4.7: Flattening and Deleting Parts of a Matrix

Flattening a Matrix

Sometimes we want to treat all elements of a matrix as one long column. In MATLAB, we can flatten a matrix using a colon inside parentheses.

Example \(\PageIndex{1}\)

myArray = [1, 2; 3, 7];
myArray(:)

Solution

ans =

    1
    3
    2
    7

MATLAB reads the matrix column by column when it flattens the matrix. This is important because many MATLAB functions use column-major order internally.

Note

Why does MATLAB flatten column-wise?

MATLAB stores arrays in memory by columns. That means it works down the first column, then down the second column, and so on.

Deleting Rows and Columns

You can delete an entire row or column by assigning an empty array, [], to that row or column.

Example \(\PageIndex{2}\)

Deleting a row and column.

sampleMatrix = [1, 2, 3; 5, 7, 11; 13, 17, 19];

sampleMatrix(2,:) = [] % delete second row
sampleMatrix(:,2) = [] % delete second column from the remaining matrix

Solution

sampleMatrix =

    1 2 3
    13 17 19


sampleMatrix =

    1 3
    13 19

However, you cannot delete a single element from the middle of a 2D array, because that would leave a hole in the rectangular shape of the matrix.

sampleMatrix(2,2) = []   % this will not work