Strange products upon running code...

[neilsonster]

Hi, I have been attempting to write up lessons for next year's programmers to practice with. The last problem I made was to write a function that measures distance driven using encoders and display the data to the screen (I'm using a 2004 RC and MPLAB 7.00). In writing the code myself I ran into something strange.

Specifically the problem came up in calculating the wheel circumference. It was a very simple calculation and it spit out 0 for the circumference upon running the program. I then toyed around with just multiplying some random numbers and 15x30 came out to be -62 somehow. Here is the actual line of code that has caused me the trouble:

unsigned int WheelCirc = (2000 * WHEEL_RAD * PI) / 1000000;

In the above I was trying to stay away from using a float or double number obviously. I used 100 for WHEEL_RAD and 3142 for PI. The answer should be 628 but when I display the value to the terminal window it says 0. I then figured it must have somehow gotten a very small number (compared to 1 000 000) for (2000 * WHEEL_RAD * PI) to come up with 0, which doesn't make sense. Sure enough it ends up getting an answer of -24704 for the product of (2000 * WHEEL_RAD * PI), which would be the cause of the 0.

I then tried just writing out the numbers themselves and multiplying them for the WheelCirc variable and got the same thing. I tried it as a float rather than int and did 200*3.142 with no success (the answer came up as 17437). Has anyone else experienced this with the robot controller, MPLAB, or even in just any C program? Am I missing something (I'm hardly an expert at this)? If you haven't experienced this please give it a try and let me know if you get the same odd products. It's really cramping my style .