R Exercises | Programming Exercises for Finance in the R Language

Matrix Algebra: Exercises

3. Matrix multiplication. Four matrices have been defined for you: A, B, C, and D. Use dim() to calculate the dimensions of each matrix.

After you have identified each matrix's dimensions, post-multiply A by the matrix that has conforming dimensions. After that, pre-multiply A by the matrix that has conforming dimensions. You can use the operator %*% to perform matrix multiplication in R.


                                  A <- matrix(1:30, nrow = 6, ncol = 5)
                                  B <- matrix(1:30, nrow = 5, ncol = 4)
                                  C <- matrix(1:30, nrow = 3, ncol = 6)
                                  D <- matrix(1:21, nrow = 3, ncol = 7)



                              # Print dimensions of A.
                              print(dim(A))

                              # Print dimensions of B.
                              print(dim(___))

                              # Print dimensions of C.
                              print(___)

                              # Print dimensions of D.
                              ___

                              # Post-multiply A by the conforming matrix.
                              A %*% ___

                              # Pre-multiply A by the conforming matrix.
                              ___ %*% A



                                # Print dimensions of A.
                                print(dim(A))

                                # Print dimensions of B.
                                print(dim(B))

                                # Print dimensions of C.
                                print(dim(C))

                                # Print dimensions of D.
                                print(dim(D))

                                # Post-multiply A by the conforming matrix.
                                A %*% B

                                # Pre-multiply A by the conforming matrix.
                                C %*% A


                              test_error()
                              success_msg("Good work. What has to be true about two matrices for the conformability requirement to be satisfied for matrix multiplication?")

Recall that the matrix() function can specify a number of rows and a number of columns.

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