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.

Q1

Match the conditional statement to the correct boolean, given x = 2.

x * 2 > 4 Choose a definition false true

x >= 2 Choose a definition false true

Check answer

Q2

Match the statement to the word that describes it best.

bool fred; Choose a definition assignment initialization variable declaration

bool test_result = false; Choose a definition assignment initialization variable declaration

fred = true; Choose a definition assignment initialization variable declaration

Check answer

Q3

What will print?

int n = 16;
int x = 4;

bool even_flag = (n % 2 == 0);
bool plus_flag = (x > 0);

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

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

n was even when I checked it x was positive when I checked it

Great!

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, But A space is after it.

x was positive when I checked itn was even when I checked it

Make sure you follow the correct order of execution.

Check answer

Q4

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.

Check answer

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More to Explore

• From cppreference.com

  • Fundamental types -- including bool

  • C++ keywords