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Strange behavior of /cumulative keyword in total() [message #63296]

Tue, 04 November 2008 02:10

Chris[5]
Messages: 16
Registered: May 2008

Junior Member

Can anybody explain this?

arr = fltarr( 500000) + .1
cumul = total(array, /cumulative)

print, (cumul - shift(cumul, 1)) [1 : 10]
print, (cumul - shift(cumul,1)) [499990:499999]

> 0.100000 0.100000 0.100000 0.100000 0.100000 0.100000
0.100000 0.100000 0.100000 0.100000

> 0.101562 0.101562 0.101562 0.101562 0.101562 0.101562
0.101562 0.101562 0.101562 0.101562

Plotting cumul - shift(cumul,1) is even weirder. I can understand the
net error of cumul growing over time, as floating point precision
errors accumulate. However, shouldn't the error between any two
entries in a cumulative sum not accumulate over the array?

chris

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