Why is this ending my matrices before they are filled, and how do I print the answer of the multiplication?

Question

David Scidmore 2021-1-30

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https://ww2.mathworks.cn/matlabcentral/answers/731473-why-is-this-ending-my-matrices-before-they-are-filled-and-how-do-i-print-the-answer-of-the-multipli

编辑： Stephen23 2021-1-31

C=input('How many columns are there in P?  ');
    R=input('How many rows are there in P?  ');
    P=zeros(R:C);
    co=1; 
    ro=1;
          while co<=C+1 && ro<=R+1;
              
                  P(co)=input('What is the first value of this column of P? ')
                  co=co+1;
              
                  P(co)=input('What is the next value of this column of P?  ')
                  co=co+1;
              
              ro=ro+1;
          end 
      
B=input('How many columns are there in D?  ');
    T=input('How many rows are there in D?  ');
    D=zeros(R:C);
    co=1; 
    ro=1;
          while co<=C+1 && ro<=R+1;
              
                  D(co)=input('What is the first value of this column of D? ')
                  co=co+1;
              
                  D(co)=input('What is the next value of this column of D?  ')
                  co=co+1;
              
              ro=ro+1;
          end 
V = P*D; 
Print(V)

When entering the values of the matrices if I make it more than 2 columns it puts zeros in for the 3rd and subsequent columns. I'm also trying to print (not sure what the command is in matlab) V, to have the value show. However using print it searches my hard drive to try and find a file (which is odd since I'm not using open).

I would assume if the matrices aren't able to be multiplied and error message shows up, but is there a way to make that a dialogue box?

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Steven Lord 2021-1-31

在 MATLAB Online 中打开

%{
 P=zeros(R:C);
%}

This doesn't do what you want.

R = 5;
C = 2;
P1 = zeros(R:C)
P1 =

     []
P2 = zeros(R,C)
P2 = 5×2
     0     0
     0     0
     0     0
     0     0
     0     0

The expression 5:2 returns the empty array. When you pass that into zeros as the size input you get a 0-by-0. But even worse, if C was much larger than R at best MATLAB would tell you you can't make that large an array. At worst it would try and potentially take a long time to allocate.

R = 2;
C = 30;
numElements = prod(R:C)
numElements = 2.6525e+32
P = zeros(R:C)
Error using zeros
Requested array exceeds the maximum possible variable size.

You can't make a 2-by-3-by-4-by-5-by-6-by-...-by-29-by-30 array because it would have over 2e32 elements.

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Answer 1

Walter Roberson 2021-1-30

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https://ww2.mathworks.cn/matlabcentral/answers/731473-why-is-this-ending-my-matrices-before-they-are-filled-and-how-do-i-print-the-answer-of-the-multipli#answer_610418

编辑：Walter Roberson 2021-1-30

在 MATLAB Online 中打开

print() of a value is Python. MATLAB uses disp()

To make a dialog box for that error:

try
  V = P*D;
catch ME
  errordlg(getReport(ME)) ;
end

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David Scidmore 2021-1-31

在 MATLAB Online 中打开

Got it,

My intent is to let users form what ever size matrix they wish and fill it with their own values. I thought the zeros would clear out the matrix to be filled.

Like they want P to be a 3*5 Matrix they enter the values they want and T could be a 5*3 matrix and they enter the values they wish. Not to be a zero matrix.

My new code only puts everything in a single row.

C=input('How many columns are there in P?  ');
R=input('How many rows are there in P?  ');
    P=(R:C);
    co=1; 
    ro=1;
          while co<=C+1 && ro<=R+1;
              
                  P(co)=input('What is the first value of this column of P? ')
                  co=co+1;
              
                  P(co)=input('What is the next value of this column of P?  ')
                  ro=ro+1;
              
              ro=ro+1;
          end 