Why are these two sums() not the same?

Asked Oct 02 '18 at 16:47

Active Oct 02 '18 at 16:48

Viewed 20 times

I have two vectors:

parentvec<-c(1.4982, 18.18734, 901.14744)
childvec<-c(0.90268, 13.77532, 613.38298, 0.01845, 0.0681, 7.19925, 3.75315, 0.14333, 136.54008, 0.04766, 0.59077, 28.97255, 0.38608, 115.05258)

When I sum each of them, I get the same result:

> sum(childvec)
[1] 920.833

> sum(parentvec)
[1] 920.833

But when I subtract one from the other, I get a non-zero result:

> sum(childvec)-sum(parentvec)
[1] 1.136868e-13

What on earth is going on and how do I stop it?

In case this is relevant, I'm using Win10, with R 3.4.3.

edited Oct 02 '18 at 16:48

asked Oct 02 '18 at 16:47

iod

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`1.136868e-13` is basically zero. Do you actually need more precision than that? – Carcigenicate Oct 02 '18 at 16:48
Yes, because I'm using parentvec-childvec to get a TRUE or FALSE vector – iod Oct 02 '18 at 16:49
You could add a step that checks if the difference is less than some threshold value like 1e-13, and make it zero if it is. – Carcigenicate Oct 02 '18 at 16:50

Why are these two sums() not the same?

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