memcpy elements of an array

[JamesB]

Dear All,

I started trying to learn C++ about a week ago, so please bear with me here. I'm trying to build a function that will insert one element before another in an array but I keep getting "Segmentation Faults".

Here's my first effort:

void ins_b_b4_a(double *arr, int a, int b)
{
  double temp = arr[b];
  memcpy(arr + a + 1, arr + a, sizeof(double) * (b-a));
  arr[a] = temp;
}

I want element b inserted before element a, e.g. I want [3, 4, 5, 6, 7, 8] to become [3, 7, 4, 5, 6, 8] where b == 4 and a == 1. Can anyone tell me where I'm going wrong? Is it that my two blocks of memory aren't allowed to overlap initially or something like that?

[hamsterman]

It works fine for me. Could it be that your array is not long enough? If no, post the whole code.

[JamesB]

Really? Could you show me the code that worked for you please? For me, the following

#include <iostream>
using namespace std;

void ins_b_b4_a(double *arr, int b, int a);
void swap(double *arr, int b, int a);
void print(double *arr, int n);

int main ()
{
  double arr[] = {4, 6, 3, 8, 9, 2, 7, 3, 5, 1};
  print(arr, 10);
  ins_b_b4_a(arr, 6, 3);
  print(arr, 10);
  cout << "\n";
  return 0;
}

void ins_b_b4_a(double *arr, int a, int b)
{
  double temp = arr[b];
  memcpy(arr + a + 1, arr + a, sizeof(double) * (b-a));
  arr[a] = temp;
}

void swap(double *arr, int b, int a)
{
  double temp = arr[a];
  arr[a] = arr[b];
  arr[b] = temp;
}

void print(double *arr, int n)
{
  cout << "\n" << arr[0];
  for (int i=1; i<n; i++) cout << ", " << arr[i];
}

gives me a segmentation fault

[JamesB]

It's OK. I've seen why; I'm being an idiot and putting my args the wrong way round. Sorry

[JamesB]

That said, I'm still getting strange results. So the following

#include <iostream>
using namespace std;

void ins_b_b4_a(double *arr, int b, int a);
void swap(double *arr, int b, int a);
void print(double *arr, int n);

int main ()
{
  double arr[] = {4, 6, 3, 8, 9, 2, 7, 3, 5, 1};
  print(arr, 10);
  ins_b_b4_a(arr, 6, 3);
  print(arr, 10);
  cout << "\n";
  return 0;
}

void ins_b_b4_a(double *arr, int b, int a)
{
  double temp = arr[b];
  memcpy(arr + a + 1, arr + a, sizeof(double) * (b-a));
  arr[a] = temp;
}

void swap(double *arr, int a, int b)
{
  double temp = arr[a];
  arr[a] = arr[b];
  arr[b] = temp;
}

void print(double *arr, int n)
{
  cout << "\n" << arr[0];
  for (int i=1; i<n; i++) cout << ", " << arr[i];
}

gives me output of:

4, 6, 3, 8, 9, 2, 7, 3, 5, 1
4, 6, 3, 7, 8, 8, 8, 3, 5, 1

Why is that please? That is, why the repeated 8s?

[JamesB]

Ah, gotcha. It seems memmove is the function for me. This works as if a buffer is being used, which is what I want I think. Thanks for your help though