For loop is overwriting loop counter? - c++

Question

The code below is my source code that was part of a lab test that I recently took. I was counted off points because the program did not properly display the line and color it as it was supposed to. I found this incredible, as I had tested it for 3 points (to draw two lines), in order to save time on a timed test, and it works perfectly. The example input for the test was for 5 points (four lines). When I downloaded my code and tested it with 5 points, the graphical display does indeed go haywire, drawing seemingly random lines. I have debugged it, and after the third iteration (fourth time through the loop) of the first for loop where the program is collecting the x and y coordinates from the user, whatever is entered for the x coordinate value appears to be overwriting the loop control variableno_points[0], for no apparent reason. My thoughts are that the loop control variable, and the fourth x coordinate value are sharing an address somehow. As I said, I have already taken the test and received my grade, so I am not looking for a handout to cheat on anything. I simply am not able to understand why this occurring. Any help would be appreciated.
#include <iostream>
#include "graph1.h"
#include <cstdlib>
using namespace std;
// declaring prototypes
void getData(int* no_points, int* x, int* y, int* r, int* g, int* b);
void drawPolyLine(int* objects, int*x, int* y, int* no_points);
void colorPolyLine(int* objects, int* no_points, int r, int g, int b);
// declaring main
int main()
{
int no_points = NULL;
int x = NULL;
int y = NULL;
int r = NULL;
int g = NULL;
int b = NULL;
int objects[50] = {};
int again = 1;
do
{
displayGraphics();
clearGraphics();
getData(&no_points, &x, &y, &r, &g, &b);
drawPolyLine(objects, &x, &y, &no_points);
colorPolyLine(objects, &no_points, r, g, b);
cout << "Please enter a 0 to exit the program..." << endl;
cin >> again;
} while (again == 1);
return 0;
}
// declaring functions
void getData(int* no_points, int* x, int* y, int* r, int* g, int* b)
{
cout << "Enter # of points: " << endl;
cin >> *no_points;
cout << "Number of points entered is " << *no_points << endl;
cout << "Enter r/g/b colors..." << endl;
do
{
cout << "Enter a red value between 0 and 255 " << endl;
cin >> *r;
} while (*r < 0 || *r > 255);
do
{
cout << "Enter a green value between 0 and 255 " << endl;
cin >> *g;
} while (*g < 0 || *g > 255);
do
{
cout << "Enter a blue value between 0 and 255 " << endl;
cin >> *b;
} while (*b < 0 || *b > 255);
for (int i = 0; i < no_points[0]; i++)
{
cout << "Enter the x/y coord for Point #" << i + 1 << endl;
cin >> x[i]; cin >> y[i];
}
}
void drawPolyLine(int* objects, int* x, int* y, int* no_points)
{
for (int i = 0; i < no_points[0] -1; i++)
objects[i] = drawLine((x[i]), (y[i]), (x[i + 1]), (y[i + 1]), 3);
}
void colorPolyLine(int* objects, int* no_points, int r, int g, int b)
{
for (int i = 0; i < no_points[0] - 1; i++)
{
setColor(objects[i], r, g, b);
}
}

Answer 1

the x coordinate value appears to be overwriting the loop control variable "no_points[0]", for no apparent reason.
Well, for no reason that is apparent to you anyway.
In your main program you declare all your variables no_points, x, y, etc. as scalars, not arrays. That is, each variable accommodates one int. Your other functions treat the pointers to those variables (that you provide as arguments) as if they pointed into arrays at least no_points elements in length. Accessing elements past the first (at index 0) produces undefined behavior.
Although one cannot actually predict the outcome of undefined behavior from the code and the standard, memory corruption is a common outcome of the kind of incorrect code you present.