在R中，是否有一种方法可以将一个数据帧中的列与另一数据帧中的值重新编码？

debugcn 发表于 Dev

格兰特

我对使用R还是比较陌生，我不确定如何解决此问题。任何帮助或建议，不胜感激！！！

我的问题是我正在使用两个数据帧，并且需要使用第二个数据帧中的值重新编码第一个数据帧。第一个数据帧（df1）包含从受访者到调查的数据，另一个数据帧（df2）是df1的数据字典。

数据如下所示：

df1 <-  data.frame(a = c(1,2,3), 
            b = c(4,5,6), 
            c = c(7,8,9))

df2 <- data.frame(columnIndicator = c("a","a","a","b","b","b","c","c","c" ),
              df1_value = c(1,2,3,4,5,6,7,8,9),
              new_value = c("a1","a2","a3","b1","b2","b3","c1","c2","c3"))

到目前为止，我可以通过执行以下操作手动重新编码df1以获得预期的输出：

df1 <- within(df1,{
  a[a==1] <- "a1"
  a[a==2] <- "a2"
  a[a==3] <- "a3"
  b[b==4] <- "b4"
  b[b==5] <- "b5"
  b[b==6] <- "b6"
  c[c==7] <- "c7"
  c[c==8] <- "c8"
  c[c==9] <- "c9"
})

但是，我的真实数据集大约有42列需要重新编码，而该方法需要花费一些时间。R中还有另一种方法可以将df1中的值重新编码为df2中的值吗？

谢谢！

张Zhang

只需将形状变形一下即可。

library(data.table)
df1 <-  data.frame(a = c(1,2,3), 
                   b = c(4,5,6), 
                   c = c(7,8,9))

df2 <- data.frame(columnIndicator = c("a","a","a","b","b","b","c","c","c" ),
                  df1_value = c(1,2,3,4,5,6,7,8,9),
                  new_value = c("a1","a2","a3","b4","b5","b6","c7","c8","c9"),stringsAsFactors = FALSE)



setDT(df1)
setDT(df2)

df1[,ID:=.I]

ldf1 <- melt(df1,measure.vars = c("a","b","c"),variable.name = "columnIndicator",value.name = "df1_value")


ldf1[df2,"new_value":=i.new_value,on=.(columnIndicator,df1_value)]

ldf1
#>    ID columnIndicator df1_value new_value
#> 1:  1               a         1        a1
#> 2:  2               a         2        a2
#> 3:  3               a         3        a3
#> 4:  1               b         4        b4
#> 5:  2               b         5        b5
#> 6:  3               b         6        b6
#> 7:  1               c         7        c7
#> 8:  2               c         8        c8
#> 9:  3               c         9        c9

dcast(ldf1,ID~columnIndicator,value.var = "new_value")
#>    ID  a  b  c
#> 1:  1 a1 b4 c7
#> 2:  2 a2 b5 c8
#> 3:  3 a3 b6 c9

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