Find the index of the column in data frame that contains the string as value

Question

I have data frame like this :

df <- data.frame(col1 = c(letters[1:4],\"a\"),col2 = 1:5,col3 = letters[10:14])
 df
  col1 col2 col3
1    a    1    j
2    b         


        

Answer 1

Since you mention you were trying to use sapply() but were unsuccessful, here's how you can do it:

> sapply(df, function(x) any(x == "a"))
 col1  col2  col3 
 TRUE FALSE FALSE 
> which(sapply(df, function(x) any(x == "a")))
 col1 
    1

Of course, you can also use the grep()/grepl() approach if you prefer string matching. You can also wrap your which() function with unname() if you want just the column number.

Answer 2

Something like this?

 which(apply(df, 2, function(x) any(grepl("a", x))))

The steps are:

1. With apply go over each column
2. Search if a is in this column with grepl
3. Since we get a vector back, use any to get TRUE if any element has been matched to a
4. Finally check which elements (columns) are TRUE (i.e. contain the searched letter a).