[R] How do I get the row indices?

[Marc Schwartz (via MN)]

On Fri, 2005-09-16 at 10:34 -0700, Martin Lam wrote:
> Hi,
> 
> I was wondering if it's possible to get the row
> numbers from a filtering. Here's an example:
> 
> # give me the rows with sepal.length == 6.2
> iris[(iris[,1]==6.2),]
> 
> # output
>     Sepal.Length Sepal.Width Petal.Length Petal.Width 
>   Species
> 69           6.2         2.2          4.5         1.5
> versicolor
> 98           6.2         2.9          4.3         1.3
> versicolor
> 127          6.2         2.8          4.8         1.8 
> virginica
> 149          6.2         3.4          5.4         2.3 
> virginica
> 
> What I really want is that it return the row numbers:
> 69, 98, 127, 149.
> 
> Thanks in advance,
> 
> Martin

Martin,

First: Be very, very careful when performing exact equalities on
floating point numbers. They won't always result in the answer you
expect. For more information see R FAQ 7.31: Why doesn't R think these
numbers are equal?

Second:

See ?all.equal, ?sapply and ?which. Here is a possible vectorized
solution:

> which(sapply(iris[, 1], function(x) isTRUE(all.equal(x, 6.2))))
[1]  69  98 127 149


The above applies isTRUE(all.equal(x, 6.2)) for each element 'x' in 
iris[, 1], returning the indices of the TRUE results for the near
equality comparison, based upon the tolerance argument in all.equal().

HTH,

Marc Schwartz