Error using * Inner matrix dimensions must agree.

I have a problem that is already solved (the problem in the teacher's book) wrong, when I add * it gives me the following error:

"Matrix dimensions must agree."

I understand that it is about the size of the 2 matrices, what I don't understand ii how can I correct it?

The example statement is as follows:

Consider a sinusoidal signal having a frequency of 3 Hz. Write a program that will display the continuous signal and the samples when sampling with sampling frequencies of 100 Hz, 50 Hz, 10 Hz and 6 Hz.

In the model in the teacher's book there is the written version as a commentary.

clc
clear all
close all
F = 13;
T = 0.1;
n = (0:T:1);
xn = cos(2*pi*F*n);
t = linspace(-0.5, 1.5, 500);
% ya = sinc((1/T)*t(:,ones(size(n)))(1/T)*n(:,ones(size(t))))*xn;
ya = sinc((1/T).*t(:,ones(size(n)))*(1/T).*n(:,ones(size(t)))).*xn;

Arrays have incompatible sizes for this operation.

stem(n,xn,'r')
hold on
plot(t,ya)
grid on
xlabel('Timp, msec');
ylabel('Amplitudine');
axis([0 1 -1.2 1.2]);

[Torsten]

I don't recover the assignment in the code.

Answer 1

[Steven Lord]

F = 13;
T = 0.1;
n = (0:T:1);
xn = cos(2*pi*F*n);
t = linspace(-0.5, 1.5, 500);

Now let's look at the terms you're multiplying on the next line.

term1 = (1/T);
term2 = t(:,ones(size(n)))*(1/T);
term3 = n(:,ones(size(t)));
term4 = xn;

What are their sizes?

whos

  Name        Size             Bytes  Class     Attributes

  F           1x1                  8  double              
  T           1x1                  8  double              
  cmdout      1x33                66  char                
  n           1x11                88  double              
  t           1x500             4000  double              
  term1       1x1                  8  double              
  term2       1x11                88  double              
  term3       1x500             4000  double              
  term4       1x11                88  double              
  xn          1x11                88  double              

Elementwise multiplication of a scalar (term1) and an array (term2) is allowed. That product will be the same size as the array term2, namely 1-by-11.

What should be the size of the result of multiplying a 1-by-11 and a 1-by-500 array element-wise be? That operation is not defined mathematically, nor does it satisfy the requirements for implicit expansion to apply. See also this blog post from 2016 for more information on implicit expansion.

If that product in the sinc call returned a something-by-11 then implicit expansion would apply and ya would be something-by-11. But right now it's not defined. If you wanted that result to be 500-by-11, transposing term3 would work.

innerSection = term1.*term2.*term3.';
ya1 = sinc(innerSection).*term4;
whos innerSection ya1

  Name                Size            Bytes  Class     Attributes

  innerSection      500x11            44000  double              
  ya1               500x11            44000  double              

But as written in your original code, MATLAB cannot multiply term2 and term3.

ya = sinc(term1.*term2.*term3).*term4;

Arrays have incompatible sizes for this operation.

Answer 2

[Star Strider]

Change the ‘t’ assignment to:

t = linspace(-0.5, 1.5, numel(n));

and it works —

% clc
% clear all
% close all
F = 13;
T = 0.1;
n = (0:T:1);
xn = cos(2*pi*F*n);
t = linspace(-0.5, 1.5, numel(n));
% ya = sinc((1/T)*t(:,ones(size(n)))(1/T)*n(:,ones(size(t))))*xn;
ya = sinc((1/T).*t(:,ones(size(n)))*(1/T).*n(:,ones(size(t)))).*xn;
stem(n,xn,'r')
hold on
plot(t,ya)
grid on
xlabel('Timp, msec');
ylabel('Amplitudine');
axis([0 1 -1.2 1.2]);

.