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

Hi all,

I am very new to Matlab and CVX. I have only been using them for the last few months and my background is in engineering not mathematics. I am having the following problem. I am trying to minimise the following objective function:

sum(sum_square(p1(:,2:30)-p1(:,1:29)))

Based on the requirements of the following equations:

for i = 1:29

    V = 900;
    CL(i) = (W(i).*g)/(0.5*density*(V^2)*S);
    CD(i) = CDo + ((CL(i).^2)/(3.14*AR*e));
    T = ((CD(i)/CL(i))*(W(i)*g))/1000;                      %convert to KN
    cp(i) = {p1(1,i), p1(2,i); p1(1,i+1), p1(2,i+1)};
    disp(i) = pdist(cp(i),'euclidean')*1000;                 %convert to m    
    Wf(i) = SFC*T*(disp(i)/V);
    W(i+1) = W(i)-Wf(i);
end

Given variables: W, g, V, S, CDo, SFC, density

When I run this I get the following error:

Error using cvx/subsasgn (line 39)

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

Error in fueloptimal (line 79) cp(i) = {p1(1,i), p1(2,i); p1(1,i+1), p1(2,i+1)};

Can anybody help me with this problem?

Thank you in advance.

Answer 1

[Image Analyst]

In this line

cp(i) = {p1(1,i), p1(2,i); p1(1,i+1), p1(2,i+1)};

you're setting cp(i) to a cell. That's what the braces mean - it's a cell. Inside the cell you can have an array, a string, a structure, another cell, just about anything. See the FAQ for a good intuitive explanation: http://matlab.wikia.com/wiki/FAQ#What_is_a_cell_array.3F

But you can't have p1(1,i), p1(2,i); p1(1,i+1), p1(2,i+1) inside a cell because that's not an array. Those values need to be enclosed by brackets to turn them into an array. And once they're an array, THEN you can stick them into a cell and make that the i'th cell of the cell array called "cp". Understand? You will if you read the FAQ.

I'll do it in two steps just to be explicit:

thisArray = [p1(1,i), p1(2,i); p1(1,i+1), p1(2,i+1)];
cp(i) = {thisArray};  % Equivalent to cp{i} = thisArray;

[Zena Assaad]

I have tried what you mentioned but I get the following error:

Undefined function 'sparse' for input arguments of type 'cell'.

Error in fueloptimal (line 80)

    cp(i) = {thisArray};
    % Equivalent to cp{i} = thisArray;

[Image Analyst]

Try preallocating cp before the for loop:

cp = cell(1, 29);

[Zena Assaad]

I pre-allocated cp but then I get the following error:

Conversion to double from cell is not possible.

Error in pdist (line 142) X = double(X);

Error in fueloptimal (line 82)

    disp(i) = pdist(cp(i),'euclidean')*1000;
    %convert to m

[Image Analyst]

If cp is a cell, you need to get the contents of the cell, not the cell itself. So use cp{i}, not cp(i). See the FAQ: http://matlab.wikia.com/wiki/FAQ#What_is_a_cell_array.3F

[Zena Assaad]

Firstly thank you for taking the time to help me. I am very new to Matlab and as I said my background is in engineering. I read the FAQ and changed cp(i) to cp{i} but I was still left with the same error. Do you think the entire equation is wrong or am I just making errors in notation?

[Image Analyst]

What is CVX? I don't have enough information or code to run this, so until we have that we can't help.

Nevermind - found it: http://cvxr.com/cvx/

I'm not going to download that and install it, so just give nominal values for all those variables you have so we have something that will at least run without saying "W undefined", etc.

[Zena Assaad]

W(1) = 77000;

g = 9.81;

SFC = 0.94;

g = 9.81;

m = 77000;

S = 122.60;

CDo = 0.024;

Cd2 = 0.0375;

b = 34.10;

e = 0.8;

AR = (b^2)/S;

density = 0.3494;

These are all the known values defined before I run the above for loop. I do intend to make velocity a variable at a later stage with the equation being: V = sqrt(2*g*displacement). I have a 3x30 matrix (p1) that defines the three dimensional position at different intervals (row 1 = x, row 2 = y, row 3 = z). The displacement will be taken from that matrix by finding the Euclidean distance between consecutive x and y points (row 1 and row 2).