Vfrdtyky

I am new to C++ and working for an assignment, using C++98. I am trying to loop through an array of objects using pointer to array.

It is printing first point correctly, and after that some value.

Here are the code snippets:

struct Point { int x, y; };

int randomNumber(int low, int high ) {

   int randomNumber = low + (rand())/ (RAND_MAX/(high-low));

   cout << setprecision(2);

   return randomNumber;

}



Point *generateCoordinates(int nodesPerBlock) {

    Point cities[nodesPerBlock];

    for (int i = 0; i < nodesPerBlock; i++) {

        cities[i].x = randomNumber(1, 50);

        cities[i].y = randomNumber(1, 50);

    }

    return cities;

}

int main() {

  int nodesPerBlock = 5;

  Point *points = generateCoordinates(nodesPerBlock);

  for (int n = 0; n < (nodesPerBlock-2); n++) {

    cout << "n: x=" << points[n].x << ", y=" << points[n].y << endl;

    cout << "n+1: x=" << points[n+1].x << ", y=" << points[n+1].y << endl;

  }

}

this prints:

n: x=1, y=4

n+1: x=18, y=11

n: x=2049417976, y=32767

n+1: x=2049417976, y=32767

n: x=2049417976, y=32767

n+1: x=2049417984, y=167804927

while the actual Points printed were:

Point : x=1, y=4.

Point : x=18, y=11.

Point : x=13, y=6.

Point : x=2, y=16.

Point : x=16, y=22.

referred this questions and this, but no success so far.

cities[nodesPerBlock] is a local variable in generateCoordinates function and it goes out of scope when the function exits.

You are returning an address to it and are accessing that address in main. This is undefined behavior to do so.

You have to allocate cities on the the heap using new (since you are using C++98) and then return that address to main. Then you will be able to reliably access that address.

After your processing, do not forget to delete the memory you have allocated at the end of main.

You can avoid memory allocation and deletion by changing your function to take an extra parameter which you can pass from main. Then cities can be an array of Points on the stack.

void generateCoordinates(Point cities, int nodesPerBlock);

There are three mistakes in your code:

• In generateCoordinates you declare an array cities with a size determined by the parameter nodesPerBlock. This is not allowed, because c++ forbids to declare an array of variable length. When you have an array with a variable size you have to explicitly allocate the memory. Though, the code may still compile because of a gnu extension (if you are using gcc), but still does not follows the standard.




• In generateCoordinates you return a pointer to a local variable cities which is deallocated after you exit the function. This is why you get random number.




• You forgot to include the relevant headers.

A correct way to do the task could be to:

• Allocate an array in main()




• Pass its pointer to generateCoordinates




• Also, do not forget to deallocate the array

Example of code:

#include <cstdlib>

#include <iomanip>

#include <iostream>



struct Point { int x, y; };

    int randomNumber(int low, int high ) {

       int randomNumber = low + (rand())/ (RAND_MAX/(high-low));

       std::cout << std::setprecision(2);

       return randomNumber;

    }



    void generateCoordinates(int nodesPerBlock, Point *cities) {

        for (int i = 0; i < nodesPerBlock; i++) {

            cities[i].x = randomNumber(1, 50);

            cities[i].y = randomNumber(1, 50);

        }

    }

    int main() {

      int nodesPerBlock = 5;

      Point *points = new Point[nodesPerBlock];

      generateCoordinates(nodesPerBlock, points);

      for (int n = 0; n < (nodesPerBlock-2); n++) {

         std::cout << "n: x=" << points[n].x << ", y=" << points[n].y << std::endl;

         std::cout << "n+1: x=" << points[n+1].x << ", y=" << points[n+1].y << std::endl;

      }

      delete points;

    }

As a final comment: in the header part of a code you should avoid using namespace std because, if you ever split the header into a separate file, every time you include it you will bring out all the names of std into the global space, thus possibly colliding with names from other #include.