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Substitute Scalars with Matrices

Create the following expression representing the sine function.

syms w t
f = sin(w*t);

Suppose, your task involves creating a matrix whose elements are sine functions with angular velocities represented by a Toeplitz matrix. First, create a 4-by-4 Toeplitz matrix.

W = toeplitz(sym([3 2 1 0]))
W =
[ 3, 2, 1, 0]
[ 2, 3, 2, 1]
[ 1, 2, 3, 2]
[ 0, 1, 2, 3]

Next, replace the variable w in the expression f with the Toeplitz matrix W. When you replace a scalar in a symbolic expression with a matrix, subs expands the expression into a matrix. In this example, subs expands f = sin(w*t) into a 4-by-4 matrix whose elements are sin(w*t). Then it replaces w in that matrix with the corresponding elements of the Toeplitz matrix W.

F = subs(f, w, W)
F =
[ sin(3*t), sin(2*t),   sin(t),        0]
[ sin(2*t), sin(3*t), sin(2*t),   sin(t)]
[   sin(t), sin(2*t), sin(3*t), sin(2*t)]
[        0,   sin(t), sin(2*t), sin(3*t)]

Find the sum of these sine waves at t = π, t = π/2, t = π/3, t = π/4, t = π/5, and t = π/6. First, find the sum of all elements of matrix F. Here, the first call to sum returns a row vector containing sums of elements in each column. The second call to sum returns the sum of elements of that row vector.

S = sum(sum(F))
S =
6*sin(2*t) + 4*sin(3*t) + 4*sin(t)

Now, use subs to evaluate S for particular values of the variable t.

subs(S, t, sym(pi)./[1:6])
[ 0,...
  0,...
  5*3^(1/2), 4*2^(1/2) + 6,...
  2^(1/2)*(5 - 5^(1/2))^(1/2) + (5*2^(1/2)*(5^(1/2) + 5)^(1/2))/2,...
  3*3^(1/2) + 6]

You also can use subs to replace a scalar element of a matrix with another matrix. In this case, subs expands the matrix to accommodate new elements. For example, replace zero elements of the matrix F with a column vector [1;2]. The original 4-by-4 matrix F expands to an 8-by-4 matrix. The subs function duplicates each row of the original matrix, not only the rows containing zero elements.

F = subs(F, 0, [1;2])
F =
[ sin(3*t), sin(2*t),   sin(t),        1]
[ sin(3*t), sin(2*t),   sin(t),        2]
[ sin(2*t), sin(3*t), sin(2*t),   sin(t)]
[ sin(2*t), sin(3*t), sin(2*t),   sin(t)]
[   sin(t), sin(2*t), sin(3*t), sin(2*t)]
[   sin(t), sin(2*t), sin(3*t), sin(2*t)]
[        1,   sin(t), sin(2*t), sin(3*t)]
[        2,   sin(t), sin(2*t), sin(3*t)]