Nr=nonzeros(union(Nr,Nk));

This line of code is used in MATLAB to create a new array that contains all unique non-zero elements from two existing arrays, Nr and Nk.

The function union() is used to find the union of two arrays, which means it returns a new array that contains all elements from both arrays, with no duplicates.

The function nonzeros() is used to create a new array that contains only the non-zero elements from an existing array.

Therefore, by using union(Nr,Nk) we get a new array that contains all unique elements from Nr and Nk, and then by using nonzeros() we get a new array that contains only the non-zero elements from the union of Nr and Nk. Finally, this new array is assigned to the variable Nr, effectively updating its value.