OPERATORS: && || !
The logical operators are generally used with Boolean values and, when
they are, they return a Boolean value. However, both && and || actually
return the value of one of their operands, so if the relevent operand
is not a Boolean value, the operator may return a non-Boolean value.
This is the logical AND operator, which returns a Boolean true if both
the operands are true. Logically it follows that if the first operand
is false, then the whole expression is false, and this is how the operator
works; It first evaluates the left hand operand, and if this returns
false then, without going any further, it returns false for the whole
expression. Otherwise it returns the value of the second operand: true
or false for a Boolean value, or the actual value itself if non-Boolean.
Assuming 'a' to be 3, 'b' to be 5, and 'c' to be 3, the following examples
all return true:
if((a == c) && (b == 5))
x = "bread" && (c == 3)
...while the following return 'cheese':
x = (b > c) && "cheese"
x = "bread" && "cheese"
This is the logical OR operator and it returns a value of true if
one or both of the operands is true. It works by first evaluating the
left-hand operand and, if this is true, disregarding the right-hand
one and returning true for the whole expression. If, however, the left-hand
operand is false, then it returns the value of the right-hand operand:
true or false if Boolean, or else the value itself. These next examples
both return true:
if((a == c) || (b == 9))
x = (a > b) || (c == 3)
...while this one returns 'cheese':
x = (a > b) || "cheese"
If, however, the first operand is not a Boolean value, the OR operator
returns the value of the first operand whether the second is true or
not; in the next example 'bread':
x = "bread" || (c == 3)
This is the logical NOT operator which returns false if its single
operand can be converted to true, or if it is a non-Boolean value:
x = !(a == c)
x = !"cheese"
...and true if its operand can be converted to false:
x = !(a > b)