Kahan summation: Difference between revisions

</pre>

=={{header|FutureBasic}}==

FB has proper decimal numbers supporting mantissas and exponents. But conversion to and from floating point numbers (or strings) makes it easier and more readable for this task to be completed with doubles as are many other examples here.

<syntaxhighlight lang="futurebasic">

_elements = 3

local fn Epsilon as double

double eps = 1.0

while ( 1.0 + eps != 1.0 )

eps = eps / 2.0

wend

end fn = eps

local fn KahanSum( nums(_elements) as double, count as long ) as double

double sum = 0.0

double c = 0.0

double t, y

long i

for i = 0 to count - 1

y = nums(i) - c

t = sum + y

c = (t - sum) - y

sum = t

next

end fn = sum

local fn DoKahan

double a = 1.0

double b = fn Epsilon

double c = -b

double fa[_elements]

fa(0) = a : fa(1) = b : fa(2) = c

printf @"Epsilon = %.9e", b

printF @"(a + b) + c = %.9e", (a + b) + c

printf @"Kahan sum = %.9e", fn KahanSum( fa(0), 3 )

printf @"Delta = %.9e", fn KahanSum( fa(0), 3 ) - ((a + b) + c)

end fn

fn DoKahan

HandleEvents

</syntaxhighlight>

{{output}}

<pre>

Epsilon = 1.110223025e-16

(a + b) + c = 1.000000000e+00

Kahan sum = 1.000000000e+00

Delta = 1.110223025e-16