MATLAB: How to check if all the numbers in a column of a matrix are equal and return that value

columnsequal numbers

I am trying to construct a way to find a column of equal numbers in matrix of any size. There will only be one column where the numbers are equal. I then need to extract that number (the number that is the same throughout the column) to a variable that can be called on. I have tried for loops, but I cannot get it to work. Any help would be greatly appreciated.

Best Answer

If A is your matrix

idx=find(all(~diff(A)))
out=A(1,idx)

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Two things:

1) Using a loop for what you want is inefficient and probably will lead to bugs if you're not careful. What you want can be achieved much more easily without a loop. I'm not exactly sure what you mean by the the first instant of a negative number. If you mean _get all values in column 19 for which column 17 are negative, it's simply:

s_dis = yourmatrix(yourmatrix(:, 17) < 0, 19);  %get all values in column 19 for which column 17 is negative

If you mean the first negative value of a continuous run of negative values, it's a bit more complicated

isstartofarun = diff([false; yourmatrix(:, 17) < 0]) == 1; %find transitions from nonnegative to negative
s_dis = yourmatrix(isstartofarun, 19);   %get matching values in column 19

If you mean something else, you'll have to be clearer.

2) The way matlab displays number has no bearing on what is truly stored in the matrix. Whatever is stored in the matrix will be unchanged after the above (or your for loop). However, depending on the magnitude of the numbers and the display format that you're using some numbers may display as 0. Their actual value will still be used for all calculation

You can change the display format of the command with the format command, personally I use format longg, and the display format of the variable browser in the view tab of matlab toolbar.

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x(:,4) = all(bsxfun(@eq,x(:,1),x(:,2:3)),2)

The fourth column will not be true where the first three columns are equal.

I believe this is what you want. If it's not, exactly what do you want? A small example with inputs->operation->expected output would be ideal

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