beginnings of a transpose() function

Author: eddelbuettel

ChangeLog

@@ -1,3 +1,8 @@
+2015-11-07  Dirk Eddelbuettel  <[email protected]>
+
+        * inst/include/Rcpp/vector/Matrix.h (Rcpp): Beginnings of a Matrix
+        transpose operator
+
 2015-11-06  Kevin Ushey <kevinushey@gmailcom>
 
         * inst/include/Rcpp/vector/Subsetter.h: Add sugar math operators

inst/NEWS.Rd

@@ -15,6 +15,8 @@
       by Tianqi, fixing \ghit{380})
       \item An overflow in Matrix column indexing was corrected (PR \ghpr{390}
       by Qiang, fixing a bug reported by Allessandro on the list)
+      \item Matrix classes now have a \code{transpose()} function (fixing
+      \ghit{383}) 
     }
     \item Changes in Rcpp Attributes:
     \itemize{

inst/include/Rcpp/vector/Matrix.h

@@ -161,6 +161,36 @@ class Matrix : public Vector<RTYPE, StoragePolicy>, public MatrixBase<RTYPE, tru
       return Sub( const_cast<Matrix&>(*this), row_range, Range(0,ncol()-1) ) ;
     }
 
+    Matrix transpose() {
+        typedef Vector<CHARSXP, StoragePolicy> CHARVECTOR;
+        int nrow = VECTOR::dims()[0], ncol = VECTOR::dims()[1];
+        R_xlen_t len = XLENGTH(*this);
+
+        Matrix r(Dimension(ncol, nrow));
+        R_xlen_t i, j, l_1 = len-1;
+
+        // similar approach as in R: fill by in column, "accessing row-wise"
+        VECTOR s = VECTOR(r.get__());
+        for (i = 0, j = 0; i < len; i++, j += nrow) {
+            if (j > l_1) j -= l_1;
+            s[i] = (*this)[j];
+        }
+
+        Rf_copyMostAttrib((*this), r);
+
+        // there must be a simpler, more C++-ish way for this ...
+        SEXP rnames = internal::DimNameProxy((*this), 0);
+        SEXP cnames = internal::DimNameProxy((*this), 1);
+        SEXP dimnames;
+        PROTECT(dimnames = Rf_allocVector(VECSXP, 2));
+        SET_VECTOR_ELT(dimnames, 0, cnames);
+        SET_VECTOR_ELT(dimnames, 1, rnames);
+        // do we need dimnamesnames ?
+        Rf_setAttrib(r, R_DimNamesSymbol, dimnames);
+        UNPROTECT(1); /* dimnames */
+
+        return r;
+    }
 
 private:
     inline R_xlen_t offset(const int i, const int j) const { return i + static_cast<R_xlen_t>(nrows) * j ; }