Matrix manipulation and replacements

I have a large matrix A (size 116x116) and a small matrix B (64x64)...

in B i only have all 2s or 5s ... In A i have 2s 3s etc..

I know the vector of rows and collumns that match matrix B..ie Irow 1by64 and Jcol 1by64;

I want to replace A(Irow,Jcol) with matrix elements in B; but only if the parent matrix A location is not equal to 2..

I.e. if A already has a 5 it should not be replace.

If I do A(Irow, Jcol) = B; then some of the 5s I have in A are getting replaced by 2s in B which I do not want.

Please help.

here is a smaller e.g. for illustration

A =[2 2 3 1
    5 5 5 5
    2 2 2 2
    2 3 5 5];
B = [2 5
     5 5];

Need to rplace A(Irow,Jcol) where Irow is a vector of size 2 and Jcol is vector of size 2) however, it can be arbitrary and located anywhere within the domain of A.

Answer 1

[Niels]

there is an easy solution if the matrices have the same size, so you coule just crewate a new matrix C which is A(Irow,Jcol)

A =
     2     2     2     2
     5     5     5     5
     2     2     2     2
     2     2     5     5
>> B=[inf nan; nan inf] % replaced 3 and 4s by nan and inf, so that you can see the replacements
B =
   Inf   NaN
   NaN   Inf

>> [Irow,Jcol]=size(B);
>> C=A(1:Irow,1:Jcol)
C =
     2     2
     5     5
% now check where C (which is A(...) ) is not equal to 2 and replace with matching index of matrix B
>> C(C~=2)=B(C~=2)
C =
     2     2
   NaN   Inf
>> A(1:2,1:2)=C
A =
     2     2     2     2
   NaN   Inf     5     5
     2     2     2     2
     2     2     5     5

[Pappu Murthy]

This solution is very nice and cute and compact. I accept it as the most direct answer. Thanks for the help.

Answer 2

[Wilson A N]

I think you can use the 'sub2ind' command as shown in the code below:

A = rand(3,3)

A =

      0.8059    0.8305    0.1025
      0.0777    0.6046    0.6690
      0.0537    0.4764    0.0370

A(sub2ind([3 3],[1,2],[2,3]))= [2, 5]

A =

      0.8059    2.0000    0.1025
      0.0777    0.6046    5.0000
      0.0537    0.4764    0.0370

Here I replaced the locations A(1,2) and A(2,3) with 2 and 5 respectively. More information on sub2ind command can be found in the link given below:

https://www.mathworks.com/help/matlab/ref/sub2ind.html