导致访问冲突错误的阵列条目删除

Question

I have been working on an assignment for university the past few hours and I am pretty much finished save for one error. I managed to track down the error to a loop inside this function. The function is supposed to remove a desired entry from the call_DB array, then shift the following entries up one index number, then subtract the count by one so that no empty space is left in the array which is later printed into a text file. That function is here:

void remove_DB(call_record call_DB[], int & count, string cell_number)
{

   int location = search_DB(call_DB, count, cell_number);

   while (location != -1)
   {
        location = search_DB(call_DB, count, cell_number);

        if (location != -1)
        {
          for (int i = location; i < count; i++)
          {
            call_DB[i] = call_DB[i + 1];
          }

          count--;
        }

   }
 }

In case it is relevant the search function looks through a user defined class array (sorry if that is the wrong terminology) and returns the index 'i' of the location for the searched entry. It returns a -1 if nothing is found. That code is here:

int search_DB(call_record call_DB[], int count, string cell_number)
{

 int i;

   for (i = 0; i < count; i++)
   {

      if (call_DB[i].cell_num == cell_number)
      {
        return i;
      }
      return -1;
   }

}

Specifically, the error occurs after the for loop exits when it hits the 'count - -;' statement inside remove_DB. The last time the loop executes before the error the value for call_DB[i +1] becomes:

"call_DB[i+1] {cell_num= relays=1 minutes=7116664 ...}"

according to the value monitor in Visual Studio. If the code proceeds after this iteration a pop-up error which displays the message:

"Unhandled exception at 0x0F6131CA (msvcr120d.dll) in my_program.exe: 0xC0000005: Access violation reading location 0x68FD7339."

Followed by a tab opening in Visual Studio which tells me memcpy.asm is not found.

I hope this isn't something obvious I'm not seeing but I'm really stumped here so any help would be appreciated.

Answer 1

If i == count-1 , and DB is an array that has count entries, then DB[i + 1] is out of bounds:

for (int i = location; i < count; i++)
{
  call_DB[i] = call_DB[i + 1];
}

Should be:

for (int i = location; i < count-1; i++)
  call_DB[i] = call_DB[i + 1];

Second, your loop that searches is wrong. If the item isn't at position 0 in the array, you will always return -1.

It should be:

   for (int i = 0; i < count; i++)
   {
        if (call_DB[i].cell_num == cell_number)
        return i;
   }
   return -1;

But you have another issue, and that is you'll have trouble deleting the last item in DB if DB consists of purely data, and not a dummy or sentinel value at the end of the array. The easiest fix for that would be to declare your array 1 entry larger than what you expect, and have the last entry serve as a dummy value.

Answer 2

Basically, all you need to do, is find the index of the data you want to remove and move everything below that one slot up, thus overwriting the data you want to delete.

void remove_DB(call_record call_DB[], int & count, string cell_number)
{

   int location = search_DB(call_DB, count, cell_number);

   if (location < 0)
      return;

   for (location; location < count - 1; location++)
   {
       call_DB[location] = call_DB[location+1];
   }
   count--;
   return;
}