Find value in a matrix

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Zenia Askar 2020-1-9

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https://ww2.mathworks.cn/matlabcentral/answers/499665-find-value-in-a-matrix

评论： Edvin Sandulf 2021-9-28

Hello! I wanted to check if a value exists inside a matrix without the use of a loop. To be more spesific I have a matrix, D, which is 50x50 and I want to check if a variable A, is in D. I used ismember(A,D) but it doesn't work. This function works only if D is an array? What can I do to look through a matrix without a loop? Thank you in advance.

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Stephen23 2020-1-9

"I used ismember(A,D) but it doesn't work."

Taking a guess: you have floating point number issues.

"What can I do to look through a matrix without a loop?"

ismembertol

Adam Danz 2020-1-9

在 MATLAB Online 中打开

I second the ismembertol() suggestion.

A lower-level approach would be to simply subtract the value from each element of the matrix and look if any of the results are very close to zero.

% M is the matrix
% v is the scalar value
[row,col] = find(abs(M-v) < 0.000001)

row and col will be empty if there are no "matches". Otherwise they will contain the row and column numbers of all "matches".

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Answer 1

James Browne 2020-1-10

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Why not use a loop? You can create one with very little code and you can easily modify it to do whatever you want. For example, I wrote the following code, which not only searches for a value in a matrix, D, of arbitrary size and sets a boolean variable to TRUE, if the value os found in D, but also reports the number of times it is found. You can also add lots of things, easily, such as a tolerence range or something...

%Create a 50x50 matrix of random integers between 1 and 10
D = randi(10,50);
%Determine the dimensions of the matrix D
NRows = size(D,1);
NCols = size(D,2);
%Value to be searched for in the matrix D
CompareValue = 5;
%Variable which indicates whether or not "CompareValue" was found in the
%matrix D
ValueFound = false;
%Number of times that CompareValue was found in the matrix D
NumTimesFound = 0;
%Search the matrix D to see if the value in "CompareValue" is present 
for i = 1:NRows
    for j = 1:NCols
        if D(i,j) == CompareValue
            ValueFound = true;
            NumTimesFound = NumTimesFound + 1;
        end
    end
end
fprintf('The value %4.2f was found in the matrix D %i times', CompareValue, NumTimesFound)