Question

I am working on a program which generates a segmentation fault and I cannot understand why. If I remove the pointer declaration from “lowest” and “largest” variables and use them as pointerless integers the program works fine.

However, as soon as I try to use pointers, problems arise and I receive a segmentation fault. I realize that this is probably a very easy and well addressed issue, but I’ve tried to understand the code by looking on other similar problems. I haven’t found a solution for my problem yet. Nor do I understand what’s going wrong.

This is the code generating the problem (link to full source is below):

     cout << "This is the array containing the random numbers:\n";
 for(int *i=numbers; i != numbers + arrLength; i++) {
     if((*i % 200) == 0 && *i > 200) {
         cin.get();
         cout << endl;
     }
     else
         cout << *i << ' ';

     // Get statistics
     // In the continuation of getting, lowest, largest then adding to sum.
     // THIS PART IS MAKING SEGMENTATION FAULT.
     if(*i < *lowest)
         lowest =  i;
     if(*i > *largest)
         largest = i;
     sum += *i;
 }

The i variable points to an old reference which is declared after user input:

cout << "You entered: " << arrLength << "\n\n";

 int *numbers = new int[arrLength];

 // Fill the array with random numbers
 srand(time(NULL));
 int x;
 int range = 5001;
 for(int index=0; index<arrLength; index++){
     *(numbers + index) = rand() % range;
     x = rand() % 2;
     if(x > 0) {
         *(numbers + index) = *(numbers + index) * -1;
     }
 }

Please explain to be why my program isn’t working and what I am doing wrong. As I’ve said earlier everything works except the:

if(*i < *lowest)
         lowest =  i;
     if(*i > *largest)
         largest = i;

Full source:
http://pastie.org/2105963

Thanks in advance on this matter!

Answer 1

You are initialising lowest and largest to 0 and the dereference is accessing memory via a null pointer and so it falls over. Try this instead:

if (!lowest || *i < *lowest)
    lowest =  i;
if (!largest || *i > *largest)
    largest = i;

or:

int *numbers = new int[arrLength];
lowest = largers = numbers; // initialise to something non-null