Kahan summation: Difference between revisions
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Kahan sum = 1
Delta = 1.110223024625157e-016
</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
</pre>
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