gistfile1.cpp

# include <iostream>
# include <string>
# include <cstdlib>

void readValue(int& v, const std::string& str)
{
    std::cout << "Please enter " << str << " and press ENTER: ";
    std::cin >> v;
    
    if( v <= 0)
    {
        std::cout << str << " must be a non-negative value. Try again" << std::endl;
        readValue(v, str);
    }
}

int main()
{
    std::cout << "This program calculates area" << std::endl;
    
    int w;
    while (42)
    {
        std::cout << "0 to exit; 1 for triangle; 2 for trapeze" << std::endl;
        std::cin >> w;
        if (w == 0) break;
        
        if (w == 1)
        {
            int a,h;
            readValue(a, "Base length");
            readValue(h, "Vertical height");

            std::cout << "Area of triangle is: " << a*h/2. << std::endl; // Дабл, не теряй результат
        }
        else if (w == 2)
        {
            int a,b,h;
            readValue(a, "Base length of A");
            readValue(b, "Base length of B");
            readValue(h, "Vertical height(h)");

            std::cout << "Area of trapeze is:"  // У тебя тут Треугольник написано
                << (a+b)/2.*h << std::endl;
        }
        else std::cout << "Wrong value. Try again." << std::endl;
    }
    
    std::cout << "Thank's for using" << std::endl;    	
    exit(0);
}