title: Cannot get ratings from Recommenderlab for sparse binary ratings matrix
body: <p>The data used is a ratings matrix generated from simple 0-1 yes/no click data based on whether or not a user visited a section of a website. This is implicit voting since if a user is interested in a section he will click it and thus rate it as a "1" and if not rate it as a "0". The ratings matrix thus has 32711 users in the rows and 285 site sections in the columns, after removing 9 that had no user visits.</p>
  
  <p>I run:</p>
  
  <pre><code>library("recommenderlab")
  ratingmatrix &lt;- read.csv("ratingmatrix3.txt",",",header=TRUE, row.names=1 )
  ratingmatrix &lt;- as.matrix(ratingmatrix)
  ratingmatrix &lt;- as(ratingmatrix, "realRatingMatrix")
  
  Rec1 &lt;- Recommender(ratingmatrix[1:1000], method="POPULAR")
  pred1 &lt;- predict(Rec1, ratingmatrix[1001:1011], type="ratings")
  as(pred1,"matrix")[,1:10]
  </code></pre>
  
  <p>I only get a prediction matrix with NAs implying that no predictions were made. It is the same with any other method; above POPULAR is used.</p>
  
  <p>Does recommenderlab only deal with denser matrices? Does anyone have any ideas on how to deal with matrices with 0-1 votes, and no NAs? Thanks.</p>
  
  <p>Output:</p>
  
  <pre><code>      South.Africa Site.Builder.Workshop Netherlands..Holland. Europe
  11001           NA                    NA                    NA     NA
  11002           NA                    NA                    NA     NA
  11003           NA                    NA                    NA     NA
  11004           NA                    NA                    NA     NA
  11005           NA                    NA                    NA     NA
  11006           NA                    NA                    NA     NA
  11007           NA                    NA                    NA     NA
  11008           NA                    NA                    NA     NA
  11009           NA                    NA                    NA     NA
  11010           NA                    NA                    NA     NA
  11011           NA                    NA                    NA     NA
        Microsoft.Solution.Providers Microsoft.Financial.Forum For.Developers.Only.News
  11001                           NA                        NA                       NA
  11002                           NA                        NA                       NA
  11003                           NA                        NA                       NA
  11004                           NA                        NA                       NA
  11005                           NA                        NA                       NA
  11006                           NA                        NA                       NA
  11007                           NA                        NA                       NA
  11008                           NA                        NA                       NA
  11009                           NA                        NA                       NA
  11010                           NA                        NA                       NA
  11011                           NA                        NA                       NA
        Visual.Fox.Pro.Support Educational.Services...Programs Chile
  11001                     NA                              NA    NA
  11002                     NA                              NA    NA
  11003                     NA                              NA    NA
  11004                     NA                              NA    NA
  11005                     NA                              NA    NA
  11006                     NA                              NA    NA
  11007                     NA                              NA    NA
  11008                     NA                              NA    NA
  11009                     NA                              NA    NA
  11010                     NA                              NA    NA
  11011                     NA                              NA    NA
  </code></pre>