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    2    k
3    c    3    l
4    d    4    m
5    a    5    n

I want to find the index of the column of df that has values matching to string "a". i.e. it should give me 1 as result. I tried using which in sapply but its not working. Anybody knows how to do it without a loop ??

Answer 1

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).