Question

I am having some issues doing basic math with Java. I don’t know why I am getting 0 as a result of multiplying n*n*n only in one case. (see below) I need this not to be zero because I have to divide timing/n*n*n to get that big O performance. Also if I can get that working the output may look like 0.00000 but I want to multiply that by like 100,000 just so I can see numbers and find any trends in performance.

You can see the values of n and timing in the first two number columns.

• n is Integer
• timing is Long

This is my output statement,

System.out.println(fmt.format("%20s %20d %20d %20d %20d %20d%n", "Alg. 1", n, timing, n*n*n, timing/(n*n), timing /*((double)timing/((double)n*Math.log((double)n)))*/));

My results,

 Alg. 1                  256                    4             16777216                    0                    4
 Alg. 1                  512                   22            134217728                    0                   22
 Alg. 1                 1024                  173           1073741824                    0                  173
 Alg. 1                 2048                 1362                    0                    0                 1362

Please keep in mind I need to perform this log math also. Any tips or fixes for that would also be appreciated!

Note: I am not dividing at all in the statement n*n*n and I am getting 0 in column four row four.

Can someone also please tell me how to get this to output decimal places that are accurate not just 0.000000. My new arithmetic is ((float)(timing/((long)n)*n*n)*100000. I am multiplying by 100000 as said above because I want to see something in the decimal places. I should be seeing 0.0159139 with this equation when n is 2048 and timing is 1362. I just see 0.000000 though. Any suggestions?

Answer 1

2048³ is 2³³ which overflows using 32-bit arithmetic. Use a long or a double to handle numbers this big.

((long) n) * n * n

(Demo at ideone.com)