Error approximation as a function

I have this error approximation

and I am trying to create two functions. One which loops over the values and the second which loops over input x values. So far I have

function b = erff(x,N); 
xx=-x^2; 
a=x; b=a; c=1; 
while(abs(a)>N); 
a=a*xx*(2*c-1)/(c*(2*c+1)); 
b=b+a; c=c+1; 
end 
b=b*2/sqrt(pi);

I'm not sure if my code is correct or how to only make it loop over n values and x values seperately.