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

Question

I'm getting this error
Invalid operations to binary expression ('float' and 'id')
NSMutableArray *redValues = [NSMutableArray array];
NSUInteger *redValuesLength = [redValues count];
NSMutableArray *arrayOne = [NSMutableArray array];
NSInteger j;
float diffForAverage, totalOne;
__block int counter = 0;
float average = totalOne / amount;
int amount = 1;
for (j = (counter + 25); j < (redValuesLength - 25); j++)
{
diffForAverage = average - redValues[j + 1]; // error occurs here
if (diffForAverage > -1 && diffForAverage < 1)
{
totalOne += redValues[j + 1]; // error occurs here
amount++;
[arrayOne addObject:[NSNumber numberWithInt:(j - 25)]];
counter++;
}
}
How can I solve this?

Answer 1

Please don't retype error messages (or code) when you post a question. Copy and paste. Otherwise you make mistakes. You can copy Xcode error messages out of the Issue Navigator. Default shortcut: ⌘4.
The actual error message is
Invalid operands to binary expression ('float' and 'id')
The problem is that redValues is an NSMutableArray, so its elements are objects (like NSObject), not primitive numbers (like float and int). The id type is “object identity”, and can refer to any Objective-C object, even those (extremely rare) objects that are not instances of NSObject.
You cannot add a float to an object reference, and you cannot store primitive numbers in an NSMutableArray.
You already know about the NSNumber wrapper object because you create one later in the code. You need to store NSNumber instances in redValues. Perhaps you already are doing that. Then when you get an NSNumber back out of redValues, you need to turn it back into a float to do math on it, like this:
diffForAverage = average - [redValues[j + 1] floatValue]; // error occurs here
If you declare your NSMutableArray like
NSMutableArray<NSNumber *> *redValues = [NSMutableArray array];
then the compiler knows that each element is an NSNumber and you can use the dot syntax (which some people prefer) like
diffForAverage = average - redValues[j + 1].floatValue; // error occurs here