C++ program to select a shape (rectangle, square, or circle), input dimensions, and calculate the area:


#include <iostream>
#include <iomanip>


using namespace std;


const double Pi = 3.14;


int main()
{
char shape = ' ';
double l = 0.0;
double w = 0.0;
double A = 0.0;
double r = 0.0;        


while (shape != '0')
{
cout << "This program will find the area of a shape." << endl << endl;
cout << "Shape Menu: \n (r) rectangle \n (s) square \n (c) circle \n (0) exit" << endl;
cout << "Type the letter corresponding to your desired option: ";
cin >> shape;
cout << endl;


switch(shape)
{
case 'r':
case 'R':
cout << "You selected a rectangle." << endl << endl;
cout << "Enter the length of the rectangle: ";
cin >> l;
cout << "Enter the width: ";
cin >> w;
        
if (cin.fail())
{
cout << "Invalid input data. Expected a floating point value, but a character was entered." << endl;
cout << "Program terminating; please re-execute the program if you wish to begin again." << endl;
return 1;
}


A = l*w; //Formula to calculate area of rectangle


cout << "The area is " << A << " units squared." << endl;
cout << endl;
break;


case 's':
case 'S':
cout << "You selected a square." << endl << endl;
cout << "Enter the length of the square's side: ";
cin >> l;
                                
if (cin.fail())
{
cout << "Invalid input data. Expected a floating point value, but a character was entered." << endl;
cout << "Program terminating; please re-execute the program if you wish to begin again." << endl;
return 1;
}


A = l*l; //Formula to calculate area of square


cout << "The area is " << A << " units squared." << endl;
cout << endl;
break;


case 'c':
case 'C':
cout << "You selected a circle." << endl << endl;
cout << "Enter the length of the circle's radius: ";
cin >> r;


if (cin.fail())
{
cout << "Invalid input data. Expected a floating point value, but a character was entered." << endl;
cout << "Program terminating; please re-execute the program if you wish to begin again." << endl;
return 1;
}


A = Pi*r*r; //Formula to calculate area of circle
        
cout << "The area is " << A << " units squared." << endl;
cout << endl;
break;


case '0':
cout << "Terminating program. Have a nice day!" << endl;
return 1;


default:
cout << "Invalid menu input. Type t to try again or 0 to exit." << endl;
cin >> shape;
cout << endl; 
}
}
return 0;
}