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sum array

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  sum array Andrew Poulos <ap_prog@hotmail.com> - 2017-11-12 11:09 +1100
    Re: sum array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-11-12 01:20 +0000
    Re: sum array Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2018-02-14 09:57 +0100
      Re: sum array "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2018-02-15 12:12 -0800

#34310 — sum array

To sum the integer elements in an array I'm using

var sum = arr.reduce(function(a, b) {
   return a + b;
});

which is fine. When I pass it
   var arr = [,0, 1, -1, 3,, null];
I get the correct answer though I'm expecting it to error. Why doesn't 
it throw an error or at least not the right answer?

Andrew Poulos

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#34311

Andrew Poulos <ap_prog@hotmail.com> writes:

> To sum the integer elements in an array I'm using
>
> var sum = arr.reduce(function(a, b) {
>   return a + b;
> });
>
> which is fine. When I pass it
>   var arr = [,0, 1, -1, 3,, null];
> I get the correct answer though I'm expecting it to error. Why doesn't
>   it throw an error or at least not the right answer?

First, reduce only considers the elements that are present in the array,
and second, null is converted to +0 before the addition (non-string operands
of + are converted to number using the the ToNumber abstract operation).

-- 
Ben.

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#34734

Andrew Poulos wrote:

> To sum the integer elements in an array I'm using
> 
> var sum = arr.reduce(function(a, b) {
>    return a + b;
> });
> 
> which is fine. 

Actually, it is NOT.

> […]
>    var arr = [,0, 1, -1, 3,, …];

This approach is (more or less) fine with *those* numbers and in general 
with numbers that require little precision.

But as I learned recently, it is prone to “catastrophic cancellation”
in ECMAScript implementations, too, because they use IEEE-754 “double 
precision” *floating-point* values for numbers by default.

Therefore, I have added implementations of the Kahan–Babuška, Kahan–Babuška–
Neumaier and pairwise summation algorithms to JSX:math/float.js yesterday, 
and I am going to refactor the corresponding methods (e.g. 
JSX:math/algebra.js#jsx.math.Vector.prototype.dot()) to use Kahan–Babuška–
Neumaier summation.

<https://en.wikipedia.org/wiki/Loss_of_significance>

-- 
PointedEars
FAQ: <http://PointedEars.de/faq> | <http://PointedEars.de/es-matrix>
<https://github.com/PointedEars> | <http://PointedEars.de/wsvn/>
Twitter: @PointedEars2 | Please do not cc me./Bitte keine Kopien per E-Mail.

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#34735

Thomas 'PointedEars' Lahn wrote:

> But as I learned recently, it is prone to “catastrophic cancellation”
> in ECMAScript implementations, too, because they use IEEE-754 “double 
> precision” *floating-point* values for numbers by default.
> 
> Therefore, I have added implementations of the Kahan–Babuška, Kahan–Babuška–
> Neumaier and pairwise summation algorithms to JSX:math/float.js yesterday, 
> and I am going to refactor the corresponding methods (e.g. 
> JSX:math/algebra.js#jsx.math.Vector.prototype.dot()) to use Kahan–Babuška–
> Neumaier summation.
> 
> <https://en.wikipedia.org/wiki/Loss_of_significance>

+1

Aside:

"Nothing brings fear to my heart more than a floating point number. Numerical analysis is the blackest art I know." — Gerald Jay Sussman

Ref: <https://www.youtube.com/watch?v=O3tVctB_VSU>

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