How to resolve issue while dealing with larger matrices?

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Ammy 2022-3-25

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评论： Arif Hoq 2022-3-25

A=randi(16,[4,4]);

B=A(:)';

P=randi(16,[1 4]);

V=randi(16,[1 4]);

B(P)=V; % replace entries at position P with values in V

C=B(1,P)

C=V satisfied in this case

But I'm not getting the right answer in case of larger size matrices.

A=randi(255,[512,512]);

>> B=A(:)';

>> P=randi(255,[1 256]);

>> V=randi(256,[1 256]);

>> B(P)=V; %

>> C=B(1,P);

C should be equal to B , but I'm not getting it right,

Request for help regarding this.

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Ammy 2022-3-25

@Arif Hoq Got the point why I'm facing the problem, thank you very much. How can I vote your answers?

Arif Hoq 2022-3-25

i am posting my previous response as an answer

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回答（2 个）

Voss 2022-3-25

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在 MATLAB Online 中打开

As @Arif Hoq points out, the problem is not with the size of the matrices. The problem is that the elements of P are not guaranteed to be unique. Look at what happens when a number occurs more than once in P, in this case 8 appears twice in P:

A=randi(16,[4,4]);
B=A(:)'
B = 1×16
     8     9    11     8     9     4     5    11     1    14    13     5     1    15    13     5
P=randi(16,[1 4])
P = 1×4
     7     1     8     8
V=randi(16,[1 4])
V = 1×4
     2    11    13     4
B(P)=V % replace entries at position P with values in V
B = 1×16
    11     9    11     8     9     4     2     4     1    14    13     5     1    15    13     5
C=B(1,P) 
C = 1×4
     2    11     4     4
C==V 
ans = 1×4 logical array
   1   1   0   1

In this case, B(P)=V is B([7 1 8 8]) = [2 11 13 4]; which means that B(8) is assigned the value 13 and then immediately assigned the value 4, so that B(8) is 4 after that step.

Then C = B(1,P) is C = B(1,[7 1 8 8]) = [2 11 4 4] not [2 11 13 4] = V

Any time P has repeated values, then V will not be recoverable from B and P.

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Ammy 2022-3-25

Got the point, thank you very much.

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Arif Hoq 2022-3-25

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1680204-how-to-resolve-issue-while-dealing-with-larger-matrices#answer_926589

在 MATLAB Online 中打开

A=randi(16,[4,4]);
B=A(:)';
P=randi(16,[1 4]);
V=randi(16,[1 4]);
[C out]=ismember(V,B);
if numel(out(out~=0)) ==4
    disp(out)
else
    disp('V is not the member of B')
end