Управляващи оператори - I

** Програмиране на алтернативи (избор на един от няколко варианта).
Пример: Да се намери стойността (в долари) на купчина американски монети (1 penny = 1 цент, 1 nickel = 5 цента, 1 dime = 10 цента, 1 quarter = 25 цента). Купчината съдържа еднакви монети.

// coins5.cpp
#include <iostream>
 #include <string>
 using namespace std;

 int main()
 {  cout << "Enter coin name: ";
    string name;
    cin >> name;
    double value = 0;

    if (name == "penny") value = 0.01;
    else if (name == "nickel") value = 0.05;
    else if (name == "dime") value = 0.10;
    else if (name == "quarter") value = 0.25;
    else
       cout << name << " is not a valid coin name\n";

    cout << "Value = " << value << "\n";
    return 0

// richter.cpp
#include <iostream>
#include <string>
using namespace std;

int main()
{  cout << "Enter a magnitude on the Richter scale: ";
   double richter;
   cin >> richter;

   if (richter >= 8.0)
      cout << "Most structures fall\n";
   else if (richter >= 7.0)
      cout << "Many buildings destroyed\n";
   else if (richter >= 6.0)
      cout << "Many buildings considerably damaged, "
         << "some collapse\n";
   else if (richter >= 4.5)
      cout << "Damage to poorly constructed buildings\n";
   else if (richter >= 3.5)
      cout << "Felt by many people, no destruction\n";
   else if (richter >= 0)
      cout << "Generally not felt by people\n";
   else
      cout << "Negative numbers are not valid\n";
   return 0;
}

// tax.cpp
#include <iostream>
#include <string>
using namespace std;

int main()
{  const double SINGLE_LEVEL1 = 21450.00;
   const double SINGLE_LEVEL2 = 51900.00;

   const double SINGLE_TAX1 = 3217.50;
   const double SINGLE_TAX2 = 11743.50;

   const double MARRIED_LEVEL1 = 35800.00;
   const double MARRIED_LEVEL2 = 86500.00;

   const double MARRIED_TAX1 = 5370.00;
   const double MARRIED_TAX2 = 19566.00;

   const double RATE1 = 0.15;
   const double RATE2 = 0.28;
   const double RATE3 = 0.31;

   double income;
   cout << "Please enter your income: ";
   cin >> income;

   cout << "Please enter s for single, m for married: ";
   string marital_status;
   cin >> marital_status;

   double tax;
   if (marital_status == "s")
   {  if (income <= SINGLE_LEVEL1)
         tax =  RATE1 * income;
      else if (income <= SINGLE_LEVEL2)
         tax = SINGLE_TAX1 + RATE2*(income - SINGLE_LEVEL1);
      else
         tax = SINGLE_TAX2 + RATE3*(income - SINGLE_LEVEL2);
   }
   else
   {  if (income <= MARRIED_LEVEL1)
         tax =  RATE1 * income;
      else if (income <= MARRIED_LEVEL2)
         tax = MARRIED_TAX1 + RATE2*(income - MARRIED_LEVEL1);
      else
         tax = MARRIED_TAX2 + RATE3*(income - MARRIED_LEVEL2);
   }
   cout << "The tax is $" << tax << "\n";
   return 0;
}

if (now.get_hours() == homework.get_hours() &&
    now.get_minutes() == homework.get_minutes())
       cout << "The homework is due right now!\n";

if (state == "HI" || state == "AK")
    shipping_charge = 10.00;

Таблици за стойностите на логическите операции:

** Отложени пресмятания

• За логическата операция and - ако първият аргумент е false и стойността на операцията е false, независимо от стойността на втория аргумент.
• За логическата операция or  - ако първият аргумент е true и стойността на операцията е true, независимо от стойността на втория аргумент.

Затова и двата случая не се проверява стойността на втория аргумент - нарича се отложено пресмятане.

Пример.

cin >> area; 
if (cin.fail() || area < 0) cout << "Input error.\n";

if (!(country == "USA" && state != "AK" && state != "HI"))
                      shipping_charge = 20.00;

state != "AK" && state != "HI"

!(state == "AK") && !(state == "HI")

!(state == "AK") && !(state == "HI")

!(state == "AK" || state == "HI")

!(country == "USA" && 
!(state == "AK" || state == "HI"))

!(country == "USA") || 
(state == "AK" || state == "HI")

country != "USA" || state == "AK" || state == "HI"

i = start;
while (i <= end)
{  . . . 
   i++;
}

for (i = start; i <= end; i++)

// forfac.cpp
#include <iostream>
using namespace std;

long forfac(int n)
{  long product = 1;
   for (int i = 1; i <= n; i++) product = product * i;
   return product;
}

int main()
{  cout << "Please enter a number: ";
   int n;
   cin >> n;  
   cout << n << "! = " << forfac(n) << "\n";
   return 0;
}

for (int n = 10; n >= 0; n--) ...

for (x = -10; x <= 10; x = x + 0.5) ...

for (rate = 6; month--; cout << balance) ... // BAD

for (;;) ...
while(true) ...