How to multiply two matrices where A= 1415×6 and B=6×128

Matrices A=1415×6 and B=6×128

[Stephen23]

Did you try:

A*B

[Nandita Sarkar]

No..but the two matrices are of different dimensions.

[Walter Roberson]

What size of output are you expecting?

A = randi(9, 5, 6)

A = 5×6

1 3 4 5 2 6 6 4 6 6 2 4 4 8 1 3 2 1 7 5 3 9 6 1 5 5 3 5 1 5

B = randi(9, 6, 8)

B = 6×8

9 3 8 6 4 7 4 6 6 6 7 6 2 2 5 7 3 9 4 1 8 2 2 8 2 2 3 6 9 3 2 2 1 7 2 3 5 5 7 5 6 8 6 3 9 4 6 3

C = A * B

C = 5×8

87 129 100 82 151 70 87 97 134 154 146 120 180 106 106 146 101 97 111 100 86 69 84 107 132 146 148 150 182 126 125 152 125 129 134 111 149 91 98 119

whos

  Name      Size            Bytes  Class     Attributes

  A         5x6               240  double              
  B         6x8               384  double              
  C         5x8               320  double              

A and B are different sizes, but A*B (as suggested by @Stephen) works to do something that is referred to as "multiplying" (also known as "inner product")

[Cris LaPierre]

You may want to review matrix multiplication.

[the cyclist]

Can I suggest we make a smaller example, so we can visualize everything?

Suppose instead of A=1415×6 and B=6×128, we instead have A = 3x2 and B=2x4:

A = [1 2;
     3 4;
     5 6];

B = [1 2 3 4;
     5 6 7 8];

Now, we can certainly do matrix multiplication (because the "inner" dimensions are the same), and the result will be a 3x4 matrix:

C = A * B

C = 3×4

11 14 17 20 23 30 37 44 35 46 57 68

But perhaps that is not what you wanted? Please explain in more detail!

[Nandita Sarkar]

I want my output 1415×128.

[John D'Errico]

Then A*B is EXACTLY what you do. Why do you think that is not what you get?

A = rand(1415,6);B = rand(6,128);
C = A*B;
size(C)

ans = 1×2

1415 128

Maybe you need to do as @Cris LaPierre suggests, and review what a matrix multiplication means, and then consider what the * operator does?

help mtimes

 *   Matrix multiply.
    X*Y is the matrix product of X and Y.  Any scalar (a 1-by-1 matrix)
    may multiply anything.  Otherwise, the number of columns of X must
    equal the number of rows of Y.
 
    C = MTIMES(A,B) is called for the syntax 'A * B' when A or B is an
    object.
 
    See MATLAB Operators and Special Characters for more details.
 
    See also TIMES, PAGEMTIMES.

    Documentation for mtimes
       doc mtimes

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       calendarDuration/mtimes    InputOutputModel/mtimes
       codistributed/mtimes       LagOp/mtimes
       dlarray/mtimes             laurmat/mtimes
       duration/mtimes            laurpoly/mtimes
       fints/mtimes               tall/mtimes
       gpuArray/mtimes            timeseries/mtimes
       icsignal/mtimes            tsdata.datametadata/mtimes