Create vector matrix of movie ratings using R project

Question

Suppose I am using this data set of movie ratings: http://www.grouplens.org/node/73

It contains ratings in a file formatted as userID::movieID::rating::timestamp

Given this, I want to construct a feature matrix in R project, where each row corresponds to a user and each column indicates the rating that the user gave to the movie (if any).

Example, if the data file contains

1::1::1::10
2::2::2::11
1::2::3::12
2::1::5::13
3::3::4::14

Then the output matrix would look like:

UserID, Movie1, Movie2, Movie3
1, 1, 3, NA
2, 5, 2, NA
3, NA, NA, 3

So is there some built-in way to achieve this in R project. I wrote a simple python script to do the same thing but I bet there are more efficient ways to accomplish this.

Answer 1

Why do'nt you try the dcast function, in the reshape2 package.

d <- read.delim(
  "u1.base", 
  col.names = c("user", "film", "rating", "timestamp")
)
library(reshape2)
d <- dcast( d, user ~ film, value.var = "rating" )

Just in case your fields are separated by double colons, you cannot use the sep argument of read.delim

So, you just have to read the file as a single column, split the strings, and concatenate the result.

d <- read.delim("a")
d <- as.character( d[,1] )   # vector of strings
d <- strsplit( d, "::" )     # List of vectors of strings of characters
d <- lapply( d, as.numeric ) # List of vectors of numbers
d <- do.call( rbind, d )     # Matrix
d <- as.data.frame( d )
colnames( d ) <- c( "user", "movie", "rating", "timestamp" )