MATLAB: Find elements in vector that satisfy condition

MATLABmatrix

say that I have the following matrix X:

x1 =[1 2]
x2 = [3 2]
x3 = [3 4]
X = [x1;x2;x3]

I need to get a matrix consisting of the rows in X that that satisfy the following condition:

x(1) == 3.

So my resulting matrix will be:

S = [x2;x3] = [3 2; 3 4]

How can I do this (for any dimension index, not necessarily 1)

Thank you

Best Answer

X = your big matrix
k = column number you want to examine
value = some number you are looking for
idx = (X(:,k) == value); % logical indexes of the rows where the element in column k is value
S = X(idx,:); % pick off the subset of rows that have value in column k

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MATLAB: Assign data satisfied rules given

variant for rules 1

szAA = [22 7];
n2233 = randi(floor(szAA(1)*.2),2,szAA(2));
N = [22-sum(n2233);n2233];
B = cell2mat(arrayfun(@(x)cat(3,randperm(22)',[11*ones(N(1,x),1);22*ones(N(2,x),1);33*ones(N(3,x),1)]),1:size(N,2),'un',0));
AA = B(sub2ind(szAA,B(:,:,1),cumsum(ones(szAA),2))+prod(szAA));

rules 2, so?

AA(sub2ind(szAA,1:szAA(1),randi(szAA(2),1,szAA(1))))=44;

MATLAB: How to do matrix indexing using for loop that involved sampling at time k

Hey Eric

Assuming that you have 2 arrays to start with (x1 and x2, as given in your question), you can declare an empty matrix x (16 x 2), and assign the values as here:

x = zeros(16,2)
for i=1:16
x(i,:)=[x1(i) x2(i)]
end

For kth element, x(k) = [x1(k) x2(k)].

For X matrix, you will have to declare the matrix with variable dimensions:

X = cell(16,1)
X{1} = [x1(1) x2(1)]
for i=2:16    
X{i} = [x1(i) x2(i) X{i-1}]
end

Here, X{k} = [x1(k) x2(k) x1(k-1) x2(k-1).... x2(1)].

If you have more arrays (x1, x2, x3,...., xm), you can loop in the inner values.

After obtaining the matrices, you can transpose each value (x(k) or X(k)) to get the final answer.

Best Answer

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MATLAB: Assign data satisfied rules given

MATLAB: How to do matrix indexing using for loop that involved sampling at time k

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