I am trying to implement some code to calculate Stirling numbers.  The code shown below provides the correct calculation but throws a Segmentation fault: 11 once it is done running. I suspect there is something with the way I am setting up the multidimensional array.



import std.stdio;
import std.datetime ;
import std.conv ;
import std.file;
import std.string;
import std.regex;
import std.bigint ;
import std.range : enumerate;


int stirling1(int n, int k)
{
        auto matrix = new int[][](n+1,k+1) ;
        for(int i = 0; i <= n; i++)
        {
            matrix[i][0] = 0 ;
        }
        for(int i = 0; i <= k; i++)
        {
            matrix[0][k] = 0 ;
        }
        for(int i = 1; i <= n ; i++)
        {
            for(int q = 1; q <= i ; q++)
            {
                if(q == 1 || i == q)
                {
                    matrix[i][q] = 1 ;
                }
                else
                {
                    matrix[i][q] = q*matrix[i-1][q] + matrix[i-1][q-1] ;
                }
            }
        }
        return(matrix[n][k]) ;
}

void main()
{
    writeln("s(n,k) for s(7,2)") ;
    writeln(stirling1(7,2)) ;
}

On 04/08/2018 06:15 PM, popgen wrote:
> I am trying to implement some code to calculate Stirling numbers.  The code shown below provides the correct calculation but throws a Segmentation fault: 11 once it is done running. I suspect there is something with the way I am setting up the multidimensional array.
[...]
> int stirling1(int n, int k)
> {
>          auto matrix = new int[][](n+1,k+1) ;
[...]
>          for(int i = 1; i <= n ; i++)
>          {
>              for(int q = 1; q <= i ; q++)

Should it be `q <= k` here? You're using q as an index into an array of length k + 1. If you go up to i, you'll exceed that and go out of bounds.

That you're seeing a segfault instead of a range error indicates that you're compiling with -release. Better not do that when debugging.

On Sunday, 8 April 2018 at 16:51:14 UTC, ag0aep6g wrote:
> On 04/08/2018 06:15 PM, popgen wrote:
>> [...]
> [...]
>>  [...]
> [...]
>>  [...]
>
> Should it be `q <= k` here? You're using q as an index into an array of length k + 1. If you go up to i, you'll exceed that and go out of bounds.
>
> That you're seeing a segfault instead of a range error indicates that you're compiling with -release. Better not do that when debugging.

Thank you for your help! And I learned two things, the error in the code and not to use -release while debugging.