Chap.2. Types, Operators, and Expressions

1
Chap.2. Types, Operators, and Expressions

• 2.1 Variable Names
• 2.2 Data Types and Sizes
• 2.3 Constants
• 2.4 Declarations
• 2.5 Arithmetic Operators
• 2.6 Relational and Logical Operators
• 2.7 Type Conversations
• 2.8 Increment and Decrement Operators
• 2.9 Bitwise Operators
• 2.10 Assignment Operators and Expressions
• 2.11 Conditional Expressions
• 2.12 Precedence and Order of Evaluation

Session 2 - 25 March 2003
2
2.1 Variable Names

• Names are made of letters and digits, but
• the first character must be a letter
• the underscore _ counts as a letter (useful for
  improving readability)
• dont begin variable names with underscore (since
  library routines often use such names)
• Case sensitivity
• Upper and lower case are distinct, so x and X are
  two different names
• Traditional C pratice is to use
• lower case for variable names
• upper case for symbolic constants
• Miscellaneous
• At least the first 31 characters of an internal
  name are significant (!!!)
• For external names, the standard guarantees
  uniqueness only for 6 characters (!!!)
• Keywords like if, else, int, float, etc., are
  reserved you cant use them as variable names

3
2.2 Data Types and Sizes

• Basic data types
• char, int, float, double
• Qualifiers
• short int, long int
• short is often 16 bits, long 32 bits, and int
  either 16 or 32 bits
• signed, unsigned apply to char and any integer
• if chars are 8 bits, unsigned chars have values
  between 0 and 255, while signed chars between
  -128 and 127 (in a twos complement machine)
• long double
• Standard Header Files
• ltlimits.hgt and ltfloat.hgt contain symbolic
  constants for all these sizes

4
Signed integer (1/3)

• Signed magnitude
• The leftmost bit is the sign bit

• Ones complement
• To negate a number, replace each 1 by 0 and
  each 0 by 1

5
Signed integer (2/3)

• Twos complement
• First step as ones complement
• Second step add one to the result

6
Signed integer (3/3)
000
001
111
110
010
101
011
100

• Excess 2m-1
• a number is stored as its true value plus 2m-1

7
2.3 Constants

• 1234 integer constant
• 123456789L long constant
• 1234U unsigned constant
• 123456789UL unsigned long
• 01234 octal (with a leading zero)
• 0X1234 hexadecimal
• x character constant
• (The value of x is the numeric value of x in
  the machines character set)
• \ooo ooo is one to three octal digits
• \xhh hh is 1 or 2 hexadecimal digits

• Complete set of escape sequences
• \a \b \f \n \r \t \v
• \\ \? \ \ \ooo \xhh
• I am a string string constant
• hello world string concatenation
• (a string constant is a array of characters, with
  a null character \0 at the end \0 represents
  the character with value zero)
• Be careful x is a character constant and x
  is string constant
• Enumeration constant
• enum booelean NO, YES
• enum escape BELL \a, TAB \t

8
2.4 Declarations

• All variables must be declared before use
• A declaration specifies a type, and contains a
  list of one or more variables of that type, as in
  
• int lower, upper, step
• char c, line1000
• A variable may be initialized in its declaration,
  as in
• char esc \\
• int i 0, limit MAXLINE 1
• float eps 1.0e-5
• External and static variables are intialized to
  zero by default
• Automatic variables for which there is no
  explicit initializer have undefined (garbage)
  values.
• const qualifier can be applied to the declaration
  of any variable to specify that its value will
  not be changed
• const double e 2.71828182845905
• const char msg warning
• int strlen(const char)

9
2.5 Arithmetic Operators

• , -, , /,
• The integer division / truncates any fractional
  part
• The expresssion x y, which reads x modulus y,
  produces the remainder when x is divided by y
• Machine dependance for negative operands
• The direction of truncation for /
• The sign of the result for
• The action taken on overflow or underflow
• Precedence rules
• The binary and - operators have the same
  precedence, which is lower than the precedence of
  , /, and , which is in turn lower than the
  unary and -.
• Arithmetic operators associate left to right.

10
2.6 Relational and Logical Operators

• Relational operators gt, gt, lt, lt
• Equality operators , !
• Logical operators ,
• Expressions connected by and are evaluated
  left to right, and evaluation stops as soon as
  the truth or falsehood of the result is known
• Example
• for (i0 iltlim-1 (cgetchar()) ! \n c
  ! EOF i)
• si c
• By definition, the numeric value of a relational
  or logical expression is 1 if the relation is
  true, and 0 if the relational is false
• Unary negation operator !
• Converts a non-zero operand into 0, and a zero
  operand into 1
• Example if (!valid) is equivalent to if (valid
  0)

11
2.7 Type Conversation

• When an operator has operands of different types,
  they are converted to a common type according to
  a small natural number of rules
• Hierarchy
• short int -gt int -gt long -gt float -gt double -gt
  long double
• Remark
• char is a (signed or unsigned) short int (!!!)
• Explicit type conversions cast unary operator
• (type-name) expression
• Example
• sqrt((double) n)

12
2.8 Increment and Decrement Operators

• The increment operator adds 1 to its operand,
  while the decrement opartor -- substracts 1
• and -- may be used either as prefixor postfix
  operators
• n increments n before its value is used
• n increments n after its value has been used
• Example
• x n
• y n
• sets x to 6 and y to 5, but in both cases n
  becomes 6

13
2.9 Bitwise Operators

• bitwise AND
• bitwise inclusive OR
• bitwise exclusive OR
• ltlt left shift
• gtgt right shift
• ones complement (unary)
• Examples
• n n 01777 (sets to zero all but the low-order
  7 bits of n)
• n n 077 (sets the last 6 bits of n to zero)

14
2.10 Assignment Operators and Expressions

• Most binary operators have a corresponding
  assignment operator op, where op is one of
• / ltlt gtgt
• If expr1 and expr2 are expressions, then
• expr1 op expr2
• is equivalent to
• expr1 (expr1) op (expr2)
• except that expr1 is computed only once
• Assignment expression has the same value as the
  left expression of the assignment
• Example
• while ((c getchar()) ! EOF)

15
2.11 Conditional Expressions

• The conditional expression
• expr1 ? expr2 expr3
• provides an alternate way to write the if-else
  statement.
• Note that the operator ? has three operands.
  Such an operator is commonly called a ternary
  operator.
• Example
• z (a gt b) ? a b / z max(a,b) /

16
2.12 Precedence and Order of Evaluation

• The rules for precedence and associativity of all
  operators are summarized in Table 2.1.
• C, like most languages, does not specify the
  order in which the operands of an operator are
  evaluated. The exceptions are
• ?
• ,