How can I create a vector of ratios from columns in a matrix?

I need to determine the ratio of elements above 30 in each column of a matrix and save in a vector. My current code looks like this:

function output = ratiomatrix(a)

%address all rows, by each column do previous ratio
%but divide by number of elements in column
%probably do it inside for loop and put in vector

[rows,columns] = size(a); %gets number of rows and columns (divide by rows for ratios)
greater = 0; %initialise variable for loop
output = zeros(1,columns); %preallocate variable for greater speeeeeed

for i = 1:columns 
    for j = 1:rows
        if a(j) > 30
            greater = greater + 1;
            output(i) = greater/rows;
        end
    end
    %output(i) = greater/rows;
    %greater = 0;
end

end

The problem i am currently having is that with each iteration, the value of greater includes the previous column and so gives my the wrong ratio. I have tried setting greater back to zero in a number of different places but this does not help. I would appreciate some hints as to how I might fix this, maybe without a complete answer if that's possible?

[KSSV]

Can you show us an numerical example with output..what you are expecting? You need not struggle with loops to achieve this. Welcome to MATLAB.

[Gerard Carroll]

Sure. Inputting:

StrengthMatrix =[39.0900   39.0900   31.1700;...
        25.2700   30.3100   34.0300;...
       35.2500   31.1500   36.2500;...
       36.7500   39.7600   33.7500;...
       33.0100   39.1800   33.2700;...
       38.2800   35.1400   33.7500;...
       38.4800   39.8200   32.6700;...
       34.3800   36.5000   26.2300;...
       27.0600   30.0500   35.7900;...
       28.2600   34.9300   39.9400;...
       27.7300   28.6600   30.3100;...
        25.6200   29.4300   39.5600;...
       26.6000   35.2000   30.1900;...
       34.2400   32.9100   38.2900]
ratiomatrix(StrengthMatrix)

Should give: 0.5714 0.8571 0.9286

Answer 1

[Star Strider]

Since ‘a’ is a matrix, you likely have to use two subscripts to it in your if statement, not one.

[Gerard Carroll]

I have tried a(i, j) and a(i(j)) but they just give me array indexing errors

[Star Strider]

If I understand correctly what you want to do, you can probably do what you want in one line:

output = sum(StrengthMatrix > 30) / size(StrengthMatrix,1);

output =
    0.5714    0.8571    0.9286

[Gerard Carroll]

ok that works! So that sums the number of elements in each column that are above 30, then divides by the size of rows in strengthmatrix right?

[Star Strider]

Correct!

The sum function (and most others in MATLAB) operate by default on the columns (dimension 1), so here it sums each column. You can specify the dimension to operate over as the second argument.

Similarly, since the number of rows are dimension 1,

size(StrengthMatrix,1)

returns that number.