Hi! I have a problem in my function, Error in MATLAB

Use Wellis product= pi/2=2/1∗2/3∗4/3∗4/5∗6/5∗ … in a function in MATLAB that receives the relative error willing to accept and return the user a vector with all values of π and other vector estimated with relative errors associated with each value of π. (This is in arrays). Thanks a lot. But, MATLAB gives me an error:

Attempted to access ret(0); index must be a positive integer or logical.

Error in pi_value (line 13)
        ret(i) = 2 * (ret(i - 1) / 2) * temp;

The function is the following:

function[e, ret] = pi_value(err)
    ret = [4];
    value = 0;
    e = inf;
    i = 1;
    while e > err
        ++i;
        temp = 0;
        if mod(i, 2) == 0
            temp = i / (i + 1);
        else
            temp = (i + 1) / i;
        end
        ret(i) = 2 * (ret(i - 1) / 2) * temp;
        e = abs(ret(i) - ret(i - 1));
    end
end

Thanks a lot!

[dpb]

What is index of the RH ret value in

ret(i) = 2*(ret(i-1)/2)*temp;

?? Matlab arrays, as the error message says, only begin at 1. You'll have to special-case the end condition to avoid explicitly referencing a 'zeroth' element.

Also, note that numerically the expression as written above is simply

ret(i) = ret(i-1)*temp;

the two 2's cancel each other out...

[John D'Errico]

Not that the code as posted is NOT valid MATLAB syntax.

       ++i;

NOT in MATLAB.

Answer 1

[KSSV]

function[e, ret] = pi_value(err)
    ret = 4;
    e = inf;
    i = 1;
    while e > err
        i = i+1 ;
        if mod(i, 2) == 0
            temp = i / (i + 1);
        else
            temp = (i + 1) / i;
        end
        ret(i) = 2 * (ret(i - 1) / 2) * temp;
        e = abs(ret(i) - ret(i - 1));
    end
end