On 07/18/2014 12:00 PM, Georgiy Rakov wrote:
On 18.07.2014 20:14, Joe Darcy wrote:
Hello Georgiy,
On 07/18/2014 05:29 AM, Georgiy Rakov wrote:
Hello Joe,
could you please clarify by short example following assertion:
154 * If the exact sum is infinite, a properly-signed infinity is
155 * returned.
I'm afraid I don't quite understand what you mean here by 'exact sum'.
By "exact sum," the sum absent any floating-point rounding, the sum
you would get using infinite precision to operate on the values in
question.
The sentence in question is intended to be a short way of saying "If
you have same-signed infinities in your input, the result will be an
infinity of that sign." In particular, this disallows the behavior
that was fixed before JDK 8 GA where having infinities in the input
would cause a NaN to be returned because of how the compensated
summation code manipulated those values.
Thanks, I see,
however it seems to me a bit confusing, since the term "infinite exact
sum" seems to me not obvious and I believe it needs some definition.
I'd like to suggest to use more straightforward approach, that is as
you've said: "If you have same-signed infinities in your input, the
result will be an infinity of that sign.". I believe it would be more
clear for end user (at least for me :)) and from conformance point of
view.
Besides it seems to me a bit questionable. For instance "inexact some"
looks like more appropriate, since overflowing to infinity occurs when
_actual _sum exceeds the limit. By actual sum I mean sum resulted from
actual summation with all the rounding happened. There wouldn't be
such questions, provided straightforward approach is used.
Thanks,
Georgiy.
In response to previous feedback, I propose this revised change to the
specification:
--- a/src/share/classes/java/util/DoubleSummaryStatistics.java Sat Jul
19 11:22:08 2014 +0800
+++ b/src/share/classes/java/util/DoubleSummaryStatistics.java Mon Jul
21 18:02:54 2014 -0700
@@ -1,5 +1,5 @@
/*
- * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights
reserved.
+ * Copyright (c) 2012, 2014, Oracle and/or its affiliates. All rights
reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
@@ -129,9 +129,6 @@
* Returns the sum of values recorded, or zero if no values have been
* recorded.
*
- * If any recorded value is a NaN or the sum is at any point a NaN
- * then the sum will be NaN.
- *
* <p> The value of a floating-point sum is a function both of the
* input values as well as the order of addition operations. The
* order of addition operations of this method is intentionally
@@ -143,6 +140,44 @@
* numerical sum compared to a simple summation of {@code double}
* values.
*
+ * Because of the unspecified order of operations and the
+ * possibility of using differing summation schemes, the output of
+ * this method may vary on the same input values.
+ *
+ * <p>Various conditions can result in a non-finite sum being
+ * computed. This can occur even if the all the recorded values
+ * being summed are finite. If any recorded value is non-finite,
+ * the sum will be non-finite:
+ *
+ * <ul>
+ *
+ * <li>If any recorded value is a NaN, then the final sum will be
+ * NaN.
+ *
+ * <li>If the recorded values contain one or more infinities, the
+ * sum will be infinite or NaN.
+ *
+ * <ul>
+ *
+ * <li>If the recorded values contain infinities of opposite sign,
+ * the sum will be NaN.
+ *
+ * <li>If the recorded values contain infinities of one sign and
+ * an intermediate sum overflows to an infinity of the opposite
+ * sign, the sum may be NaN.
+ *
+ * </ul>
+ *
+ * </ul>
+ *
+ * It is possible for intermediate sums of finite values to
+ * overflow into opposite-signed infinities; if that occurs, the
+ * final sum will be NaN even if the recorded values are all
+ * finite.
+ *
+ * If all the recorded values are zero, the sign of zero is
+ * <em>not</em> guaranteed to be preserved in the final sum.
+ *
* @apiNote Values sorted by increasing absolute magnitude tend to
yield
* more accurate results.
*
@@ -193,15 +228,9 @@
* Returns the arithmetic mean of values recorded, or zero if no
* values have been recorded.
*
- * If any recorded value is a NaN or the sum is at any point a NaN
- * then the average will be code NaN.
- *
- * <p>The average returned can vary depending upon the order in
- * which values are recorded.
- *
- * This method may be implemented using compensated summation or
- * other technique to reduce the error bound in the {@link #getSum
- * numerical sum} used to compute the average.
+ * <p> The computed average can vary numerically and have the
+ * special case behavior as computing the sum; see {@link #getSum}
+ * for details.
*
* @apiNote Values sorted by increasing absolute magnitude tend to
yield
* more accurate results.
(With analogous changes in java/util/stream/DoubleStream.java.)
Thanks,
-Joe
