4. EXPRESSIONS

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4. EXPRESSIONS
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Arithmetic Operators

• The arithmetic operators are
• unary plus
• - unary minus
• addition
• - subtraction
• multiplication
• / division
• remainder (modulus)

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Arithmetic Operators

• The value of i j is the remainder when i is
  divided by j. For example, the value of 10 3 is
  1 the value of 12 4 is 0. The operator
  requires integer operands all other operators
  allow either integer or floating-point operands.
• If both operands are integers, / truncates the
  result toward 0.
• There is no exponentiation operator.
• Must use pow function defined in ltmath.hgt

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Operator Precedence

• When an expression contains more than one
  operator, the meaning of the expression may not
  be immediately clear
• Does i j k mean (i j) k or
• i
  (j k) ?
• In C, this potential ambiguity is resolved by
  operator precedence.
• Precedence of the arithmetic operators
• Highest - (unary)
• /
• Lowest - (binary)

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Operator Precedence

• Examples
• i j k means i (j k)
• -i -j means (-i) (-j)
• i j / k means (i) (j / k)

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Associativity

• Operator precedence rules alone arent enough
  when an expression contains two or more operators
  at the same level of precedence. The
  associativity of the operators now comes into
  play.
• The binary arithmetic operators are all left
  associative (they group from left to right) the
  unary operators are right associative.
• Examples of associativity
• i - j - k means (i - j) - k
• i j / k means (i j) / k
• i - j i k means (i - (j i)) k
• - - i means -(-i)

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Increment and Decrement Operators

• The and -- operators increment and decrement
  variables.
• Both operators have the same precedence as
  negation.
• Either operator can be prefix or postfix
• i
• i
• --i
• i--
• When used as a prefix operator, increments the
  variable before its value is fetched
• i 1
• printf("i is d\n", i) / prints "i is 2" /

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Increment and Decrement Operators

• When used as a postfix operator, increments
  the variable after its value is fetched
• i 1
• printf("i is d\n", i) / prints "i is 1" /
• printf("i is d\n", i) / prints "i is 2" /
• The -- operator has similar properties
• i 1
• printf("i is d\n", --i) / prints "i is 0" /
• i 1
• printf("i is d\n", i--) / prints "i is 1" /
• printf("i is d\n", i) / prints "i is 0" /

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Lvalues

• The increment and decrement operators require an
  lvalue (pronounced Lvalue) as their operand.
• An expression has an lvalue if it represents a
  storage location in computer memory.
• Variables are lvalues, since a variable is just
  a name for a storage location.
• Constants and most expressions containing
  operators are not lvalues, including the
  following
• 12
• i j
• -i

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Simple Assignment

• One way to change the value of a variable is by
  assignment.
• Simple assignment has the form v e, where v is
  an lvalue and e is an expression. Examples
• i 5 / i is now 5 /
• j i / j is now 5 /
• k 10 i j / k is now 55 /
• If v and e dont have the same type, then the
  value of e is converted to the type of v before
  assignment
• int i
• float f
• i 72.99 / i is now 72 /
• f 136 / f is now 136.0 /

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Simple Assignment (Continued)

• Assignment is a true operator, just like and
  . The value of v e is the value of v after the
  assignment.
• Assignments can be chained together
• i j k 0
• Since the operator is right associative, this
  statement is
• equivalent to
• i (j (k 0))
• The assignment operator may appear in any
  expression
• i 1
• k 1 (j i)
• printf("d d d\n", i, j, k) / prints "1 1 2"
  /
• Hiding an assignment inside a larger expression
  usually isnt a good idea.

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Compound Assignment

• In addition to the simple assignment operator ,
  C provides ten compound assignment operators,
  including these five
• - /
• The expression v e is equivalent to
• v v e,
• except that v is evaluated only once in the
  former expression. The other operators have
  similar meanings.

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Compound Assignment
Examples i 5 j 2 i j / i is now 7
/ i 5 j 2 i - j / i is now 3 / i
5 j 2 i j / i is now 10 / i 5 j
2 i / j / i is now 2 / i 5 j 2 i
j / i is now 1 /
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Compound Assignment
The compound assignment operators have the same
properties as the operator in particular,
theyre right associative and can be used in
arbitrary expressions.
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Expression Statements

• Any expression can be used as a statement
• i
• The value of the expression is discarded the
  statement has no effect unless the expression
  modifies one of its operands
• i 1 / stores 1 into i, then evaluates i
  and discard the result /
• i-- / fetches i, discards the result,
  then decrements i /
• i j / adds i and j, then discards the
  result /

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Expression Statements

• Expression statements are typically used to
  perform assignments or to increment or decrement
  variables.
• Warning A minor typing mistake can turn a
  meaningful statement into a meaningless
  expression statement. For example, the statement
• i j
• could easily become
• i j