R: How to retrieve a column name of a data frame [duplicate]

Question

I am trying to extract the colnames of a data frame, based on the values in the cells. My data is a series of a couple hundred categories, with a simple binary 0 or 1 in the cells to indicate which column name I want in my new df.

To illustrate my point:

year cat1 cat2 cat3 ... catN
2000  0    0    1        0
2001  1    0    0        0
2002  0    0    0        1
....
2018  0    1    0        0

I am trying to get a df like:

year category 
2000  cat3
2001  cat1  
2002  catN  
....
2018  cat2  

My code:

newdf <- as.data.frame(colnames(mydf)[which(mydf == "1", arr.ind = TRUE)[2]])

But alas this only returns one category name!

Any help would be greatly appreciated!

Answer 1

A base R solution:

Using sapply to find which are the ones and get the names.

out <- data.frame(year = df1$year, category = names(sapply(df1[, -1], function(x) which(x == 1))))

out
 year category
1 2000     cat1
2 2001     cat2
3 2002     cat3
4 2018     catN

data:

df1 <- structure(list(year = c(2000L, 2001L, 2002L, 2018L), cat1 = c(0L, 
1L, 0L, 0L), cat2 = c(0L, 0L, 0L, 1L), cat3 = c(1L, 0L, 0L, 0L
), catN = c(0L, 0L, 1L, 0L)), class = "data.frame", row.names = c(NA, 
-4L))

Answer 2

A possible solution is this:

library(tidyverse)

df = data.frame(year = 2000:2002,
                cat1 = c(0,0,1),
                cat2 = c(1,0,0),
                cat3 = c(0,1,0))

df %>%
  gather(category, value, -year) %>%  # reshape data
  filter(value == 1) %>%              # keep rows with 1s
  select(-value) %>%                  # remove that column
  arrange(year)                       # order that column (if needed)

#   year category
# 1 2000     cat2
# 2 2001     cat3
# 3 2002     cat1