The logical operators are the boolean operators && (AND), ||(OR), and !(NOT).
They can create compound conditions and perform conditional execution of a program depending on multiple conditions.
A logical expression results in a value false or true.
"Truth" table for logical operators &&, ||
| A | B | A && B | A || B |
|---|---|---|---|
| true | true | true | true |
| true | false | false | true |
| false | true | false | true |
| false | false | false | false |
"Truth" table for logical operators !
| A | !A |
|---|---|
| true | false |
| false | true |
| x | y | Logical Expression | Result |
|---|---|---|---|
| 1 | -1 | x <= y || y >=0 | false |
| 0 | 0 | x > -2 && y == 0 | true |
| -1 | 0 | x && !y | true |
| 0 | 1 | !(x+1) || y - 1 > 0 | false |
A numeric value, such as x or x+1, is interpreted as "false" if its value is 0.
Any value other than 0 is interpreted as "true."
The OR operator || will return true only if at least one operand is true, so the value of the expression
(length < 0.2) || (length > 9.8)
is true if length is less than 0.2or greater than 9.8.
The AND operator && will return true only if both operands are true, so the logical expression
(index < max) && (cin >> number)
is true, provided index is less than max and a number is successfully input.
If the condition index < max is not met, the program will not attempt to read a number!
For logical operators && and ||, the left operand is evaluated first and if a result has already been decided, the right operand will not be evaluated!