Why is the result of this product operation wrong?

Question

I had a problem in a program where the result of product is not what it's supposed to be. For example: 775*775 gives 10801 instead of 6006025.

To try to isolate and resolve the problem, I made a simple sketch to test the result of different products. What I found is that if I do the same product in this way: 775/100.00*775/100.00 then multiply the result by 10000 I get the right value: 600625.00

The attached pictures illustrate what I said.

While I can use this solution in my initial program by I still want to do the multiplication directly as more operations means more time.

I would appreciate any help. Thank you.

Answer 1

Integer overflow. You're multiplying an int by an int then assigning it to a long. Instead if you multiply a long by a long, or an int by a long, or a long by an int, it should work.

unsigned long res = 775L * 775;

The L suffix tells the compiler to treat it as a long, not an int.