C ++ Armadillo：最近的邻居

Question

I'm using C++ with the Armadillo library.

Suppose I have anx 1 column matrix which is sorted in numerical order. For example

mat X; X.randn(100,1);
mat X_sorted; X_sorted = sort(X);
cout << X_sorted << endl;

and suppose I have a variable

double y = 0.5;

What I want: is a way of finding the index, z , of x_sorted , for which x_sorted(z) is closest to y. In the case of a tie (which doesn't actually matter in my case), simply choose the larger one.

Answer 1

One way of doing it is like this:

int z = as_scalar(sort_index(abs(X_sorted - y)).row(0));

Please feel free to criticise this solution and suggest an improvement.

Here's an example program of it in action:

int main(int argc, char** argv)
{
using namespace arma;
using namespace std;

mat X; X.randn(100,1);
mat X2; X2.zeros(100,1);
for(int i=0; i<100; i++){X2(i) = i;}
mat X_sorted; X_sorted = sort(X);

mat XX; XX=join_rows(X2,X_sorted);
cout << XX << endl;

double y = 0.5;
int z = as_scalar(sort_index(abs(X_sorted - y)).row(0));
mat XX_z; XX_z = XX.row(z);
cout << XX_z << endl;

return 0;
}