Creating a logic vector out of 2D matrix

Question

Lama Hamadeh 2022-4-27

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1706350-creating-a-logic-vector-out-of-2d-matrix

评论： Jon 2022-4-27

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Hi,

How can I create a logic vector v out of a

matrix M in a way that each row of v equals to 1 when the second element of Mper row equals to 0, as illustrated below:

My attempt in coding this is as following:

%M matrix
M = [1 2;
    2 0;
    3 1;
    4 0;
    5 3;
    6 0;
    7 2;
    8 4;
    9 0;
    10 1];
%initialise v vector
v = [];  
%create v vector
for i =1:size(M,1)
    if M(i,i) == M(i,0)
        v(i) = 1;
    else
        v(i) = 0;
    end
end

An error message of "Index in position 2 is invalid. Array indices must be positive integers or logical values." apears.

Any help would be appreicted.

Thanks.

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Answer 1

Dyuman Joshi 2022-4-27

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https://ww2.mathworks.cn/matlabcentral/answers/1706350-creating-a-logic-vector-out-of-2d-matrix#answer_952060

在 MATLAB Online 中打开

Indexing in MATLAB starts with 1 and not 0.

M = [1 2;
    2 0;
    3 1;
    4 0;
    5 3;
    6 0;
    7 2;
    8 4;
    9 0;
    10 1];
%initialise v vector
v = []; 
%create v vector
for i =1:size(M,1)
    if M(i,2) == 0 %Comparing 2nd element of ith row to 0 as you mentioned
        v(i,1) = 1;
    else
        v(i,1) = 0;
    end
end
v
v = 10×1
     0
     1
     0
     1
     0
     1
     0
     0
     1
     0

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Answer 2

Fangjun Jiang 2022-4-27

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1706350-creating-a-logic-vector-out-of-2d-matrix#answer_952050

编辑：Fangjun Jiang 2022-4-27

M(i,i) == M(i,0)

should be

M(i,2) == 0

When you have this type of error on a very simple script, put a break point and run the script line by line. You will be able to debug the problem by yourself.

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Answer 3

Jon 2022-4-27

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https://ww2.mathworks.cn/matlabcentral/answers/1706350-creating-a-logic-vector-out-of-2d-matrix#answer_952055

在 MATLAB Online 中打开

%M matrix
M = [1 2;
    2 0;
    3 1;
    4 0;
    5 3;
    6 0;
    7 2;
    8 4;
    9 0;
    10 1];
% logic vector
v = ~M(:,2)

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Jon 2022-4-27

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The above uses the fact that MATLAB treats double values which are zero as if they were logical false. You could also do what you want, maybe more clearly using

v = M(:,2)==0

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