Exp random sample

[Gyanendra Pokharel]

Hi,

I want to get random sample from the exponential distribution exp(10^5). I made following function 

real function logprior(z)

 real :: z, logprior

  if(z < 0)then

    logprior = 0.0

    if(z >= 0.0) then

      logprior =(1/(10.00**5)) * exp(-z/(10.00**5))

     logprior = log(logprior)

  endif

 endif

end function logprior

But it returns zero values.

Can some one suggest why it is not working and how how it works?

GP

[Robert Miles]

Have you tried steeping through the code with z set to 0.0?

Does it give the result you want?

[Larry Baker]

I thin your conditional should not be nested.  You probably want code like this:

    if ( z >= 0.0 ) then

      logprior = ( 1/(10.00**5) ) * exp( -z/(10.00**5) )

      logprior = log( logprior )

    else

      logprior = 0.0

    end if

This handles the case of z<0 and z=NaN or +-Inf in the else clause.

Larry Baker

US Geological Survey

650-329-5608

ba...@usgs.gov

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