MATLAB: What controls of this error ‘Inner matrix dimensions must agree’

dimesionserrormatrix

Sometimes I use these steps but without error

N=5;
s=linspace(0,2);
C=exp(s*(0:5));

Why sometimes other times this error appears ? Inner matrix dimensions must agree.

Best Answer

When using * in Matlab with non-scalars, matlab follows the calculation rules of linear algebra:

>> A = randi(10,3,2)
A =
       4     4
       2     6
       8     2
>> B = randi(10,2,5)
B =
       7     7     8     1    10
       3     7     5     3     2
>> C = A * B
C =
      40    56    52    16    48
      32    56    46    20    32
      62    70    74    14    84
>> whos A B C
  Name      Size            Bytes  Class     Attributes
    A         3x2                48  double              
    B         2x5                80  double              
    C         3x5               120  double

Multiply a 3x2 Matrix with a 2x5 is possible and results in a 3x5 Matrix. If you try the same "backwards" it will give the error:

>> D = B * A
Error using  * 
Incorrect dimensions for matrix multiplication. Check that the number of columns in the first matrix
matches the number of rows in the second matrix. To perform elementwise multiplication, use '.*'.

Read more in the documentation for Matrix operations, where this behavior is described in detail.

Best regards

Stephan

Related Solutions

MATLAB: Query regarding a sparse complex matrix

Hi,

the resulting size of the sparse matrix depends from the number of zero entries. Consider this:

A = randi(10,100);
A(A<=7) = 0;
B = sparse(A);
whos A B
Name        Size             Bytes  Class     Attributes
A         100x100            80000  double              
B         100x100            49096  double    sparse

The code above produced a sparse matrix which has about 60% size then the normal one. There should be about 70% zeros in it.

If we only have about 30% zeros the situation changes:

A = randi(10,100);
A(A<=3) = 0;
B = sparse(A);
whos A B
Name        Size              Bytes  Class     Attributes
A         100x100             80000  double              
B         100x100            113240  double    sparse

Now the size of B is about 40% bigger than A.

EDIT - Now the same example using complex numbers:

about 70% zeros:

R = randi(10,100);
I = ones(100);
C = complex(R,I);
C(real(C)<=7) = 0;
D = sparse(C);
whos C D

gives:

Name        Size              Bytes  Class     Attributes     
C         100x100            160000  double    complex        
D         100x100             71944  double    sparse, complex

and the same with about 30% zeros:

R = randi(10,100);
I = ones(100);
C = complex(R,I);
C(real(C)<=3) = 0;
D = sparse(C);
whos C D

gives:

Name        Size              Bytes  Class     Attributes     
C         100x100            160000  double    complex        
D         100x100            168928  double    sparse, complex

The values differ, but the fundamental effect is the same.

Best regards

Stephan

MATLAB: Error using * matrix dimensions must agree

Your question states that 1st dim should correspond to the x, and the 2nd dim to t. But where are you specifying that? Both x and t are row vectors (and of different lengths), so neither of them specify anything along the 1st dim:

>> size(t)
ans =
     1     5
>> size(x)
ans =
     1    17

Multiply two row vectors of different lengths will be an error, no matter how many different matrix multiply methods you try:

>> tan(t)*x
??? Error using ==> mtimes
Inner matrix dimensions must agree.
>> tan(t).*x
??? Error using ==> times
Matrix dimensions must agree.

There is no standard mathematical definition for multiplying row vectors of different lengths. In this example, what should the 4 multiply with?:

>> [1,2,3,4].*[5,6,7]
??? Error using ==> mtimes
Inner matrix dimensions must agree.

Instead of imagining what the code should be doing, try to pay attention to what the code is really doing. For example, when you read about matrix multiplication then you would realize that you can trivially orient the x as a column, and you get a matrix output when it is multiplied with the row vector t:

 >> x(:)*tan(t)
 ans =
            0            0            0            0            0
        -4.28      -32.027       8.0989       1.6002      -2.1035
      -8.5599      -64.053       16.198       3.2004       -4.207
       -12.84       -96.08       24.297       4.8006      -6.3105
   ... etc

And then this leads directly to one simple solution to your question:

>> g = 9.81;
>> t = 15:15:75;
>> x = 0:5:80;
>> v = 28;
>> yo = 0;
>> y = x(:)*tan(t) - (x(:).^2) * (g./(2*v^2*(cos(t)).^2)) + yo
y =
          0            0            0            0            0
     -4.551        -38.6       7.5321       1.4278      -2.2876
     -9.644      -90.348       13.931       2.5107      -4.9434
    -15.279      -155.24       19.196       3.2487      -7.9673
    -21.456      -233.28       23.327       3.6419      -11.359
    -28.175      -324.47       26.325       3.6903       -15.12
    -35.436      -428.81       28.189       3.3937      -19.248
    -43.239      -546.29        28.92       2.7524      -23.745
    -51.585      -676.92       28.517       1.7661       -28.61
    -60.472       -820.7       26.981        0.435      -33.843
    -69.901      -977.63       24.311       -1.241      -39.444
    -79.872      -1147.7       20.508      -3.2618      -45.414
    -90.386      -1330.9       15.571      -5.6275      -51.751
    -101.44      -1527.3       9.5002       -8.338      -58.457
    -113.04      -1736.8       2.2961      -11.393      -65.531
    -125.18      -1959.5      -6.0416      -14.794      -72.973
    -137.86      -2195.3      -15.513      -18.539      -80.784

Multiplying vectors (and matrices) is covered quite well in thousands of online high-school math tutorials, so there is no point in repeating it all here. Writing code without understanding what it is doing is a guarantee that the code will be buggy.

Best Answer

Related Solutions

MATLAB: Query regarding a sparse complex matrix

MATLAB: Error using * matrix dimensions must agree

Related Question