Notes on Linear Algebra Part 3

As seen in Part 1 calling outer() on two vectors does indeed give the cross product (technically corresponding to tcrossprod()). This works because the defaults carry out multiplication.³ However, looking through the R source code for uses of outer(), the function should really be thought of in simple terms as creating all possible combinations of the two inputs. In that way it is similar to expand.grid(). Here are two illustrations of the flexibility of outer():

# generate a grid of x,y values modified by a function
# from ?colorRamp
m <- outer(1:20, 1:20, function(x,y) sin(sqrt(x*y)/3))
str(m)

 num [1:20, 1:20] 0.327 0.454 0.546 0.618 0.678 ...

# generate all combinations of month and year
# modified from ?outer; any function accepting 2 args can be used
outer(month.abb, 2000:2002, FUN = paste)

      [,1]       [,2]       [,3]      
 [1,] "Jan 2000" "Jan 2001" "Jan 2002"
 [2,] "Feb 2000" "Feb 2001" "Feb 2002"
 [3,] "Mar 2000" "Mar 2001" "Mar 2002"
 [4,] "Apr 2000" "Apr 2001" "Apr 2002"
 [5,] "May 2000" "May 2001" "May 2002"
 [6,] "Jun 2000" "Jun 2001" "Jun 2002"
 [7,] "Jul 2000" "Jul 2001" "Jul 2002"
 [8,] "Aug 2000" "Aug 2001" "Aug 2002"
 [9,] "Sep 2000" "Sep 2001" "Sep 2002"
[10,] "Oct 2000" "Oct 2001" "Oct 2002"
[11,] "Nov 2000" "Nov 2001" "Nov 2002"
[12,] "Dec 2000" "Dec 2001" "Dec 2002"

Bottom line: outer() can be used for linear algebra but its main uses lie elsewhere. You don’t need it for linear algebra!

Here’s an interesting connection discussed in this Wikipedia entry. In Part 1 we demonstrated how the repeated application of the dot product underpins matrix multiplication. The first row of the first matrix is multiplied element-wise by the first column of the second matrix, shown in red, to give the first element of the answer matrix. This process is then repeated so that every row (first matrix) has been multiplied by every column (second matrix).

If instead, we treat the first column of the first matrix as a column vector and cross multiply it by the first row of the second matrix as a row vector, we get the following matrix:

Now if you repeat this process for the second column of the first matrix and the second row of the second matrix, you get another matrix. And if you do it one more time using the third column/third row, you get a third matrix. If you then add these three matrices together, you get as seen in Equation 1. Notice how each element in in Equation 1 is a sum of three terms? Each of those terms comes from one of the three matrices just described.

To sum up, one can use the dot product on each row (first matrix) by each column (second matrix) to get the answer, or you can use the cross product on the columns sequentially (first matrix) by rows sequentially (second matrix) to get several matrices, which one then sums to get the answer. It’s pretty clear which option is less work and easier to follow, but I think it’s an interesting connection between operations.