Using the C++ Math Library

Deleting Elements from an Array

You can use indexing expressions to delete elements from an array. Deletion is a special case of using indexing expressions in assignment statements. Instead of assigning a new value to an element in an array, you assign the null array to a position in the array. The MATLAB C++ Math Library interprets that assignment as a deletion of the element and shrinks the array.

For example, to delete an element from example matrix A, you assign the null array to that element. You create a null array with the empty() function.

When you delete a single element from a matrix, the matrix is converted into a row vector that contains one fewer element than the original matrix. For example, when element (8) is deleted from matrix A

A(8) = empty();

matrix A becomes this row vector with element 8 missing:

1 2 3 4 5 6 7 9

You can also delete more than one element from a matrix, shrinking the matrix by that number of elements. To retain the rectangularity of the matrix, however, you must delete one or more entire rows or columns. For example,

A(2,colon()) = empty();

produces this rectangular result:

1 4 7
3 6 9

Note that the right side of an assignment statement that expresses a deletion is always a call to empty(). The left side of the assignment statement must be a valid indexing expression. The null array is applied to each element selected by the subscript.

Note    An N-dimensional subscript on the left side of the assignment statement can contain only one scalar, vector, or matrix index. The other indices used in deletion operations must be colon indices. For example, if an array is three-dimensional and you delete row 2, you must delete row 2 from all pages.

Similar to reference and assignment, two-dimensional deletion extends to N dimensions. If A has more than two dimensions, simply specify more than two dimensions as indices in the subscript.

Extending Two-Dimensional Assignment to N Dimensions

Indexing into Cell Arrays