How to select certain columns of a matrix only when the values in the 4th row are bigger than three values of the other four rows in that particular column?

Hi everybody.

I have a Matrix with 5 rows and 27 columns M(5,27).

-465016.671990511	-7739.99191635794	-341535.491595863	-85371.1969301907
-5903.99881269675	-7859.03942441580	1327.06121931661	-2689.99261151688
-9775.84903413543	-24436.8700789172	10631.1822509721	-5457.55830920490
-115990.998278705	-14946.7388544833	-102.707785969593	-22516.0893903143
-49215.4580227008	-42337.1765610354	-22878.6399931591	-17776.5021930945

looks like above. I don't want the columns (using 0 index) where the absolute value in the fourth row is bigger than at least 3 absolute values of the other 4 values in the particular column (values in row 1:3 and 5). From the above shown columns it should give me a result like a=[0,1,1,0] because only in the first and fourth column the values in the 4th row are bigger than at least three other values in that particulare column ( first column: 4th value 115990 > 465016, 5903, 9775, 49215; fourth column: 4th value 22516> 2689, 5457, 17116)

What is the easiest way to write a code for this? Using the if else statement? Thanks already in advance!

[Stephen23]

"What is the easiest way to write a code for this?"

Not using loops.

>> idx = sum(abs(A(4,:))>abs(A([1:3,5],:)),1)<3
idx =
  0  1  1  0

[Kim Arnold]

Thank you very much for this short line of code, i thought there is no need for loops but as a beginner i really appreciate any help!

Answer 1

[ME]

For the example in your question, the following will work:

[~,c] = size(A);
a = ones(1,c);
for col = 1:c
    if((sum(abs(A(4,col)) > abs(A([1:3,5],col)))) >=3)
        a(col) = 0;
    end
end

You don't need an else statement if you pre-allocate the a array as being all ones.

[Kim Arnold]

Thanks a lot for this nice answer, gives me exactly what i need!

[ME]

No problem! If this answer was helpful to you, I don't suppose you could accept it?

[Kim Arnold]

Apologies, im quite new here :) it's done!

[ME]

Thank you!

Answer 2

[Bhaskar R]

mat = abs(your matrix say urmat);
ind = mat(4,:)>mat([1:3, 5], :);
ind_4 = ind(4,:);
desired_col = urmat(:, ind_4);    % or mat(:, ind(4,:))

[Kim Arnold]

Thanks as well for your answer but this gives me exactly the columns i don't want, it should be the other way around =) anyway, many thanks for your help

[Bhaskar R]

" it should give me a result like a=[0,1,1,0]" - As you mentioned thats why I had to perform < operation, now it is corrected.

[Kim Arnold]

thank you.