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

问题

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!

回答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))

回答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

回答3:

Another option using max.col from base R

data.frame(year = dat$year, category = names(dat[-1])[max.col(dat[-1])])
#  year category
#1 2000     cat3
#2 2001     cat1
#3 2002     catN
#4 2018     cat2

max.col finds the positions of the 1s in your data (first column excluded) and returns a numeric vector of column positions that we use to subset the column names, again first column excluded.

Data as supplied by Rui Barradas.

回答4:

This is basicaly a Reshaping data.frame from wide to long format trick. To do it I will use package reshape2.

Then select the rows with value == 1.

result <- reshape2::melt(dat, id.vars = "year")
result <- result[result$value == 1, 1:2]
result <- result[order(result[[1]]), ]
names(result)[2] <- "category"
row.names(result) <- NULL

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

DATA.

dat <- read.table(text = "
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
", header = TRUE)

回答5:

A simple solution:

# your data
data <-data.frame(year =c(2016,2017,2018),
                  cat.1=c(0,1,0),
                  cat.2=c(0,0,1),
                  cat.3 =c(1,0,0))

# a nice library 
library(reshape2)

# from wide to long
filtered <- melt(data, id = 'year') %>% filter(value>0) 

# remove the useless column
filtered <- filtered[,1:2]
    > filtered
  year variable
1 2017    cat.1
2 2018    cat.2
3 2016    cat.3

回答6:

Yet another base R option using stack

subset(data.frame(year = df$year, stack(df, select = -year)), values == 1)[-2]
#   year  ind
#2  2001 cat1
#8  2018 cat2
#9  2000 cat3
#15 2002 catN

来源：https://stackoverflow.com/questions/51613634/r-how-to-retrieve-a-column-name-of-a-data-frame

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