While relational (comparison) operators can be used to test whether a particular condition is true or false, they can only test one condition at a time. Often we need to know whether multiple conditions are true simultaneously. For example, to check whether we’ve won the lottery, we have to compare whether all of the multiple numbers we picked match the winning numbers. In a lottery with 6 numbers, this would involve 6 comparisons, *all* of which have to be true. In other cases, we need to know whether any one of multiple conditions is true. For example, we may decide to skip work today if we’re sick, or if we’re too tired, or if won the lottery in our previous example. This would involve checking whether *any* of 3 comparisons is true.

Logical operators provide us with the capability to test multiple conditions.

C++ has 3 logical operators:

Operator | Symbol | Form | Operation |
---|---|---|---|

Logical NOT | ! | !x | true if x is false, or false if x is true |

Logical AND | && | x && y | true if both x and y are true, false otherwise |

Logical OR | || | x || y | true if either x or y are true, false otherwise |

**Logical NOT**

You have already run across the logical NOT unary operator in lesson 4.9 -- Boolean values ^{[1]}. We can summarize the effects of logical NOT like so:

Logical NOT (operator !) | |
---|---|

Operand | Result |

true | false |

false | true |

If *logical NOT’s* operand evaluates to true, *logical NOT* evaluates to false. If *logical NOT’s* operand evaluates to false, *logical NOT* evaluates to true. In other words, *logical NOT* flips a Boolean value from true to false, and vice-versa.

Logical NOT is often used in conditionals:

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bool tooLarge { x > 100 }; // tooLarge is true if x > 100 if (!tooLarge) // do something with x else // print an error |

One thing to be wary of is that *logical NOT* has a very high level of precedence. New programmers often make the following mistake:

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#include <iostream> int main() { int x { 5 }; int y { 7 }; if (! x > y) std::cout << "x is not greater than y\n"; else std::cout << "x is equal to or greater than y\n"; return 0; } |

This program prints:

x is equal to or greater than y

But *x* is not equal to or greater than *y*, so how is this possible? The answer is that because the *logical NOT* operator has higher precedence than the greater than operator, the expression `! x > y`

actually evaluates as `(!x) > y`

. Since *x* is 5, !x evaluates to *0*, and `0 > y`

is false, so the *else* statement executes!

The correct way to write the above snippet is:

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#include <iostream> int main() { int x { 5 }; int y { 7 }; if (!(x > y)) std::cout << "x is not greater than y\n"; else std::cout << "x is equal to or greater than y\n"; return 0; } |

This way, `x > y`

will be evaluated first, and then logical NOT will flip the Boolean result.

Best practice

If *logical NOT* is intended to operate on the result of other operators, the other operators and their operands need to be enclosed in parenthesis.

Simple uses of *logical NOT*, such as `if (!value)`

do not need parenthesis because precedence does not come into play.

Logical OR

The *logical OR* operator is used to test whether either of two conditions is true. If the left operand evaluates to true, or the right operand evaluates to true, or both are true, then the *logical OR* operator returns true. Otherwise it will return false.

Logical OR (operator ||) | ||
---|---|---|

Left operand | Right operand | Result |

false | false | false |

false | true | true |

true | false | true |

true | true | true |

For example, consider the following program:

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#include <iostream> int main() { std::cout << "Enter a number: "; int value {}; std::cin >> value; if (value == 0 || value == 1) std::cout << "You picked 0 or 1\n"; else std::cout << "You did not pick 0 or 1\n"; return 0; } |

In this case, we use the logical OR operator to test whether either the left condition (value == 0) or the right condition (value == 1) is true. If either (or both) are true, the *logical OR* operator evaluates to true, which means the *if statement* executes. If neither are true, the *logical OR* operator evaluates to false, which means the *else statement* executes.

You can string together many *logical OR* statements:

1 2 |
if (value == 0 || value == 1 || value == 2 || value == 3) std::cout << "You picked 0, 1, 2, or 3\n"; |

New programmers sometimes confuse the *logical OR* operator (||) with the *bitwise OR* operator (|). Even though they both have *OR* in the name, they perform different functions. Mixing them up will probably lead to incorrect results.

Logical AND

The *logical AND* operator is used to test whether both operands are true. If both operands are true, *logical AND* returns true. Otherwise, it returns false.

Logical AND (operator &&) | ||
---|---|---|

Left operand | Right operand | Result |

false | false | false |

false | true | false |

true | false | false |

true | true | true |

For example, we might want to know if the value of variable *x* is between *10* and *20*. This is actually two conditions: we need to know if *x* is greater than *10*, and also whether *x* is less than *20*.

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#include <iostream> int main() { std::cout << "Enter a number: "; int value {}; std::cin >> value; if (value > 10 && value < 20) std::cout << "Your value is between 10 and 20\n"; else std::cout << "Your value is not between 10 and 20\n"; return 0; } |

In this case, we use the *logical AND* operator to test whether the left condition (value > 10) AND the right condition (value < 20) are both true. If both are true, the *logical AND* operator evaluates to true, and the *if statement* executes. If neither are true, or only one is true, the *logical AND* operator evaluates to false, and the *else statement* executes.

