Boolean Expressions

​Boolean Value

• Logically

  • True

  • False

• C++

  • Represented by an integer in the background

    • false is 0

    • true is literal 1 by default

      • any non-zero value is truthy

Where it is a problem

• Some functions that are expected to be Boolean actually return an int.

  • int isalnum (int c);

  • check if character is alphanumeric (a decimal digit or an uppercase or lowercase letter)

• What some students in the past have done

char c = 'z';
if(isalnum(c) == true) {
    //do something
}
//Dose not always work

Solution

• Don't compare Boolean-ish functions with true or false.

  • Boolean-ish: return a value which is not strictly a Boolean value (e.g. int), but is used as if it returns a Boolean.

• Just use the value directly as a Boolean without comparing it.

• What you should do

char c = 'z';
if (isalnum(c)) {
    //do something
}
//Always works

Boolean Operators

• And: &&

• Or: ||

• Not: !

Boolean Expressions

• p && q is true if and only if both p and q are truthy (not 0/false)

  • && is Boolean AND

  • & is bitwise AND operation (does not produce a Boolean value)

• P || q is true if and only if either p or q, or both , is truthy (not 0/false)

  • || is Boolean OR

  • | is bitwise OR operation (does not produce a Boolean value)

• !p is true if and only if p is false

  • !p is false if and only if p is truthy (not 0/false)

  • ~ is bitwise negation (1's complement, does not produce a Boolean value)

Example

Boolean Operator-Notes

• ! has highest precedence, followed by &&, then ||

• If the value of an expression can be determined by evaluating just the sub-expression on left side of a logical operator, then the sub-expression on the right side will not be evaluated (short circuit evaluation)