I'm just curious why my application actually crashes after it compute the area of a cross.

I output is correct but it crashes after it calculation is done.

cross.cpp

``````void Cross::setCrossCord()
{
for (int i=0; i<=12; i++)
{
cout << "Please enter x-ordinate of pt " << i+1 << ": ";
cin >> xVal;
xvalue[i] = xVal;
cout << endl;
cout << "Please enter y-ordinate of pt " << i+1 << ": ";
cin >> yVal;
yvalue[i] = yVal;
cout << endl;
}
}

double Cross::computeArea()
{
int points = 12;
int running_total = 0;

for (int i=0; i<12-1; i++)
{
running_total = (xvalue[i]*yvalue[i+1]) - (xvalue[i+1]*yvalue[i]);  //cross calculation of coord in a cross
}                                                                     //(x1*y2)-(y1*x1)

running_total = (xvalue[points-1]*yvalue[0]) - (xvalue[0]*yvalue[points-1]);   // traverse back to the origin point
// (xn*y1)-(yn*x1)

area = abs(running_total / 2); //purpose of absolute is to make sure result is positive.
//polygon are specified in counter-clockwise order (i.e. by the right-hand rule), then the area will be positive.

return (area);
}

int main()
{
Cross cross;
string shapetype;

cout << "enter shape type: " << endl;
cin >> shapetype;

if(shapetype == "cross")
{
cross.setCrossCord();
}else
{cout << "error" << endl;};

cout << "area of cross is: " << cross.computeArea() << endl;
return 0;
}
``````

however if I change my for loop to

for(int 1=0; i<12; i++)

the calculation would actually be wrong.. why is this so?

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Last Post by kal_crazy

There is an access violation actually. See at line 5 you have added 1 to i. So the storage of the xvalue array starts from 1 instead of 0. I know you were trying to show the output for what point the user inputting co-ordinates for. However you can start i from 1 in the for loop condition.
So like this:

``````void Cross::setCrossCord()
{
for (int i=1; i<=12; i++)
{
cout << "Please enter x-ordinate of pt " << i << ": ";
cin >> xVal;
xvalue[i] = xVal;
cout << endl;
cout << "Please enter y-ordinate of pt " << i << ": ";
cin >> yVal;
yvalue[i] = yVal;
cout << endl;
}
}
``````

Then in the computeArea function start for loop also from 1 till 12.

Hope that works :)

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