submatrix(Matrix,VisibleList) -- select columns

Synopsis

• Function: submatrix
• Usage:
submatrix(f, cols)
• Inputs:
• f, , a map between free modules
• cols, , a list of integers denoting the choice of columns
• Outputs:
• , the submatrix of f corresponding to the columns cols

Description

Yields an r by c matrix, where r is the number of rows of f, and c is the length of the list of integers cols. The (i,j)-th entry of the result is f_(i, cols_j).

The list of integers, cols, may contain ranges and repetitions, as in {3, 5..7, 3:0}

If the list of column indices is a permutation of 0 .. n-1, where n is the number of columns, then the result is the corresponding permutation of the columns of f.

 i1 : R = ZZ/101[a .. o]; i2 : f = genericMatrix(R, a, 3, 5) o2 = | a d g j m | | b e h k n | | c f i l o | 3 5 o2 : Matrix R <--- R i3 : submatrix(f, {1..3,0}) o3 = | d g j a | | e h k b | | f i l c | 3 4 o3 : Matrix R <--- R