C++ application crash after computing area - c++

Question

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=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;
}
}
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.
cout << "area of cross is: " << area << endl;
return (area);
}
int main()
{
Cross cross;
string shapetype;
cout << "enter shape type: " << endl;
cin >> shapetype;
if(shapetype == "cross")
{
cross.setCrossCord();
}else
{cout << "error" << endl;};
cross.computeArea();
}
this is the error I'm getting from windows, and I'm puzzled why this is happening.

Answer 1

You must change the for loop in setCrossCord to be zero - based:
// change
for (int i=1; i<=12; i++)
// to
for (int i=0; i<12; i++)
Because arrays (and vectors etc.) are zerobased in C++
In fact you can see that this it the intended range because of the loop in computeArea.
The reason that the program crashes only after the computation is because out-of-bounds processing (specifically, writing) invoked Undefined Behaviour: it may crash or do random stuff: it's not error detection.
Here's a 'technically' fixed version of your code: Live on Coliru
However, you'd do good to (re)design your class to have only 1 responsibility (namely: represent a Cross, not do input or output. Also, the temporaries xVal and yVal should not live beyond the input procedure (and could be merged)).
#include <iostream>
#include <cmath>
#include <string>
using namespace std;
struct Cross
{
double xvalue[12], yvalue[12];
void setCrossCord()
{
for (int i=0; i<12; i++)
{
cout << "Please enter x-ordinate of pt " << i << ": ";
double xVal, yVal;
cin >> xVal;
xvalue[i] = xVal;
cout << endl;
cout << "Please enter y-ordinate of pt " << i << ": ";
cin >> yVal;
yvalue[i] = yVal;
cout << endl;
}
}
double 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)
double 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;
}

Answer 2

I bet your indexing is wrong - C/C++ uses 0 based indexing, hence:
for (int i=1; i<=12; i++)
and
for (int i=0; i<=12-1; i++)
are fishy.
In addition index + 1 is.

Answer 3

1)your code
cin>>yval;
yvalue[i]=yval;
can be changed to
cin>>yvalue[i];
2)arrays start from subscript 0 not 1
3)return something in main function(add return 0; statement in the end of main())
4)why are you returning any value from computearea() keep it void and don't return anything because you used the value in the function itself.
(i think point 3) causes the error but do correct all the four points in your code.