Question

The following example appears in the MATLAB tutorial:

X = [16  2 13;      5  11  8;      9   7 12;      4  14  1] 

Using a single subscript deletes a single element, or sequence of elements, and reshapes the remaining elements into a row vector. So:

X(2:2:10) = [] 

results in:

X = [16 9 2 7 13 12 1] 

Mysteriously, the entire 2nd row and the first two elements in the 4th row have been deleted, but I can’t see the correspondence between the position of the deleted elements and the index vector 2:2:10. Can someone please explain?

Answer 1

The example you gave shows linear indexing. When you have a multidimensional array and you give it a single scalar or vector, it indexes along each column from top to bottom and left to right. Here’s an example of indexing into each dimension:

mat = [1 4 7; ...        2 5 8; ...        3 6 9]; submat = mat(1:2, 1:2); 

submat will contain the top left corner of the matrix: [1 4; 2 5]. This is because the first 1:2 in the subindex accesses the first dimension (rows) and the second 1:2 accesses the second dimension (columns), extracting a 2-by-2 square. If you don’t supply an index for each dimension, separated by commas, but instead just one index, MATLAB will index into the matrix as though it were one big column vector:

submat = mat(3, 3);     % 'Normal' indexing: extracts element '9' submat = mat(9);        % Linear indexing: also extracts element '9' submat = mat([1 5 6]);  % Extracts elements '1', '5', and '6' 

See the MATLAB documentation for more detail.