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i thought i understood the modulo operator. guess not quite yet.  RSS feed

 
Dan D'amico
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output :
8.5 , 1.0
4.25 , 0.5
2.125 , 0.25
1.0625 , 0.125
0.53125, 1.0625


17/2 = 8.5 (have 1 remainder so its equals 1 , ok )
now, i dont get it , 8.5 / 2 = 4.25 so why 8.5 % 2 = 0.5 ? it should be 2 ? because its have two digits after thr decimal point. - > 4.25 .
and the same for all the others





 
Winston Gutkowski
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Dan D'amico wrote:now, i dont get it , 8.5 / 2 = 4.25 so why 8.5 % 2 = 0.5 ? it should be 2 ?

Should it? First-off, the idea of a modulus op (java actually calls it the "remainder operator") on a floating-point value seems somewhat odd to me.

For an explanation like this, you need to go to the horse's mouth; but unfortunately, even it doesn't provide examples of the style that you're doing. What it does do though is explain how the calculation is done - specifically:

"the floating-point remainder r from the division of a dividend n by a divisor d is defined by the mathematical relation r = n - (d ⋅ q) where q is an integer that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d."

So, in your case above (8.5%2), the calculation is 8.5 - (2 * 4), which == 0.5.

Winston
 
Liutauras Vilda
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9.25 % 2 = 1.25

1) .25 straight away goes towards final answer
2) 9 / 2 = 4 (1 remainder)
3) 1 + 0.25 = 1.25

a) Basically all the time at the very beginning discard floating part as it's not significant and use only whole part.
b) Divide whole number by given number, in your case 2, take the remainder, and add him to floating part from the very beginning.

I hope it helps to understand the main idea.
 
Campbell Ritchie
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No, I am afraid that is not how it works. The details are in the JLS link which Winston posted earlier.
Remember that the operands of binary arithmetic operators undergo promotion before the operation. So the first things which happens is that the division is converted from
9.25 % 2
to
9.25 % 2.0
9.25 / 2.0 → 4.625
This is truncated to 4.0
4.0 * 2.0 → 8.0
9.25 - 8.0 → 1.25 QED
campbell@XXXXX:~/java$ java RemainderDemo 9.25 2
9.250000 % 2.000000 = 1.250000
9.250000 BigDecimal.remainder(2.000000) = 1.250000
9.250000 IEEE% 2.000000 = -0.750000
.....
java RemainderDemo 9.25 0
9.250000 % 0.000000 = NaN
Exception in thread "main" java.lang.ArithmeticException: Division by zero
at java.math.BigDecimal.divide(BigDecimal.java:1742)
at java.math.BigDecimal.divideToIntegralValue(BigDecimal.java:1792)
at java.math.BigDecimal.divideAndRemainder(BigDecimal.java:1948)
at java.math.BigDecimal.remainder(BigDecimal.java:1890)
at RemainderDemo.showRemainders(RemainderDemo.java:27)
at RemainderDemo.main(RemainderDemo.java:7)
The IEEE remainder is described in the JLS link.
 
Liutauras Vilda
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You're right, I wasn't precise about type double.

But still, the main idea is to use modulus operator on integers.
Division of doubles gives you quite precise answer anyway.

 
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