Why is my code a matrix and not a double

Why is E_n a 4x4 matrix? I want it to be a single value.

Initial Conditions:

M_0 = [0, 30, 90, 135, 180, 200, 350;
       0, 30, 90, 135, 180, 200, 350;
       0, 30, 90, 135, 180, 200, 350;
       0, 30, 90, 135, 180, 200, 350];
M = M_0 .* (pi / 180);
 
e = [0.01;
     0.1;
     0.5;
     0.9];

E = zeros(4,7);
error = 10;
i = 2;
j = 2;

while j <= 4
    while i <= 7
        if M(i,j) > -pi && M(i,j) < 0 || M(i,j) > pi
            E_n = M(i,j) - e;
        elseif M(i,j) < pi && M(i,j) > 0
            E_n = M(i,j) + e; 
        elseif M(i,j) == 0 || M(i,j) == 180
            E_n = M(i,j);
        end
        while error >= 10^-6
            E_n_plus_1 = E_n + ((M(i,j) - E_n + e(j) * sin(E_n)) / (1 - e(j) * cos(E_n)));

            E_n = E_n_plus_1;
            error = E_n / E_n_plus_1;
        end
        E(i,j) = E_n_plus_1;        
        i = i + 1;
    end
    j = j + 1;
end

Answer 1

[David Fletcher]

The probable source of your error is the first two conditions in the if block - they result in E_n being a 4x1 vector since e is a 4x1 vector. Only the last condition in the if block will result in E_n being a scaler.

...

while j <= 4
    while i <= 7
        if M(i,j) > -pi && M(i,j) < 0 || M(i,j) > pi
            E_n = M(i,j) - e; % <-------------------------------------------- 4x1
        elseif M(i,j) < pi && M(i,j) > 0
            E_n = M(i,j) + e; % <-------------------------------------------- 4x1
        elseif M(i,j) == 0 || M(i,j) == 180
            E_n = M(i,j); % Scaler
        end
        while error >= 10^-6
            E_n_plus_1 = E_n + ((M(i,j) - E_n + e(j) * sin(E_n)) / (1 - e(j) * cos(E_n)));
            E_n = E_n_plus_1;
            error = E_n / E_n_plus_1;
        end
        E(i,j) = E_n_plus_1;        
        i = i + 1;
    end
    j = j + 1;
end

Answer 2

[Robert Brown]

1 e = [0.01;
2     0.1;
3     0.5;
4     0.9];
5while j <= 4
6    while i <= 7
7        if M(i,j) > -pi && M(i,j) < 0 || M(i,j) > pi
8            E_n = M(i,j) - e;
9        elseif M(i,j) < pi && M(i,j) > 0
10            E_n = M(i,j) + e; 

In lines 1-4, you define e as a 4x1 matrix

if you execute line 8 or line 10, E_n becomes a 4x1 matrix.

Your first loop executes line 10, and E_n = M(2,2) + e

>> M(2,2)

ans =

0.5236

e =

0.01

0.1

0.5

0.9

E_n =

0.5336

0.6236

1.0236

1.4236

So e, a 4x1 vector, automatically expands your answer (E_n) into a 4x1 vector

Your first loop executes line 10, and E_n = M(2,2) + e

Perhaps you wanted to use a single value from e, to keep E_n as a scalar value?

Did you want to add e(j) instead of just e?

[Zach Harrison]

Yes, thank you. I changed the e's to e(j)'s, but now when I run the code I get and error:

Index in position 1 exceeds array bounds (must not exceed 4).

The error is in the same line as before.

[David Fletcher]

M is four rows - the loop runs for values of i up to 7. Once i gets beyond four you will get the indexing error. Looks like you've got i and j the wrong way around

[Zach Harrison]

Thank you, I believe that helped. But now I have a different problem so I updated the code and I posted another question. https://www.mathworks.com/matlabcentral/answers/804346-code-not-working-something-with-while-loop

[David Fletcher]

You are not resetting your inner loop back to the initial i value - so the inner loop will only ever run the first time