在R中将数据框转换为矩阵

Question

我想将数据框转换为 R 中的矩阵，如下例所示：

df
row.index   column.index   matrix element
1           1              A
1           2              B
2           1              C
2           2              D

matrix
A  B
C  D

是否可以对行名做同样的事情？例如

df
row.name   column.name   matrix element
X          P             A
X          Q             B
Y          P             C
Y          Q             D

matrix
     P  Q

X    A  B
Y    C  D

感谢您的帮助！

Answer 1

我们可以使用tapply

tapply(df$matrixelement, df[1:2], FUN = I)

它也适用于第二个数据集

res <- tapply(df1$matrixelement, df1[1:2], FUN = I)
names(dimnames(res)) <- NULL
res
#   P   Q  
#X "A" "B"
#Y "C" "D"

---

如果我们需要data.frame，则可以使用dcast

library(reshape2)
dcast(df, row.index ~column.index)

数据

df <- structure(list(row.index = c(1L, 1L, 2L, 2L), column.index = c(1L, 
2L, 1L, 2L), matrixelement = c("A", "B", "C", "D")), .Names = c("row.index", 
"column.index", "matrixelement"), class = "data.frame", row.names = c(NA, 
-4L))

df1 <- structure(list(row.name = c("X", "X", "Y", "Y"), column.name = c("P", 
"Q", "P", "Q"), matrixelement = c("A", "B", "C", "D")), .Names = c("row.name", 
"column.name", "matrixelement"), class = "data.frame", row.names = c(NA, 
-4L))