R add a row number column to dataframe and handle the case that dataframe is empty-CodePudding

I have a dataframe df that I want to add a column to called row_num that represents the index of the row. This was my initial solution:

 df$row_num<-seq(1:nrow(df))

However, it doesn't work in the case that df is empty as I get the error:

Error in `$<-.data.frame`(`*tmp*`, row_num, value = 1:2) : 
  replacement has 2 rows, data has 0

One solution I have found is using row_number() from dplyr but this seems to slow down my code quite a bit so I was looking for a simpler solution.

CodePudding user response：

Two problems here:

1:nrow(df) will always return something, even if nrow(df) is 0. Try it!
```
1:0
# [1] 1 0
```
This is because 1:0 is seen as a reverse sequence, so it counts down.
seq(1:nrow(df)) is redundant. Fortunately, this isn't breaking your code, it's just sequencing along it, but it's not helping.
```
seq(1:9)
# [1] 1 2 3 4 5 6 7 8 9
seq(1:0)
# [1] 1 2
```

One fix is to just use seq_len(nrow(df)), where seq_len(0) (for a 0-row frame) will return the length vector you need (i.e., length 0). (In general, I tend to recommend seq_len(..) any time unsupervised code is meant to iterate a programmatic number of times. Another "safe" function is seq_along(..), as it will also do nothing if its input is length 0, but it isn't useful in this case.)