MATLAB: In an assignment A(:) = B, the number of elements in A and B must be the same.

Question

When trying to prove the results of convolution theorem analytically I have run into a deadwall.

The code that I am trying to run is

close all; clear all; clc;
t0=0;
tf=12;
N=5000;
dt=(tf-t0)/N;
t=t0:dt:tf;
for h=1:length(t);
    if t(h)<2
        x=0;
    else if t(h)<4
            x(h)=(-0.05/3).*(exp(-3*(t-2))-1);
        else
            x(h)=(-0.05/3).*((exp(-3.*(t-2))-exp(-3.*(t-4))));
        end;
    end;
end;
plot(t,x)
axis([t0,tf,0,0.1])
ylabel('x(t)')
xlabel('t')

The error I run into is the following

The ironic thing is if I do it this way

close all; clear all; clc;
t=0:0.0001:2;
x=0;
plot(t,x)
hold on;
t=2:0.0001:4;
x=(-0.05/3)*(exp(-3*(t-2))-1);
plot(t,x);
hold on;
t=4:0.0001:12;
x=(-0.05/3)*(exp(-3*(t-2))-exp(-3*(t-4)));
plot(t,x);
axis([0,12,0,0.05])
ylabel('x(t)')
xlabel('t')

It works and plots the following graph:

Can anyone help me understand where I am going wrong??

P.S: Please keep in mind that my knowledge of MATLAB is mediocre at best so try not to get carried away by using an unknown function.

Thanks in Advance 🙂

Answer 1

Subscript ‘t’ as ‘t(h)’ and it works:

t0=0;
tf=12;
N=5000;
dt=(tf-t0)/N;
t=t0:dt:tf;
for h=1:length(t);
    if t(h)<2
        x=0;
    else if t(h)<4
            x(h)=(-0.05/3).*(exp(-3*(t(h)-2))-1);
        else
            x(h)=(-0.05/3).*((exp(-3.*(t(h)-2))-exp(-3.*(t(h)-4))));
        end;
    end;
end;
plot(t,x)
axis([t0,tf,0,0.1])
ylabel('x(t)')
xlabel('t')

You can likely avoid the loop and use Vectorization (link).