How can i correct the subscript of a vector in a for loop ?

Hi

I want to plot X=f(wr) but i have this error

Subscript indices must either be real positive integers or logicals.

Knowing that mr,damping,kr are matrix(40*40)

And my program is :

Damping=(2*zeta*kr)/(6600*pi);

F=[zeros(39,1);ones(1,1)];

IIw=linspace(5,10,40);

wr=IIw*2*pi;

for w=wr(1):wr(40)

      X(w)=(-w.^2*mr + j*w*Damping + kr)\ F;

end

plot(wr,abs(X))

Answer 1

[Marc Jakobi]

Your problem is that you are inxexing X with w, which may be a decimal.

To correct it, use:

ind = 0;
for w=wr(1):wr(40)
    ind = ind+1;
    X(ind)=(-w.^2*mr + j*w*Damping + kr)\ F; %%you were indexing X(w)
end

[Mallouli Marwa]

I still have error :

In an assignment A(I) = B, the number of elements in B and I must be

the same.

Please help me

[Marc Jakobi]

That means somewhere you are trying to add, subtract, multiply or divide two or more matrices with different dimensions.

Keep in mind, you have to multiply with ".*" and divide with "./"

"*" and "/" are matrix operations.

[Mallouli Marwa]

mr, Damping and kr are matrix. Only w is a vector. Please help me.

[Marc Jakobi]

w is not a vector.

since

IIw=linspace(5,10,40);
wr=IIw*2*pi;

wr is a 1x40 vector. and

for w = wr(1):wr(40)

w is always a scalar. Maybe it would be easier to help you if you tell us what problem you are trying to solve. And what would you like to plot? (Just one single line or more than one?)

Answer 2

[Walter Roberson]

In your code

(-w.^2*mr + j*w*Damping + kr)\ F

your matrices in () are 40 x 40, and your F is 40 x 1. The \ operator requires that the number of rows match on the left and right (which it does in this case, both have 40 rows), and it will return a column vector with as many entries as there are columns on the left. You have 40 columns on the left so you get a column vector of length 40. And then you try to store that into the single location X(w) or X(ind) .

I suggest

num_w = length(wr);
X = zeros(size(mr,2), num_w);
for ind = 1 : num_w
  w = wr(ind);
  X(:, ind) = (-w.^2*mr + j*w*Damping + kr)\ F;
end

plot(wr, abs(X))

Note: along the way I corrected your code to loop over all of the values present in wr. Your code was not doing that: it was doing

for w = [wr(1), wr(1)+1, wr(1)+2, wr(1)+3, ... wr(40)]

which is to say, it was taking values 1 apart starting from wr(1) until wr(40).

The notation wr(1):wr(40) does not mean "all the values in wr from index 1 to 40", it means the same as wr(1):1:wr(40) which is taking values separated by 1 starting from wr(1) and ending no greater than wr(40)

for w = wr

would use all of the values in wr, but it would not be in a form suitable for using as an index into the output array.

[Mallouli Marwa]

Hi,

I really thank you for your answer.

But, I want to obtain a value of X (Tha's mean X is a real) for

each value of wr.

Please can you help me and correct my program.

[Walter Roberson]

My suggested changes do obtain the value of X for each value of wr. The values are stored in columns of X, with column K of X corresponding to the K'th value in wr.