# [R] Problem using C random generator called from R

sebastien.dejean@math.ups-tlse.fr sebastien.dejean at math.ups-tlse.fr
Wed Sep 24 10:15:51 CEST 2003

```Hello

I'm trying to use C random generator function drand48() which return
floating-point values, uniformly distributed over [0,1],
(http://www.opengroup.org/onlinepubs/007908799/xsh/drand48.html). When
values are returned to R, they are not in [0,1]. A simple C program using
drand48() gives values in [0,1] so I suppose there is a problem (type
definition ?) between C and R. Here are R and C function and an example.

# R function
simulC <- function(n)
{
simR <- runif(n)
simC <- .C("test",
as.integer(n),
res=double(n))\$res
out <- list(simR=simR,simC=simC)
out
}

// C function
void test(int *n,double *res)
{
int i;
for(i=0;i<*n;i++) {res[i] = drand48();}
}

# Result of x <- simulC(5)
> x
\$simR
[1] 0.398567942 0.866053345 0.332070718 0.535814830 0.473418784
\$simC
[1] -1222291199   211182456  1007036963  1254056690  -646279915

Any help will be greatly appreciated,
Sébastien

--
Sébastien Déjean ~~~~~~~~~~~
http://www.lsp.ups-tlse.fr/Fp/Dejean

```