subset - Subsetting R data frame results in mysterious NA rows

Question

I've been encountering what I think is a bug. It's not a big deal, but I'm curious if anyone else has seen this. Unfortunately, my data is confidential, so I have to make up an example, and it's not going to be very helpful.

When subsetting my data, I occassionally get mysterious NA rows that aren't in my original data frame. Even the rownames are NA. EG:

example <- data.frame("var1"=c("A", "B", "A"), "var2"=c("X", "Y", "Z"))
example

  var1 var2
1    A    X
2    B    Y
3    A    Z

then I run:

example[example$var1=="A",]

  var1 var2
1    A    X
3    A    Z
NA<NA> <NA>

Of course, the example above does not actually give you this mysterious NA row; I am adding it here to illustrate the problem I'm having with my data.

Maybe it has to do with the fact that I'm importing my original data set using Google's read.xlsx package and then executing wide to long reshape before subsetting.

Thanks

Answer 1

Wrap the condition in which:

df[which(df$number1 < df$number2), ]

---

How it works:

It returns the row numbers where the condition matches (where the condition is TRUE) and subsets the data frame on those rows accordingly.

Say that:

which(df$number1 < df$number2)

returns row numbers 1, 2, 3, 4 and 5.

As such, writing:

df[which(df$number1 < df$number2), ]

is the same as writing:

df[c(1, 2, 3, 4, 5), ]

Or an even simpler version is:

df[1:5, ]