Unexpected result while dividing int by int and storing result into double

Question

I'm having a weird problem.

This code simply divides an int by another int, stores result in a double variable and prints it:

int a = 200;
int b = 557;

double divisionResult = a / b;

System.out.println("Result: " + divisionResult);

After executing this code, the output is:

Result: 0

This is weird, because 200/557 is 0.3590664272890485

I noticed that if i cast a and b to double in the division line

double divisionResult = (double) a / (double) b;

It works perfectly.

Why do i have to cast my variables to double to get the real division result?

Solution 1

Because in integer division, if the answer is not a perfect integer, the digits after the decimal point will be removed (integer division yields integer value).

Note that you don't have to cast the both integers, you can cast only one, the second will be implicitly converted.

Why after the cast it works?

Because cast has higher precedence than /. So first it cast, then it divides. If this were not the case, you would get 0.0.

Solution 2

As always, any calculation on the right side of the assignment operator is executed before assigning its result to the variable specified on the left side. When the calculation is performed, it is a division between two ints, which is an integer division, which (as I'm sure you know), produces an integer result (i.e. a quotient without any remainder). Only after this calculation is performed is the result of the integer division - which is 0 - cast to a double.

As you observed, changing one or both of the ints to a double makes the calculation a double division, rather than an integer division.

Solution 3

a is int, and b is int, dividing a/b results an int value so 0.35 converted to int becomes 0.So you have to cast it like

double d = (double)a/b;

Comments

[zakinster]

Note that this only works because the cast operator has a higher precedence than the division operator, which is not obvious at first sight.

[zakinster]

Even if the cast had a lower precedence than /, the OP's division would still work since the right operand would be casted anyway.