occidentally Different behaviors between data frame and matrix in R

http://musicaludi.fr/?karmanik=site-de-rencontre-jeune-gratuit-34&fd5=2f > # Generate an artificial matrix > test.m <- matrix(1:6, nrow = 3) > row.names(test.m) <- c('x1', 'x2', 'x3') > col.names(test.m) <- c('a', 'b') Error in col.names(test.m) <- c("a", "b") : could not find function "col.names<-" > > # Generate a data frame from the matrix > test.df <- as.data.frame(test.m) > > > # Selecting elements > ## the row names can be used to select elemnts from a data frame or a matrix > test.idx <- c('x3', 'x1') > test.df[test.idx, ] V1 V2 x3 3 6 x1 1 4 > test.m[test.idx, ] [,1] [,2] x3 3 6 x1 1 4 > > # Selecting elements with index having a name which is not in the data > ## data frame returns NA rows > ## matrix returns an error > test.idx <- c('x4', 'x1') > test.df[test.idx, ] V1 V2 NA NA NA x1 1 4 > test.m[test.idx, ] Error: subscript out of bounds > > > # Duplicate row names > ## duplicate row names are not allowed in data frame. > ## duplicate row names are allowed in matrix. > test.row.names <- c('x1', 'x2', 'x1') > row.names(test.df) <- test.row.names Error in `row.names<-.data.frame`(`*tmp*`, value = c("x1", "x2", "x1")) : duplicate 'row.names' are not allowed In addition: Warning message: non-unique value when setting 'row.names': ‘x1’ > rownames(test.m) <- test.row.names > > # names > ## names() returns column name in data frame > ## names() returns NULL in matrix > names(test.df) [1] "V1" "V2" > names(test.m) NULL