As with *logical OR*, you can string together many *logical AND* statements:

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if (value > 10 && value < 20 && value != 16) // do something else // do something else |

If all of these conditions are true, the *if statement* will execute. If any of these conditions are false, the *else statement* will execute.

Short circuit evaluation

In order for *logical AND* to return true, both operands must evaluate to true. If the first operand evaluates to false, *logical AND* knows it must return false regardless of whether the second operand evaluates to true or false. In this case, the *logical AND* operator will go ahead and return false immediately without even evaluating the second operand! This is known as short circuit evaluation, and it is done primarily for optimization purposes.

Similarly, if the first operand for *logical OR* is true, then the entire OR condition has to evaluate to true, and the second operand won’t be be evaluated.

Short circuit evaluation presents another opportunity to show why operators that cause side effects should not be used in compound expressions. Consider the following snippet:

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if (x == 1 && ++y == 2) // do something |

if *x* does not equal *1*, the whole condition must be false, so ++y never gets evaluated! Thus, *y* will only be incremented if *x* evaluates to 1, which is probably not what the programmer intended!

Warning

Short circuit evaluation may cause *Logical OR* and *Logical AND* to not evaluate one operand. Avoid using expressions with side effects in conjunction with these operators.

As with logical and bitwise OR, new programmers sometimes confuse the *logical AND* operator (&&) with the *bitwise AND* operator (&).

Mixing ANDs and ORs

Mixing *logical AND* and *logical OR* operators in the same expression often can not be avoided, but it is an area full of potential dangers.

Many programmers assume that *logical AND* and *logical OR* have the same precedence (or forget that they don’t), just like addition/subtraction and multiplication/division do. However, *logical AND* has higher precedence than *logical OR*, thus *logical AND* operators will be evaluated ahead of *logical OR* operators (unless they have been parenthesized).

New programmers will often write expressions such as `value1 || value2 && value3`

. Because *logical AND* has higher precedence, this evaluates as `value1 || (value2 && value3)`

, not `(value1 || value2) && value3`

. Hopefully that’s what the programmer wanted! If the programmer was assuming left to right evaluation (as happens with addition/subtraction, or multiplication/division), the programmer will get a result he or she was not expecting!

When mixing *logical AND* and *logical OR* in the same expression, it is a good idea to explicitly parenthesize each operator and its operands. This helps prevent precedence mistakes, makes your code easier to read, and clearly defines how you intended the expression to evaluate. For example, rather than writing `value1 && value2 || value3 && value4`

, it is better to write `(value1 && value2) || (value3 && value4)`

.

Best practice

When mixing *logical AND* and *logical OR* in a single expression, explicitly parenthesize each operation to ensure they evaluate how you intend.

De Morgan’s law

Many programmers also make the mistake of thinking that `!(x && y)`

is the same thing as `!x && !y`

. Unfortunately, you can not “distribute” the *logical NOT* in that manner.

De Morgan’s law ^{[2]} tells us how the *logical NOT* should be distributed in these cases:

`!(x && y)`

is equivalent to `!x || !y`

`!(x || y)`

is equivalent to `!x && !y`

In other words, when you distribute the *logical NOT*, you also need to flip *logical AND* to *logical OR*, and vice-versa!

This can sometimes be useful when trying to make complex expressions easier to read.

Where’s the logical exclusive or (XOR) operator?

*Logical XOR* is a logical operator provided in some languages that is used to test whether an odd number of conditions is true.

Logical XOR | ||
---|---|---|

Left operand | Right operand | Result |

false | false | false |

false | true | true |

true | false | true |

true | true | false |

C++ doesn’t provide a *logical XOR* operator. Unlike *logical OR* or *logical AND*, *logical XOR* cannot be short circuit evaluated. Because of this, making an *logical XOR* operator out of *logical OR* and *logical AND* operators is challenging. However, you can easily mimic *logical XOR* using the *inequality* operator (!=):

1 |
if (a != b) ... // a XOR b, assuming a and b are Booleans |

This can be extended to multiple operands as follows:

1 |
if (a != b != c != d) ... // a XOR b XOR c XOR d, assuming a, b, c, and d are Booleans |

Note that the above XOR patterns only work if the operands are Booleans (not integers). If you need a form of *logical XOR* that works with non-Boolean operands, you can static_cast them to bool:

1 |
if (static_cast<bool>(a) != static_cast<bool>(b) != static_cast<bool>(c) != static_cast<bool>(d)) ... // a XOR b XOR c XOR d, for any type that can be converted to bool |

Quiz time

Question #1

Evaluate the following expressions.

Note: in the following answers, we “explain our work” by showing you the steps taken to get to the final answer. The steps are separated by a *=>* symbol. For example “(true || false) => true” means we evaluated “(true || false)” to arrive at the value “true”.

a) (true && true) || false

Show Solution ^{[3]}

b) (false && true) || true

Show Solution ^{[3]}

c) (false && true) || false || true

Show Solution ^{[3]}

d) (5 > 6 || 4 > 3) && (7 > 8)

Show Solution ^{[3]}

e) !(7 > 6 || 3 > 4)

Show Solution ^{[3]}

5.x -- Chapter 5 summary and quiz ^{[4]} |

Index ^{[5]} |

5.6 -- Relational operators and floating point comparisons ^{[6]} |