How to code a MEX file for a function with logical tests on array values

I have the following function coded as an m-file:

_______

function y = phiabs(x)

% returns log((e^|x| + 1)/(e^|x| - 1))

if x < 0

    x = -x;

end

if (x >= 1e-12) && (x <= 30.)

    z = exp(-x);

    y = log((1. + z)/(1. - z));

elseif x > 30

    y = 2. * exp(-x);

else

    y = log(2./(x + 1e-100));

end

______

If I try to apply it to an array or matrix, the "if" tests give trouble. To code it as a MEX file would involve individual calls to log() and exp() via mexCallMATLAB() for each element. Is there a more efficient way?

Answer 1

[David]

Solved my problem - used the C math library exp() and log() functions in the MEX .c file. Seemed like cheating to not call the Matlab functions, but it works.

Answer 2

[Jan]

Instead of calling a MEX function, you could get rid of the problems occurring for the IF statement when processing arrays:

function y = phiabs(x)
x      = abs(x);
y      = zeros(size(x));
idx    = (x > 30);
y(idx) = 2.0 * exp(-x(idx));
idx    = ~idx & x >= 1e-12;
z      = exp(-x(idx));
y(idx) = log((1 + z) ./ (1 - z));
idx    = (x < 1e-12);
y(idx) = log(2 / (x(idx) + 1e-100));
end