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]

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;`

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.

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:
>> 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;`

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 Nixon