Logical operators

Logical operators are used to perform boolean operations on values and variables. These operators are used in conditional structures and loops to combine multiple conditions and obtain a final result.

The most common logical operators are AND, OR, and NOT:

AND and OR Operators

The AND and OR operations are basic logical operations, whose results are given by the following table.

• AND returns true if both operands are true, and false otherwise.
• OR returns true if at least one of the operands is true.

If you think about it a bit, it’s the sense we usually use for the word AND and OR, respectively, in everyday usage.

For example, when you say

I take the coat if it rains AND it’s cold

You take the coat only if both conditions are met at the same time.

Whereas when you say,

I take the coat if it rains OR it’s cold

You take the coat if at least one of the two conditions is met.

NOT Operators

On the other hand, the NOT operator is very simple and just inverts a logical value.

Example in Different Languages

In languages with syntax derived from C, the operators are &&, ||, and !.

bool or_a_b = a || b;
bool and_a_b = a && b;
bool not_a_b = !a;

let or_a_b = a || b;
let and_a_b = a && b;
let not_a_b = !a;

or_a_b = a or b
and_a_b = a and b
not_a_b = not a

Short-Circuited Operators

In many languages, both AND and OR offer short-circuited versions. These operators are a mechanism to speed up the evaluation of expressions.

A short-circuited operator stops the evaluation as soon as it knows the result without needing to evaluate the rest of the operators.

This is done using the “trick” that an operation with multiple ANDs will be false as long as one of the members is false. Similarly, a series of conditions with OR will be true as long as one of its members is true.

We can use this to speed up the calculation of expressions, which is precisely what the short-circuited versions do.

• AND stops the evaluation as soon as it finds a false, and evaluates the expression to false.
• OR stops the evaluation as soon as it finds a true, and evaluates the expression to true.

That is,

For example, in this expression from C#,

// if (a > b) is true
// (c < d) will never be evaluated
bool result = (a > b) && (c < d);