WOLFRAM

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Get Parts of a Matrix

Pick out and extract parts of matrices.

Create a matrix

Use Table to set up a 5×5 matrix:

Wolfram Language code: mat = Table[Subscript[m, i, j], {i, 5}, {j, 5}]

Use MatrixForm to format the output as a regular array:

Wolfram Language code: MatrixForm[mat]
  • Matrices in the Wolfram Language are not restricted to number entries.

Extract a single part

Use Part to pick the second element of the first row:

Wolfram Language code: Part[mat, 1, 2]
  • Part has the short form notation [[... ]].

Extract a row

To take an entire row, use one index to specify the row:

Wolfram Language code: mat[[4]]

Extract a column

To take an entire column, select all the rows and specify the column:

Wolfram Language code: mat[[All, 4]]

Extract a submatrix

Use Span to specify the relevant rows and columns that make up a submatrix:

Wolfram Language code: mat[[1 ;; 2, 1 ;; 3]]//MatrixForm

Extract all elements except the outermost rows and columns using negative indices:

Wolfram Language code: mat[[2 ;; -2, 2 ;; -2]]//MatrixForm
  • Negative indices count from the end of a list.