Ifelse function for dataframe in R - r

Question

I have a dataframe in R. Either the df has two columns with values, or it has the dimensions 0,0.
If the dataframe has columns with values, I want to keep these values, but if the dimensions are 0,0, I want to create two columns with one row containing 0,0 values.
Either it looks like this:
start = c (2, 10, 20)
end = c(5, 15, 25)
dataframe = as.data.frame (cbind (start, end))
-> if the df looks like this, it should be retained
Or like this:
start = c ()
end = c()
dataframe = as.data.frame (cbind (start, end))
-> if the df looks like this, a row with (0,0) should be added in the first row.
I tried ifelse
dataframe_new = ifelse (nrow(dataframe) == 0, cbind (start = 0,end =0) , dataframe)
But if the dataframe is not empty, it remains only the value of the first row and column. If the dataframe is empty, there is only one 0.

Answer 1

Instead of the function ifelse, you should be using if...else clauses here. ifelse is used when you want to create a vector whose elements vary conditionally according to a logical vector with the same length as the output vector. That's not what you have here, but rather a simple branching condition (if the data frame has zero rows do one thing, if not then do something else). This is when you use if(condition) {do something} else {do another thing}
If you need to use the code a lot, you could save yourself time by putting it in a simple function:
fill_empty <- function(x) {
if(nrow(x) == 0) data.frame(start = 0, end = 0) else x
}
Let's test this on your examples:
start = c (2, 10, 20)
end = c(5, 15, 25)
dataframe = as.data.frame (cbind (start, end))
fill_empty(dataframe)
#> start end
#> 1 2 5
#> 2 10 15
#> 3 20 25
And
start = c ()
end = c()
dataframe = as.data.frame (cbind (start, end))
fill_empty(dataframe)
#> start end
#> 1 0 0
Created on 2022-09-19 with reprex v2.0.2