# Am I doing this right? (a Math formula to Matlab language)

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Behzad Navidi on 15 Feb 2020
I needed to calculate this formula: So I wrote a function in order to do that:
(P is x and P^ is y):
function NS = nash_sat(x, y)
% This function calculates Nash-Sutcliffe efficiency coefficient
% Detailed explanation goes here
A = sum (x-y).^2;
B = sum (x-(mean(x)).^2;
fraction = (A/B);
NS = 1-fraction;
end
This is the first time I type a math formula in Matlab. I want to know if I typed the formula in the function correctly or not.
Thank you so much.

James Tursa on 15 Feb 2020
Edited: James Tursa on 15 Feb 2020
You are on the right track, but have typos in the code due to parentheses issues.
A = sum( (x - y).^2 );
B = sum( (x - mean(x)).^2 );

#### 1 Comment

Behzad Navidi on 15 Feb 2020
Thank you so much

Giuseppe Inghilterra on 15 Feb 2020
Hi,
A and B terms are wrong.
What you wrote is the square of sum of differences. Instead formula is the sum of square of differences.
The code should be written as follow:
A = sum((x-y).^2);
B = sum((x-(mean(x))).^2);
fraction = (A/B);
NS = 1-fraction;
Pay attention also to space left between "sum" and "(x-y)" in your code: it is not correct.