Why doesn't this work?

clc,clear

prompt = 'Give a matrix of 4x4: ';

x = input(prompt);

[m,n] = size(x);

M=magic(4);

f=sum(M);

g=sum(M')';

   if m~=4 | n~= 4

     disp('The matrix is not a 4x4 please start again and fix the error.')

   else

      for i=1:16

      if x(i)<=0

         disp('Error one of the numbers put in was either a zero or negative, Fix it and start over.')

          if sum(x)~=f && sum(x)~=g

              disp('This is not a magic matrix, please start over, and do it right. ')

          end
      end
  end
  end
  disp(x)
  disp(sum(x))
  disp(sum(x')')

Made a some changes to make it a bit easier, my question is basically why does 'sum(x)~=f && sum(x)~=g' not seem to be working, 4x4s that arent magic(4) seem to still be getting passed, and not denied.

[Collin Kerr]

also I did not type in the ' with sum(x)~=g it should be sum(x')'~=g'

Answer 1

[Star Strider]

Since ‘x’ is a matrix, you need to use either any, all, sum or some other function that produces the appropriate scalar.

Consider:

x = 1:4;
f = 2;
q = x~=f

q =
     1     0     1     1

What do you want the comparison to do? I’m certain it would like to know!

EDIT — When you evaluate:

f=sum(M);
g=sum(M')';

they are still going to be (1x4) vectors.

[Collin Kerr]

Im trying to tell the code if the sum of x does not equal the sum of f then display (the error message), but if they are the same as f then keep going.

[Star Strider]

If you want the sum of a all the elements of a (4x4) (or any other array), the easiest way is:

f = sum(M(:));

in that syntax, ‘g’ is going to be the same, so only one comparison would be necessary.

Since all the row, column and diagonal sums are going to be equal in a magic matrix, you would have to test that all the row, column, and diagonal sums are equal, that is all are the same, for the matrix to be magic. (You may not have to test for all of those. I leave that choice to you.) This is not the same as the sum of all the elements in the matrix being equal to the sum of all the elements in a magic matrix being the same. Only the individual row, column, and diagonal sums matter.

To illustrate, run this:

Mg = magic(4);
q3 = sum(Mg)
q4 = sum(Mg,2)
q5 = sum(diag(Mg))