Is there a way to check if dataframe is empty and if so to add a NA row?-CodePudding

For example i have a dataframe that has nothing inside but i need it to run the full code cause it usually expects there to be data. I tried this but it did not work

ifelse(dim(df_empty)[1]==0,rbind(Shots1B_empty,NA))

CodePudding user response：

Maybe something like this:

df_empty <- data.frame(x=integer(0), y = numeric(0), a = character(0))

if(nrow(df_empty) == 0){
  df_empty <- rbind(df_empty, data.frame(x=NA, y=NA, a=NA))
}
df_empty
#   x  y  a
#1 NA NA NA

CodePudding user response：

We may need if/else instead of ifelse - ifelse requires all arguments to be of same length, which obviously will be not the case when we rbind

Shots1B_empty <- if(nrow(df_empty) == 0)  rbind(Shots1B_empty, NA)

CodePudding user response：

Simple question, OP, but actually pretty interesting. All the elements of your code should work, but the issue is that when you run as is, it will return a list, not a data frame. Let me show you with an example:

growing_df <- data.frame(
  A=rep(1, 3),
  B=1:3,
  c=LETTERS[4:6])

df_empty <- data.frame()

If we evaluate as you have written you get:

df <- ifelse(dim(df_empty)[1]==0, rbind(growing_df, NA))

with df resulting in a List:

> class(df)
[1] "list"
> df
[[1]]
[1]  1  1  1 NA

The code "worked", but the resulting class of df is wrong. It's odd because this works:

> rbind(growing_df, NA)
   A  B    c
1  1  1    D
2  1  2    E
3  1  3    F
4 NA NA <NA>

The answer is to use if and else, rather than ifelse(), just as @akrun noted in their answer. The reason is found if you dig into the documentation of ifelse():

ifelse returns a value with the same shape as test which is filled with elements selected from either yes or no depending on whether the element of test is TRUE or FALSE.

Since dim(df_empty)[1] and/or nrow(df_empty) are both vectors, the result will be saved as a list. That's why if {} works, but not ifelse() here. rbind() results in a data frame normally, but the class of the result stored into df when assigning with ifelse() is decided based on the test element, not the resulting element. Compare that to if{} statements, which have a result element decided based on whatever expression is input into {}.