but how?
NSManagedObjectContext *moc = [self managedObjectContext];
NSEntityDescription *entityDescription = [NSEntityDescription entityForName:#"Input_Details" inManagedObjectContext:moc];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entityDescription];
// Set example predicate and sort orderings...
request.propertiesToFetch = #[ #"pop_Ln" ];
request.resultType = NSDictionaryResultType;
//if you change the sort order here, please change it to SDInputDetailsList also
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc]
initWithKey:#"sewer_No" ascending:YES];
[request setSortDescriptors:#[sortDescriptor]];
NSError *error;
NSArray *pop_ln_array = [moc executeFetchRequest:request error:&error];
int arrayCount=[pop_ln_array count];
//Calculating cumulative population, flow & peak factor
NSLog(#"arrayCount is %d",arrayCount);
if (pop_ln_array == nil)
{
NSLog(#"Cumilative population line array is nil");
}
else
{
NSArray *pop_cum[arrayCount];
for(int i=0;i<[pop_ln_array count];i++)
{
NSLog(#"Cumilative array at %d is %#",i,pop_ln_array[i]);
if (i==0)
{
pop_cum[i]=pop_ln_array[i] ;
}
else
{
pop_cum[i]= (pop_cum[i-1] + pop_ln_array[i]); //getting error at this point of "Invalid operations to binary expression('id' and 'id')
}
}
}

Answer 1

Use as follows
for(int i=0;i<[pop_ln_array count];i++){
NSLog(#"Cumilative array at %d is %#",i,pop_ln_array[i]);
if (i==0){
pop_cum[i]=pop_ln_array[i];
NSLog(#"pop_cum[%d] %#",i,pop_cum[i]);
}
else {
NSLog(#"Pop_cum %#",pop_cum[i-1]);
[pop_ln_array[i]intValue]);
NSInteger first=[pop_ln_array[i-1]intValue], second=[pop_ln_array[i]intValue];
NSInteger sum=first+second;
pop_cum[i]=[NSString stringWithFormat:#"%d",sum];
}
}
You are adding two ids. Which can not be.
Convert it to integerValue or intValue or any numerical and then add.