MATLAB plotting error

Question

I am trying to plot the following function:

enter image description here

The time interval is to be from 0 to 5 seconds using a step size of 0.002. This is what I have so far...

t = 0:0.002:5;
k = [2; 4; 6; 8];
i = (1/pi) + 0.5*sin(4*t) - (2/pi)*sum((cos(4*k*t))/(k*k-1));
plot(t,i)

It gives me the error:

Error using  * 
Inner matrix dimensions must agree.

Error in lab1_5 (line 4)
i = (1/pi) + 0.5*sin(4*t) -
(2/pi)*sum((cos(4*k*t))/(k*k-1));

I then tried every use of './' and '.*' but it still gives me the same error. What am I doing wrong?

herohuyongtao · Accepted Answer · 2014-01-20 18:20:55Z

1

Change your code to:

t = 0:0.002:5;
k = [2; 4; 6; 8];

i = (1/pi) + 0.5*sin(4*t)
for j=1:4
    i = i-(2/pi)*((cos(4*k(j)*t))/(k(j)*k(j)-1));
end
plot(t,i)

The reason is that k is a col-vector (or a 4x1 matrix), so you cannot simply multiply it by k*k. For matrix multiplication, the size of left and right must follow n × m and m × p. In this case, you need to loop multiply every elements of k.

edited Jan 20, 2014 at 18:20

answered Jan 20, 2014 at 17:55

herohuyongtao

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4 Comments

John Over a year ago

You are not handling the multiple indexing at all in your answer. k'*k is a constant! That whacks the sum.

user3216081 Over a year ago

Ah that's it, thank you. Just wondering, if k is a row vector, what then should I do?

John Over a year ago

You're getting closer. The size of your summands is 1,1, not 1,2501 which is how you're treating it. You don't need the "sum" term, that's whacking your results (still)

herohuyongtao Over a year ago

@John Yeah, too careless for me. Fixed and thanks. :)

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