## digitalmars.D - compiled program error

- John Nixon (28/28) Jun 07 2015 While developing with D (DMD64 D Compiler v2.067.0 on MacOS
- Adam D. Ruppe (9/14) Jun 07 2015 These are the relevant lines: pvi_calc writes to a global array
- John Nixon (5/20) Jun 07 2015 Thank you for your concise reply. I had forgotten the distinction
- tcak (7/36) Jun 07 2015 There is only one "pvi" array at all. So, pvi_calc function

While developing with D (DMD64 D Compiler v2.067.0 on MacOS 10.10.3) I had a program with an unexpected behaviour and I reduced it to the minimal form below. The error as indicated in the comment is that the function call pvi_calc changes int_1 when I think it should not. The result copied below. I hope this helps. Kind regards John Nixon import std.stdio; int n,cp; double[] pvi,int_1,int_2; void pvi_centre(const int centre){ int_1=pvi_calc(centre); writeln("int_1 = ",int_1); int_2=pvi_calc(n-1-centre);//pvi_calc is changing int_1! writeln("int_1 = ",int_1); return;} double[] pvi_calc(const int n1){ for(int i=0;i<=n1;++i)pvi[i]= 1; return pvi;} int main(){ n=10; pvi.length=n; int_1.length=int_2.length=n; pvi_centre(cp); return 0;} ~ int_1 = [1, nan, nan, nan, nan, nan, nan, nan, nan, nan] int_1 = [1, 1, 1, 1, 1, 1, 1, 1, 1, 1]

Jun 07 2015

On Sunday, 7 June 2015 at 18:59:00 UTC, John Nixon wrote:double[] pvi,int_1,int_2; int_1=pvi_calc(centre); int_2=pvi_calc(n-1-centre);//pvi_calc is changing int_1!double[] pvi_calc(const int n1){ return pvi;}These are the relevant lines: pvi_calc writes to a global array (pvi) then returns it. So the first two lines are just setting int_1 and int_2 both to pvi, the same thing. So since they all reference the same array, writing to any of them changes all of them. You'll probably want to move double[] pvi to inside pvi_calc so it becomes a new copy, or at least `return pvi.dup;` instead of just `return pvi;`

Jun 07 2015

On Sunday, 7 June 2015 at 19:05:16 UTC, Adam D. Ruppe wrote:On Sunday, 7 June 2015 at 18:59:00 UTC, John Nixon wrote:Thank you for your concise reply. I had forgotten the distinction between arrays and slices that is described in Ali Chehreli's book Programming in D. John Nixondouble[] pvi,int_1,int_2; int_1=pvi_calc(centre); int_2=pvi_calc(n-1-centre);//pvi_calc is changing int_1!double[] pvi_calc(const int n1){ return pvi;}These are the relevant lines: pvi_calc writes to a global array (pvi) then returns it. So the first two lines are just setting int_1 and int_2 both to pvi, the same thing. So since they all reference the same array, writing to any of them changes all of them. You'll probably want to move double[] pvi to inside pvi_calc so it becomes a new copy, or at least `return pvi.dup;` instead of just `return pvi;`

Jun 07 2015

On Sunday, 7 June 2015 at 18:59:00 UTC, John Nixon wrote:While developing with D (DMD64 D Compiler v2.067.0 on MacOS 10.10.3) I had a program with an unexpected behaviour and I reduced it to the minimal form below. The error as indicated in the comment is that the function call pvi_calc changes int_1 when I think it should not. The result copied below. I hope this helps. Kind regards John Nixon import std.stdio; int n,cp; double[] pvi,int_1,int_2; void pvi_centre(const int centre){ int_1=pvi_calc(centre); writeln("int_1 = ",int_1); int_2=pvi_calc(n-1-centre);//pvi_calc is changing int_1! writeln("int_1 = ",int_1); return;} double[] pvi_calc(const int n1){ for(int i=0;i<=n1;++i)pvi[i]= 1; return pvi;} int main(){ n=10; pvi.length=n; int_1.length=int_2.length=n; pvi_centre(cp); return 0;} ~ int_1 = [1, nan, nan, nan, nan, nan, nan, nan, nan, nan] int_1 = [1, 1, 1, 1, 1, 1, 1, 1, 1, 1]There is only one "pvi" array at all. So, pvi_calc function modifying that only one, and returns it back. When you first call pvi_calc, int_1 points to pvi array. When you call it second time, int_2 points to same pvi array again. Since both of them points to same array, value of int_1 (actually value of pvi array) changes.

Jun 07 2015