[java] Integer division: How do you produce a double?

For this code block:

int num = 5;
int denom = 7;
double d = num / denom;

the value of d is 0.0. It can be forced to work by casting:

double d = ((double) num) / denom;

But is there another way to get the correct double result? I don't like casting primitives, who knows what may happen.

Type Casting Is The Only Way

May be you will not do it explicitly but it will happen.

Now, there are several ways we can try to get precise double value (where num and denom are int type, and of-course with casting)-

1. with explicit casting:

• double d = (double) num / denom;
• double d = ((double) num) / denom;
• double d = num / (double) denom;
• double d = (double) num / (double) denom;

but not double d = (double) (num / denom);

2. with implicit casting:

• double d = num * 1.0 / denom;
• double d = num / 1d / denom;
• double d = ( num + 0.0 ) / denom;
• double d = num; d /= denom;

but not double d = num / denom * 1.0;
and not double d = 0.0 + ( num / denom );

Now if you are asking- Which one is better? explicit? or implicit?

Well, lets not follow a straight answer here. Simply remember- We programmers don't like surprises or magics in a source. And we really hate Easter Eggs.

Also, an extra operation will definitely not make your code more efficient. Right?

I don't like casting primitives, who knows what may happen.

Why do you have an irrational fear of casting primitives? Nothing bad will happen when you cast an int to a double. If you're just not sure of how it works, look it up in the Java Language Specification. Casting an int to double is a widening primitive conversion.

You can get rid of the extra pair of parentheses by casting the denominator instead of the numerator:

double d = num / (double) denom;

You might consider wrapping the operations. For example:

class Utils
{
    public static double divide(int num, int denom) {
        return ((double) num) / denom;
    }
}

This allows you to look up (just once) whether the cast does exactly what you want. This method could also be subject to tests, to ensure that it continues to do what you want. It also doesn't matter what trick you use to cause the division (you could use any of the answers here), as long as it results in the correct result. Anywhere you need to divide two integers, you can now just call Utils::divide and trust that it does the right thing.

just use this.

int fxd=1;
double percent= (double)(fxd*40)/100;

Cast one of the integers/both of the integer to float to force the operation to be done with floating point Math. Otherwise integer Math is always preferred. So:

1. double d = (double)5 / 20;
2. double v = (double)5 / (double) 20;
3. double v = 5 / (double) 20;

Note that casting the result won't do it. Because first division is done as per precedence rule.

double d = (double)(5 / 20); //produces 0.0

I do not think there is any problem with casting as such you are thinking about.

If you change the type of one the variables you have to remember to sneak in a double again if your formula changes, because if this variable stops being part of the calculation the result is messed up. I make a habit of casting within the calculation, and add a comment next to it.

double d = 5 / (double) 20; //cast to double, to do floating point calculations

Note that casting the result won't do it

double d = (double)(5 / 20); //produces 0.0

use something like:

double step = 1d / 5;

(1d is a cast to double)

What's wrong with casting primitives?

If you don't want to cast for some reason, you could do

double d = num * 1.0 / denom;

Best way to do this is

int i = 3;
Double d = i * 1.0;

d is 3.0 now.

Just add "D".

int i = 6;
double d = i / 2D; // This will divide bei double.
System.out.println(d); // This will print a double. = 3D