Month: October 2011

Different behaviors between data frame and matrix in R

> # 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