sort rows

N = 3
M = 5
P = matrix(rbinom(N*M,10,.5), M)

## method 1: eval
cmd = paste0("order(", paste0("P[,", 1:ncol(P), "]", collapse=","), ",decreasing=T)")
order_P = eval(parse(text = cmd))
P2 = P[order_P,] # 你可以全部放到cmd去，無所謂

## method 2: quick sort with sort rows
library(Rcpp)
library(RcppArmadillo)

sourceCpp(code = '
// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadillo.h>
using namespace Rcpp;
using namespace arma;

inline int compare_vec(const rowvec mat_row, const rowvec pivot_row)
{
    int v = 0;
    for (uword i = 0; i < mat_row.n_elem; i++)
    {
        if (mat_row(i) < pivot_row(i))
            v = 1;
        else if (mat_row(i) > pivot_row(i))
            v = -1;
        if (v != 0)
            break;
    }
    return v;
}

void sortrows_f(mat& M, const int left, const int right)
{
    if (left < right)
    {
        int i = left, j = right;
        uword mid_loc = (uword) (left+right)/2, pivot_loc = mid_loc;
        rowvec pivot_row = M.row(pivot_loc);
        while (i <= j)
        {
            while (compare_vec(M.row( (uword) i), pivot_row) == 1)
                i++;
            while (compare_vec(M.row( (uword) j), pivot_row) == -1)
                j--;
            if (i <= j)
            {
                M.swap_rows((uword) i, (uword) j);
                i++;
                j--;
            }
        }
        if (j > 0)
            sortrows_f(M, left, j);
        if (i < (int) M.n_rows - 1)
            sortrows_f(M, i, right);
    }
}

// [[Rcpp::export]]
mat sortbyrow(mat M, bool decreasing = true){
  sortrows_f(M, (int) 0, (int) M.n_rows-1);
  if (decreasing)
    return flipud(M);
  return M;
}')
P3 = sortbyrow(P)
all.equal(P2, P3)