R not returning true when object [sqrt 2 ^ 2 ] == 2

Question

I know that running a simple line such as sqrt(2) ^ 2 == 2 will return FALSE, but when I create an object with the left part of the equation, it returns the value 2, but when logically comparing the object to the number 2, I still get FALSE. This seems to be in error. Here is the code:

> (root.of.2 <- sqrt(2))
[1] 1.414214
> (root.of.2.sqrd <- sqrt(2) ^ 2)
[1] 2
> (root.of.2.sqrd == 2)
[1] FALSE

What’s going on here?

score 3 · Accepted Answer · edited May 23 '17 at 12:32

3

Look closer at your variable:

> print(root.of.2.sqrd,18)
[1] 2.0000000000000004

root.of.2.sqrd is numeric.

> as.integer(root.of.2.sqrd)==2
[1] TRUE

Read FAQ 7.31

By the way, there are similar questions.

edited May 23 '17 at 12:32

Community

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answered Oct 12 '16 at 07:07

Katin

177
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score 1 · Answer 2 · answered Oct 12 '16 at 06:58

They are different numbers (sqrt(2) being irrational at some point of time some numerical inaccuracy, however small, will be introduced with it's rational approximation):

print(sprintf('%.20f', 2))
#[1] "2.00000000000000000000"
print(sprintf('%.20f', sqrt(2) ^ 2))
#[1] "2.00000000000000044409"

R not returning true when object [sqrt 2 ^ 2 ] == 2

2 Answers2