How to find column with first of value in R using dplyr?

Question

Consider the following example data:

d <- tibble(V1 = c(5, 3, 8, 7),
            V2 = c(4, 0, 2, 6),
            V3 = c(0, 1, 0, 3),
            V4 = c(0, 0, 0, 2))

# A tibble: 4 × 4
     V1    V2    V3    V4
  <dbl> <dbl> <dbl> <dbl>
1     5     4     0     0
2     3     0     1     0
3     8     2     0     0
4     7     6     3     2

I want to find the column name with the first 0 for each row. If there are no zeros, then I want to get the last column name.

The result should look like this:

# A tibble: 4 × 5
     V1    V2    V3    V4 firstZero
  <dbl> <dbl> <dbl> <dbl> <chr>    
1     5     4     0     0 V3       
2     3     0     1     0 V2       
3     8     2     0     0 V3       
4     7     6     3     2 V4

Answer 1

max.col from base R is fast

 d$firstZero <- ifelse(!rowSums(d == 0), names(d)[ncol(d)], 
   names(d)[max.col(d == 0, 'first')])

---

using dplyr

library(dplyr)
d %>% 
 rowwise %>% 
  mutate(firstZero = names(.)[match(0, c_across(everything()), 
       nomatch = ncol(.))]) %>% 
  ungroup

-output

# A tibble: 4 × 5
     V1    V2    V3    V4 firstZero
  <dbl> <dbl> <dbl> <dbl> <chr>    
1     5     4     0     0 V3       
2     3     0     1     0 V2       
3     8     2     0     0 V3       
4     7     6     3     2 V4       

---

Or using case_when with max.col

d %>% 
  mutate(firstZero = names(.)[case_when(!rowSums(across(c(V1, V2, V3)) == 0) ~ 
     ncol(.), TRUE ~  max.col(!across(c(V1, V2, V3)) != 0, "first"))])