Calculation in Java

Question

I have the following calculation in Java:

rightMotor = (int) (((double)speed)/((double)100)*rightMotor));

Speed and rightMotor are both integers. The problem is, it keeps returning 0. I have tried to force to divide speed and 100 as doubles, which had no different results. Can anyone give me some insight in this?

Thanks in advance.

Exactly, if the result is between -0.5 and 0.5, casting it at int will result in 0. — A. Rama, Jan 02 '15 at 14:09
Then the result is lower than 0.01 and the casting to int will approx it to 0. That's not what you wanted to do, I'm betting. — A. Rama, Jan 02 '15 at 14:11

score 2 · Accepted Answer · answered Jan 02 '15 at 14:09

The reason the calculation returns zero is that the double value computed by the expression is less than one.

It looks like you have placed the parentheses incorrectly - currently, you divide speed by hundred times rightMotor, which is bound to produce a value less than 1.

You need to fix your formula to make it work. If you wanted to compute speed / 100 * rightMotor as if it were a math formula, you could do the multiplication ahead of division, avoiding doubles altogether:

rightMotor = speed * rightMotor / 100;

score 1 · Answer 2 · answered Jan 02 '15 at 14:10

Since you are assigning the result to an int variable, there's no point in doing floating point division.

rightMotor = speed/(100*rightMotor);

would give you the same result, which would be 0 if speed < 100*rightMotor.

If your intent was to multiply speed/100 by rightMotor, you still don't need to cast to double and then back to int.

Just do:

rightMotor = (speed*rightMotor)/100;

Calculation in Java

2 Answers2