# [Rcpp-devel] Help with SubMatrix test case

Douglas Bates bates at stat.wisc.edu
Sat Dec 4 15:32:42 CET 2010

```On Fri, Dec 3, 2010 at 11:00 PM, Christian Gunning <xian at unm.edu> wrote:
>> | The first three look correct to me:
>> | i)   assign 3, 4, 5
>> | ii)  copy row 0 as 1
>> | iii) copy col 3 to 4
>> |
>> | but I am also a little puzzled by the last submatrix.
>> |
>> | I'm sure Romain will weigh in in a few hours.
>
>> In the mean time, we can of course use the trusted Armadillo library and the
>>
>> R> src2 <- '
>> +      NumericMatrix xx(4, 5);
>> +      xx(0,0) = 3;
>> +      xx(0,1) = 4;
>> +      xx(0,2) = 5;
>> +      xx(1,_) = xx(0,_);
>> +      xx(_,3) = xx(_,2);
>> +      arma::mat mm( xx.begin(), 4, 5, false );
>> +      arma::mat sm = mm.submat(0,0,2,3);
>> +      return(wrap(sm));
>> +  '
>> R> myfun2 <- cxxfunction( , src2, plugin='RcppArmadillo')
>> R> aa2 <- myfun2()
>> R> aa2
>>     [,1] [,2] [,3] [,4]
>> [1,]    3    4    5    5
>> [2,]    3    4    5    5
>> [3,]    0    0    0    0
>> R>
>>
>> That looks more like it.
>>
>
> Interesting.  Here's a fully-formed example:
>
> src3 <- '
>    int rr = 4, cc = 5;
>    List ret;
>    NumericMatrix xx(rr, cc);
>    RNGScope scope;
>    NumericVector zz = rnorm(rr*cc);
>    for (int i = 0; i<rr*cc; i++) {
>        xx[i] = zz[i];
>    }

A minor point but I think it is more STLish to write that loop as

std::copy(zz.begin(), zz.end(), xx.begin());

For large matrices, calling xx[i] and zz[i] for each element could add

>    xx(1,_) = xx(0,_);
>    xx(_,1) = xx(_,0);
>
>    ret["fullmat"] = xx;
>    NumericMatrix yy1 = xx( Range(0,2), Range(0,3) ) ;
>    ret["submat"] = yy1;
>
>    NumericVector zz2 = xx( _, 1);
>    ret["col"] = zz2;
>
>    arma::mat mm( xx.begin(), rr, cc, false );
>    arma::mat sm = mm.submat(0,0,2,3);
>    ret["arma"] = wrap(sm);
>    return(ret);
> '
>
> myfun3 <- cxxfunction( , src3, plugin='RcppArmadillo')
> aa3 <- myfun3()
>
> identical(aa3\$col, aa3\$fullmat[,2])
> [1] TRUE
> identical(aa3\$arma, aa3\$fullmat[1:3,1:4])
> [1] TRUE
> identical(aa3\$submat, aa3\$arma)
> [1] FALSE
>
> -christian
>
> --
> A man, a plan, a cat, a ham, a yak, a yam, a hat, a canal – Panama!
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```