Rcpp Quicksort error in R?

Question

I am using RCPP to build a C++ function quicksort for quicksort. I am trying to compile and load this function through the cxxfunction in R. I need to use the cxxFunction. I am getting the following message in R:

error: cannot convert 'Rcpp::IntegerVector {aka Rcpp::Vector<13, Rcpp::PreserveStorage>}' to 'int*' for argument '1' to 'int quicksort(int*, int, int)' make: *** [file15ecb338ec.o] Error 1

Can anyone tell me what is wrong with this?

incl <- 'int quicksort(int xx[], int left, int right) {

int i = left; 
int j = right;
int tmp;
int pivot = xx[(left + right) / 2];

/* partition */
while (i <= j) {
while (xx[i] < pivot)
i++;
while (xx[j] > pivot)
j--;
if (i <= j) {
tmp = xx[i];
xx[i] = xx[j];
xx[j] = tmp;
i++;
j--;
}
}

/* recursion */
if (left < j){
quicksort(xx, left, j);
}
if (i < right){
quicksort(xx, i, right);
}

return (quicksort(xx,i,j));
}
'
sortCpp <- cxxfunction(signature( x = "integer",left = "integer",
                                  right = "integer"),
                       body = 'IntegerVector arr(x);
                       int a = as<int>(left);
                       int b = as<int>(right);
                       return wrap(quicksort(arr,a,b));',
                       include = incl,
                       plugin = "Rcpp",
                       verbose = TRUE)

Just to see if I could call a recursive function using cxxfunction I created a factorial function and called this through cxxfunction and it worked fine.

incl <- 'int factorial(int n)
{
  return (n == 1 || n == 0) ? 1 : factorial(n - 1) * n;
}
'


factcpp <- cxxfunction(signature( x = "integer"),
                       plugin = "Rcpp",
                       include = incl,
                       body = 'int xnew = as<int>(x);
                       return wrap( factorial(x) );
                       ',
                       verbose = TRUE)

jcai · Accepted Answer · 2015-07-24 19:22:37Z

1

The problem is exactly what the compiler says. Your function expects an int* but you are passing it an Rcpp::IntegerVector. Looking at the documentation for Rcpp::IntegerVector, you can retrieve the raw pointer by .begin(), so you would call via quicksort(arr.begin(), a, b).

Also, unless you really need to roll your own quicksort, I would just use the Standard Library's std::sort:

src <- 'Rcpp::IntegerVector v = Rcpp::IntegerVector(vp);
        std::sort(v.begin(), v.end());
        return wrap(v);'
fun <- cxxfunction(signature(vp="integer"), body=src, plugin="Rcpp")
fun( c(5, 2, 7, -3) )

edited Jul 24, 2015 at 19:22

answered Jul 24, 2015 at 19:14

jcai

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7 Comments

oldtimetrad Over a year ago

I amended the line in the cxxfunction to 'return wrap(quicksort(arr.begin(),a,b));' I am now getting a 'error: 'quicksort' was not declared in this scope' error?

oldtimetrad Over a year ago

I placed the include before the body parameter in cxxfunction with your amendment and it compiled without errors!. when I ran sortCpp(y) on a vector of integers I got Error in sortCpp(y) : argument "left" is missing, with no default? Do you know what is the problem here?

jcai Over a year ago

You are not passing in left and right into sortCpp; and you defined sortCpp to expect those arguments.

jcai Over a year ago

When you wrote quicksort, you defined a left and right index for the part of the array to sort. I'm not sure if you defined the range to be inclusive [left, right] or half-open [left, right), but in the former case, to sort the entire array you would want to replace the quicksort(arr.begin(), a, b) with quicksort(arr.begin(), 0, arr.size() - 1) and in the latter case you want quicksort(arr.begin(), 0, arr.size()). Or, if you do intend sortCpp to sort a particular range in the array, you would call sortCpp(someVector, left, right) as you defined.

jcai Over a year ago

The expression is ill-formed as you concatenated my two expressions in a way that doesn't make any sense.. I highly recommend you get a better understanding of C++ first or you will run into a lot of frustration. I'm also 100% sure your quicksort doesn't work as it is recursive on all control paths. It will cause an infinite loop/stack overflow.

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