Matrix manipulation and replacements

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Pappu Murthy
Pappu Murthy 2017-1-17
评论： Pappu Murthy 2017-1-20
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.

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采纳的回答

Niels
Niels 2017-1-20
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
  1 个评论
Pappu Murthy
Pappu Murthy 2017-1-20
This solution is very nice and cute and compact. I accept it as the most direct answer. Thanks for the help.

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更多回答（1 个）

Wilson A N
Wilson A N 2017-1-20
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

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