[Rcpp-devel] a variable for loop

[Avraham Adler]

On Wed, Dec 14, 2016 at 1:24 AM Amina Shahzadi <aminashahzadi at gmail.com>
wrote:

> Hello Avraham --Happy to see you
>
> My code is trying to produce a cube c which is going to be constructed by
> a vector a and matrix b.
> And the number of rows in b and size of a must be same.
>
> So we can assume that if a is a vector of size 3, Then b must be 2 x 3 or
> 3 X 3 etc.
>
> Thank you Avraham for quick response. I hope this will make my question
> more clear.
>
> Best regards
>
>
> On Wed, Dec 14, 2016 at 4:46 PM, Avraham Adler <avraham.adler at gmail.com>
> wrote:
>
> On Tue, Dec 13, 2016 at 9:51 PM, Amina Shahzadi <aminashahzadi at gmail.com>
> wrote:
>
> Hello Friends and Prof. Dirk
>
> I am pasting here a code which has a for loop depending on another for
> loop.
> I am getting zeros for cube c. I tried and searched a lot but did not get
> an example of this type. Would you please help in this regard?
>
>
> #include <RcppArmadillo.h>
> using namespace Rcpp;
> using namespace RcppArmadillo;
> using namespace arma;
> //[[Rcpp::depends(RcppArmadillo)]]
> //[[Rcpp::export]]
>
>
> arma::cube  exam(arma::vec a,  arma::mat b)
> {
>   int m = a.size();
>   int n = b.n_rows;
>   arma::cube c = zeros<cube>(n, m, n);
>   for(int i=0; i<n; i++) {
>     for(int j=0; j<m; j++) {
>       for(int k=0; k<i; k++) {
>           if(k==0) {
>             c(i, j ,k) = c(i, j, k) + b(i, j);
>           }
>           else{
>             c(i, j, k) = c(i, j, k) +  c(i-1, j, k) *b(i, j);
>           }
>          }
>         }
>       }
>   return c;
> }
>
>
> Thank You
> --
> *Amina Shahzadi*
>
>
>
>
> Hello.
>
> I haven't run your code, but it strikes me
>
> that I cannot see where are you capturing the number of columns of b.
>
> It's a bit confusing as I was always taught a matrix has m rows and n
>
> columns. Be that as it may, your k==0 loop looks like it's trying to
>
> copy over the original matrix to the first slice, but how do you know
>
> that b has m columns, which is what you're assuming by letting j loop to
>
> m. Unless you are assuming a square matrix?
>
> Even if you are, if your matrix is not the same length as your vector, I
> think there is an issue with your loop boundaries, unless I've
> misunderstood something, which is certainly possible.
>
> For example, assume a is {1, 2, 3} and b is the 2 x 2 of row 1: [1 2] and
> row 2: [3 4]. Thus m = 3 and n = 2.
>
> Step 1: i = j = k = 0: c(0, 0, 0) becomes b(0, 0) or 1.
>
> Step 2: i = 0, j = 1, k = 0: c(0, 1, 0) becomes b(0, 1) or 2.
>
> Step 3: i = 0, j = 2, k = 0: c(0, 2, 0) becomes b(0, 2) ?!?! There is no
> b(0, 2), it's only a 2x2 matrix?
>
>
> Similar to your previous questions, instead of posting code, can you
> please describe in words what it is you are trying to do? That may help.
>
> Avi
>
>
>
>
>
>
> --
> *Amina Shahzadi*
>
>
> Constructed how? Can you provide a simple set of inputs and the expected
output?

Avi
-- 
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