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Next: Examples using logical operators
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Previous: Examples using Relational Operators
It is possible to specify more complex conditions than those which can be written using only the relational operators described above. Since the value of a condition has a numerical interpretation it could be operated on by the usual arithmetic operators, this is not to be recommended. There are explicit logical operators for combining the logical values true and false.
The simplest logical operator is not which is represented in
C++ by !. It operates on a single operand, and returns
false if its operand is true and true if
its operand is false.
The operator and, represented by &&, takes two
operands and is true only if both of the operands are
true. If either operand is false, the resulting
value is false.
or is the final logical operator and is represented by
||.
It results in true if either of its operands is
true. It returns false only if both its
operands are false.
The logical operators can be defined by truth tables as follows. Note that F is used for false and T is used for true in these tables.
Not ! And && Or || A !A A B A && B A B A || B F T F F F F F F T F F T F F T T T F F T F T T T T T T T
These tables show that not reverses the truth value of the operand, that the and of two operands is only true if both operands are true and that the or of two operands is true if either or both of its operands are true. Using these logical operators more complex conditions can now be written.
If i has the value 15, and j has the value 10, then the
expression (i > 10) && (j > 0) is evaluated by evaluating the
relation i > 10 (which is true), then evaluating the
relation j > 0 (which is also true), to give
true. If j has the value 
then the second
relation would be false, so the overall expression would be
false. If i has the value 5, then the first relation
would be false and the expression will be
false irrespective of the value of the second relation. C++
does not even evaluate the second relation in this situation.
Similarly, if the first relation is true in an or
(||) expression then the second relation will not be
evaluated. This short-circuit evaluation enables many
logical expressions to be efficiently evaluated.