Roundoff example

Floating point arithmetic is not associative. Which order is correct?

Think about the following example:

sum = 0.0

do i = 1, n

sum = sum + a(i)

enddo

sum2 = 0.0

do i = 1, n-1, 2

sum1 = sum1 + a(i)

sum2 = sum2 + a(i+1)

enddo

sum = sum1 + sum2

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