How to perform a calculation only when a input value is positive

I have a input file that has 1x367 inputs. I only want to calculate the following equation when the input value is positive. I thought this would be done through a for loop but that appears to not not be valid. How can I perform this calculation such that only a positive value is used? I thought a for loop would be the correct choice and also tried an if statement. Is there a better way to do this?

for chi_e > 0.0 
    j_l = ((1-chi_e)*m_dot)/(rho_L*((pi*D^2)/4));
end

Answer 1

[Abhinav Gupta]

Let your input variable be a of size = 1x367

Let b be the variable which consist of all positive elements of a.

b = a( a>= 0 );
for i = 1:1:length(b)
    j_l = ((1-b(i))*m_dot)/(rho_L*((pi*D^2)/4));
    disp(j_l);
end

Or simply,

b = a( a>= 0 );
j_l = ((1-b)*m_dot)/(rho_L*((pi*D^2)/4));

[Kevin Burg]

This is exactly what I was looking for, thanks!

Answer 2

[Scott MacKenzie]

It is not clear what you mean by "input value", but assuming you do know, then...

for chi_e > 0.0 
    if input_value > 0
        j_l = ((1-chi_e)*m_dot)/(rho_L*((pi*D^2)/4));
    end
end

[Kevin Burg]

Sorry, I have a pre-calculated matrix of positive and negative values for chi_e. I only want to use positive values of chi_e in the j_l formula.