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MANIPULATING DATA IN C - OPERATORS.
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All languages provide the necessary operators for assignment of values to
variables and for arithmetic and logical operations. C provides these and
many more.
The Assignment Operator.
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In C, assignment is a single equal sign (=), with the variable on the left
of the equal sign taking the value on the right . e.g. x = 7;
Assignments can be stacked as for example: a = b = c; In this case c is
assigned to b, which in turn is assigned to a.
Unary and Binary Operators.
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C supports the usual set of binary arithmetic operations:
Multiplication (*), division (/), modulus (%), addition (+) and
subtraction (-)
C also supports the 'unary minus' (a + (-b)), which performs an arithmetic
negation and the 'unary plus' (a + (+b)), as per the ANSI standard.
Increment (++) and Decrement (--) Operators.
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The unary operators, increment (++) and decrement (--), are single
operators that add or subtract 1 from any value. The addition or
subtraction can be done in the middle of an expression, either before or
after the expression is evaluated. The following examples illustrate the
use of the increment operator:
sum = a + b++;
This adds a and b together and assigns the result to sum and then
increments b by 1.
sum = a + ++b;
This increments b by 1, then adds a and b together and assigns the result
to sum.
The program INCDECOP.C (on diskette) illustrates the use of these operators.
Bitwise Operators.
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The following bit-level operators can be used in C:
shift left (<<) shift right (>>)
AND (&) OR (|) XOR (^) NOT (~)
These allow low-level operations on values, as shown in program BITWISE.C
which can be run from the CTUTOR menu.
Combined Operators.
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C allows a shorthand method of writing expressions that contain multiple
operators. The assignment operator (=) can be combined with the above
operators (unary, binary, increment, decrement and bitwise).
Most expressions of the form <variable> = <variable> <operator> <exp>;
can be replaced with <variable> <operator> = <exp>;
e.g. a = a + b; can be condensed to a+=b;
a = a % b; a%=b;
a = a <<b; a<<=b;
a = a & b; a&=b; etc.
The += combined form is used in the example program MATRIX.C (on diskette).
Address Operators.
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The address-of operator (&) returns the address of a given variable. Thus
&x is the address in memory of the variable x
The indirection operator (*) gives the variable to which a pointer variable
points. Thus *ptr returns the character to which the pointer (ptr), of
type char, points.
These operators are fully discussed in the notes entitled 'Arrays and
Pointers'.
Relational Operators.
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Relational operators allow the comparison of two values, yielding a result
which depends on wether the comparison is true or false. The result is 0
if false and 1 if true. The relational operators are:
> greater than <= less than or equal to
>= greater than or equal to == equal to
< less than != not equal to
These operators are most frequently used in 'if' statements, which are
fully described in the notes entitled 'Conditional Statements'.
Logical Operators.
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There are three logical operators AND (&&), OR (||) and NOT (!), which
perform differently from the bitwise operators previously described.
These logical operators work with logical values (true and false) allowing
the combination of relational expressions.
The operators && and || will short circuit, so that once the truth or
falsehood is established, the remaining expressions will not be evaluated.
The Comma Operator.
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The comma operator is used between multiple expressions inside a set of
parentheses. These expressions are then evaluated from left to right and
the entire expression assumes the value of the last one evaluated. The
example in the User's Guide on page 171 is as follows:
If 'oldch' and 'ch' are both of type char, then the expression
(oldch = ch, ch = getch());
assigns 'ch' to 'oldch', assigns the keyboard character to 'ch' so that
'oldch' also assumes that same character.