4.13. Boolean Variables

As usual, for every type of value, there is a corresponding type of variable. In C++ the boolean type is called bool. Boolean variables work just like the other types:

bool fred;
fred = true;
bool test_result = false;

The first line is a simple variable declaration; the second line is an assignment, and the third line is a combination of a declaration and as assignment, called an initialization.

As I mentioned, the result of a comparison operator is a boolean, so you can store it in a bool variable

bool even = (n % 2 == 0);     // true if n is even
bool plus = (x > 0);          // true if x is positive

and then use it as part of a conditional statement later:

if (even) {
  cout << "n was even when I checked it";
}

A variable used in this way is called a flag, since it flags the presence or absence of some condition.

Q-1: Match the conditional statement to the correct boolean, given x = 2. Try again!
• x * 2 > 4
• false
• x >= 2
• true

Q-2: Match the statement to the word that describes it best. Try again!
• bool fred;
• variable declaration
• fred = true;
• assignment
• bool testResult = false;
• initialization

Q-3: What will print?

int n = 16;
int x = 4;

bool even = (n % 2 == 0);
bool plus = (x > 0);

if (even) {
  cout << "n was even when I checked it ";
}

if (plus) {
  cout << "x was positive when I checked it";
}

• n was even when I checked it x was positive when I checked it
• A space is not automatically added.
• x was positive when I checked it n was even when I checked it
• Make sure you follow the correct order of execution. Also, a space is not automatically added.
• x was positive when I checked it
• Take another look at the result from the modulus operator.
• n was even when I checked itx was positive when I checked it
• Both flags are made, and no space is added.
• x was positive when I checked itn was even when I checked it
• Make sure you follow the correct order of execution.

Q-4: What will print?

bool low_battery=true;
bool power_outage=true;

if(low_battery){

   if(power_outage){
       power_outage=!power_outage;
   }
   else{
       low_battery=false;
   }

   if(!power_outage){

     if(low_battery){
         cout<<"Charging your phone\n";
     }
     else{
         cout<<"Battery is charged\n";
     }

   }
   else{
     cout<<"There is no power\n";
   }

}

• nothing will print
• The value of ``low_battery`` is true so we enter the first ``if`` block.
• "Charging your phone"
• correct! ``low_battery`` stays true and we set ``power_outage`` to false.
• "Battery is charged"
• ``low_battery`` is true so we don't reach this ``else``.
• "There is no power"
• We change the value of ``power_outage`` to false before hand.

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More to Explore

• From cppreference.com

  • Fundamental types – including bool

  • C++ keywords

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