Modern R Programming Cookbook

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Once you have created the matrix, then you can do matrix operations that are mathematically applicable, such as, matrix addition, subtraction, multiplication, inverse calculation, and many more. Matrix addition and subtraction are very much like the addition and subtraction of two numbers, but both matrices must have the same number of rows and columns. In other words, you cannot add or subtract two matrices if their number of rows and/or columns differ. From this recipe, matA and matB are not mathematically conformable for addition or subtraction, but matB and matE are conformable to do that operation:

    matADD <- matB + matE

To multiply one matrix by another matrix, the number of columns of the first matrix must be the same ...