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Comparisons and Logic
Six operators in C are for making logical comparisons. The relevance of these operators will quickly become clear in the next chapter, which is about decisions and comparisons. The six operators which compare values are:
is equal to
is not equal to
is greater than
is less than
is greater than or equal to
is less than or equal to
These operators belong to the second group according to the scheme above but they do actually result in values so that they could be thought of as being a part of the first group of operators too. The values which they produce are called true and false. As words, "true" and "false" are not defined normally in C, but it is easy to define them as macros and they may well be defined in a library file:
#define TRUE  1
#define FALSE 0

Falsity is assumed to have the value zero in C and truth is represented by any non-zero value. These comparison operators are used for making decisions, but they are themselves operators and expressions can be built up with them.
1 == 1

has the value "true" (which could be anything except zero). The statement:
int i;

i = (1 == 2);

would be false, so i would be false. In other words, i would be zero.
Comparisons are often made in pairs or even in groups and linked together with words like OR and AND. For instance, some test might want to find out whether:
     (A is greater than B) AND (A is greater than C)

C does not have words for these operations but gives symbols instead. The logical operators, as they are called, are as follows:
logical AND
logical OR inclusive
logical NOT
The statement which was written in words above could be translated as:
     (A > B) && (A > C)

The statement:
(A is greater than B) AND (A is not greater than C)

translates to:
    (A > B) && !(A > C)

Shakespeare might have been disappointed to learn that, whatever the value of a variable tobe the result of
  thequestion = tobe || !tobe

must always be true. The NOT operator always creates the logical opposite: !true is false and !false is true. On or the other of these must be true. thequestion is therefore always true. Fortunately this is not a matter of life or death!

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