reshape a matrix and fill empty indices by zeroes

I'm woking in encryption project.

I have the key sequare matrix A for example with dimension mxm

I need to reshape the plaintext matrix P to be with same number of rows as A which is m

Now if the size of P devided by m equals an integer, this would be fine

however if it is not the new matrix P after reshaping will need more elemnets

So, how to write a function to fill the matrix P with zeroes after reshaping.

here's my code:

A = input('Enter Your key= ','t'); %key matrix mxm
sA = size (A); 
sA = sA(1,1);
m = sA;
Ax = reshape (A , m , m );
disp (Ax);

x =input('Enter Your Message= ','s'); %plain text
sX = length(x);
n = round(sX/m);
 if rem(sX, m) == 0
  xR = reshape(x , m ,n);
 else 
???????????????

[dpb]

Can't reshape first; augment the text array with correct number of elements first. Easiest would be to turn it to vector.

nA=numel(A);
nP=numel(P);
if nP<nA
  P=reshape([reshape(P.',1,[]) blanks(nP-nA)],m,[]);
end

[Fatma Alnabrisi]

ok this gives me the plain text as a vector.

what should I do next ?

I want to represent the plain text as matrix of the same number of rows as the key matrix so I can multiply A*P

[dpb]

"... the plain text as a vector."

No, it pads the vector but returns mxm if you'll try it...

It assumes, however, the A array is always at least as large as the text...is that always so or does it have to be tiled? Having a person input() an arbitrary array seems awkward at best but I "know nuthink!" per Sgt Schultz re: the subject.

[Fatma Alnabrisi]

I have tried it and it returns my plaintext as is not in matrix form..

[dpb]

Did something wrong then...

>> b=char({'fred';'flintstone'})  % sample text array 20 char mimic a small text file
b =
  2×10 char array
    'fred      '
    'flintstone'
>> [reshape(b.',1,[]) blanks(25-numel(b))]  % the augmented vector intermediary 
ans =
    'fred      flintstone     '
>> reshape([reshape(b.',1,[]) blanks(25-numel(b))],5,[]) % the mxm (m=5) array
ans =
  5×5 char array
    'f fs '
    'r lt '
    'e io '
    'd nn '
    '  te '
>> 

Of course, in general the 25 is value of m*m and the 5 is m; just used a convenient number to illustrate.

[dpb]

In addition, the above is still a char() array, dunno what you intend to do when you speak of multiplying it with another array. You talk of augmenting w/ zero; the above used blanks instead with text, char(0) is also possible, of course.

>> double(ans)
ans =
   102    32   102   115    32
   114    32   108   116    32
   101    32   105   111    32
   100    32   110   110    32
    32    32   116   101    32
>> 

is the numeric represenation of above...as noted, I've no klew regarding what it is you're actually trying to do here.

Oh...one other thought--

>> char(ans).'
ans =
  5×5 char array
    'fred '
    '     '
    'flint'
    'stone'
    '     '
>> 

Dunno which orientation you would prefer or if it matters...you may want to transpose the previous result?