The SORTNDX subroutine creates an index to reorder a matrix by specified columns.
The SORTNDX subroutine returns the following value:
- index
-
is a vector such that index[
] is the row index of the th element of matrix when sorted according to by and descend. Consequently, matrix[index, ] is the sorted matrix.
The arguments to the SORTNDX subroutine are as follows:
- matrix
-
is the input matrix, which is not modified by the call.
- by
-
specifies the columns used to sort the matrix. The argument by is either a numeric matrix that contains column numbers, or a character matrix that contains the names of columns assigned to matrix by a MATTRIB statement or READ statement. If by is not specified, then the first column is used.
- descend
-
specifies which columns, if any, should be sorted in descending order. Any by columns not specified as descending will be ascending. If descend = by, then all by columns will be descending; if descend is skipped or is a null matrix, then all by columns will be ascending.
The SORTNDX subroutine can be used to process the rows of a matrix in a sorted order, without having to actually modify the matrix.
For example, the following statements return a vector that specifies the order of the rows in a matrix:
m = { 1 1 0,
2 0 0,
1 3 1,
2 2 2 };
call sortndx(ndx, m, {1 3}, 3);
print ndx;
The output is shown in Figure 23.324. The SORTNDX subroutine returns the vector ndx that indicates how rows of m will appear if you sort m in ascending order by column 1 and in descending order by column 3. The values of the vector ndx indicate that row 3 of m will be the first row in the sorted matrix. Row 1 of m will become the second row. Row 4 will become the third row, and row 2 will become the last row.
The matrix can be physically sorted with the SORT call), as follows:
sorted = m[ndx,];
The SORTNDX subroutine can be used with the UNIQUEBY function to extract the unique combinations of values in the by columns.