# [R] Data Checking

Ott Toomet siim at obs.ee
Mon Mar 25 11:04:20 CET 2002

``` On Sun, 24 Mar 2002, Uwe Ligges wrote:

|Ko-Kang Kevin Wang wrote:
|>
|> Hi,
|>
|> This is a simple question with if elseif....however I am having trouble
|> constructing the solution for some reason.
|>
|> Suppose I have a data set with 3 variables, a, b and c say.  Let's say c
|> is the sum of a and b.  So:
|>   a  b  c
|>   1  2  3
|>   2  3  5
|>   3  4  7
|>   .  .  .
|>   .  .  .
|>   .  .  .
|>
|> Suppose that I know there have been some data entry errors and I want to
|> check if ALL values in c is really the sum of a and b, and if not, print
|> out the whole line (i.e. all values of a, b and c in that row).
|>
|> Any help on how I can write this if elseif block will be apprecaited!
|
|Assume your "data set" is a data.frame():
|
| X <- data.frame(a=1:5, b=2:6, ce=c(3,5,7,6,11))
| # Let's call it a, b, ce --- c already is a function
| # Now get the rows with errors:
| X[X\$a + X\$b != X\$ce, ]
|

I do recommend not to use == or != to test equality.  Operators == or != are usable
== _only_ for integers.  Generally, on digital computers you can never know
when two real numbers are equal.  Here is a simle example (R 1.4.0, P-II,
linux):

> 1 + 2
[1] 3
> 1 + 2 == 3
[1] TRUE
> 1.1 + 2.2
[1] 3.3
> 1.1 + 2.2 == 3.3
[1] FALSE

So, using Uwe's example, you should write something like

tolerance= .Machine\$double.eps ^ 0.5
> X[abs(X\$a + X\$b - X\$ce) > tolerance,]
a b ce
4 4 5  6

Regards,

Ott Toomet

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