[R] problem when I Call C subfunction in void function

[eguichard]

Thank you. 
I can improve my program with your response but I have an other problem. 

/double essai (double *Px, int *tailleP)
{
	int i;
	for (i = 0; i < *tailleP; i++)
	{
		Px[i]=Px[i]*2;
		Rprintf ("I print Px %f\t", Px[i]);
	}
	Rprintf("\n");
	return *Px; 
}

void test_essai (double *Px, int *tailleP, double *res)
{
	int i;
	*res = essai(Px, tailleP); 
	for (i = 0; i < *tailleP; i++)
	{
		Rprintf ("I print res %f\t", res[i]);
	}
}/

When I compile it in R 
/trajPx <- c(2,5,5,4)
.C("test_essai",Px = as.numeric(trajPx),tailleP =
as.integer(length(trajPx)),res = numeric(4))/

I have the following result: 

I print Px 4.000000     I print Px 10.000000    I print Px 10.000000    I
print Px 8.000000     
I print res 4.000000    I print res 0.000000    I print res 0.000000    I
print res 0.000000   
 $Px
[1]  4 10 10  8

$tailleP
[1] 4

$res
[1] 4 0 0 0
 
I haven't problem in "essai" function but I can't correctly return "Px"
vector.
I d'ont understand why I get only the first number (number 4 in my exemple)
?




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