r - 根据条件提取列名和具体值

Question

假设我有以下数据框:

firstname <- c('Doug','Tom','Glenn','Billy','Angelo')
city <- c('Tulsa','Unknown','Miami','Houston','Unknown')
state <- c('OK','CA','FL','Unknown','Unknown')
job <- c('Unknown','Plumber','Professor','Unknown','Unknown')

list_test <- data.frame(firstname, city, state, job)

我想提取其中一列未知的名字和列名称。换句话说，我想要一个如下所示的表格:

  firstname  attribute
       Doug        job
        Tom       city
      Billy      state
      Billy        job
     Angelo       city
     Angelo      state
     Angelo        job

Answer 1

library(reshape2)
library(dplyr)
list_test%>%melt(id.var='firstname',variable.name='attribute')
          %>%filter(value=='Unknown')
          %>%select(-3)


firstname attribute
1       Tom      city
2    Angelo      city
3     Billy     state
4    Angelo     state
5      Doug       job
6     Billy       job
7    Angelo       job