Hi, I used the index_sort in RcppArmadillo and found it seems return a wrong result.</

Try this: don't return a List if you don't need it, return a v

index_sort gives us the wrong results about rcpparmadillo HOT 3 CLOSED

rcppcore commented on June 12, 2024

index_sort gives us the wrong results

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Comments (3)

eddelbuettel commented on June 12, 2024 1

Here is a simpler version of your code, with a repair to the R part -- you want order(), not rank():

#include <RcppArmadillo.h>

// [[Rcpp::depends(RcppArmadillo)]]

// [[Rcpp::export()]]
arma::vec cRank(arma::vec x) {
    arma::uvec rankarma  = arma::stable_sort_index(x);
    arma::vec z = arma::conv_to<arma::vec>::from(rankarma);
    return z;
}

/*** R
set.seed(123)  # reproducible
tmp <- rnorm(10)
tmp
armaind <- cRank(tmp)
rind <- order(tmp)
cbind(armaind, rind)
tmp[ armaind + 1 ]
tmp[ rind ]
*/

When I run this, the right thing happens (but note the needed +1):

R>  Rcpp::sourceCpp("/tmp/chongwu.cpp")

R> set.seed(123)  # reproducible

R> tmp <- rnorm(10)

R> tmp
 [1] -0.5604756 -0.2301775  1.5587083  0.0705084  0.1292877  1.7150650  0.4609162 -1.2650612 -0.6868529 -0.4456620

R> armaind <- cRank(tmp)

R> rind <- order(tmp)

R> cbind(armaind, rind)
        rind
 [1,] 7    8
 [2,] 8    9
 [3,] 0    1
 [4,] 9   10
 [5,] 1    2
 [6,] 3    4
 [7,] 4    5
 [8,] 6    7
 [9,] 2    3
[10,] 5    6

R> tmp[ armaind + 1 ]
 [1] -1.2650612 -0.6868529 -0.5604756 -0.4456620 -0.2301775  0.0705084  0.1292877  0.4609162  1.5587083  1.7150650

R> tmp[ rind ]
 [1] -1.2650612 -0.6868529 -0.5604756 -0.4456620 -0.2301775  0.0705084  0.1292877  0.4609162  1.5587083  1.7150650
R>

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eddelbuettel commented on June 12, 2024

Try this:

don't return a List if you don't need it, return a vector directory
important: Armadillo gives you uvec, but R only has signed vectors
so convert rankarma to either ivec (integer) ir just vec (numeric)
I think you are just getting a bad cast.

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