Chapter 4
of Programming Languages
by
Ravi Sethi
TYPES:DATA REPRESENTATION
• The Role of Types
• Basic Types
• Arrays: Sequences of Elements
• Records: Named Fields
• Unions and Variant Records
• Sets
• Pointers: Efficiency and Dynamic Allocation
• Two String Tables
• Types and Error Checking
THE ROLE OF TYPES
• Data objects and Data representation
Objects in an application have corresponding representations in a
program.
Example: correspondence between application and program
application
program
data
January 31
31
data
May 6
126
variable
d
n(integer #)
operation
tomorrow (d)
n+1
• Values and Their Types
1. Basic type
2. Constructed type
THE ROLE OF TYPES(cont’d)
• Type Expression
a) Describes how a data representation is built up
B) Can be used to
1. Represent data objects
2. Lay out values in the underlying machine
3. Check that operators are applied properly within expression
• Static Layout Decision
(next slide)
• A Preview of Type Names, Arrays, and Records
1. Type names: boolean, char, integer, real
2. Arrays: a sequence of elements
3. Records: a set of components, each with its own type
THE ROLE OF TYPES(cont’d)
BASIC TYPES
• Enumeration : a finite sequence of names written between parentheses
type day = (Mon, Tue, Wed, Thu, Fir, Sat, Sun);
• Integers and Reals
• Short-Circuit Evaluation of Boolean Expressions
Example : While (i >= 0 && x != A[ i ]) i = i-1 ;
• Subranges : a special case of basic types that restrict the range of
values of an existing type - weekends, weekdays
BASIC TYPES (cont’d)
• Programming Style: Characters and Type Conversion
Example :
C : characters are implicitly coerced to integers
int c;
c = getchar(); -> coerced to an int
Pascal : Conversion will be done explicitly
c = chr(ord(c))
i = ord(chr(i))
ARRAYS: SEQUENCES OF ELEMENTS
• Efficient access and storage allocation
• Indexed by integers or enumerations
• Array layout
1. Elements appear in consecutive location in the underlying machine
2. Elements can be computed in two parts
a) Pre-computation
b) Run time
3. Arrays of Arrays : row-major layout, column-major layout
ARRAYS: SEQUENCES OF ELEMENTS (cont’d)
• Array Bounds
a) Computed at compile time (Static evaluation)
b) Computed at run time (Dynamic evaluation)
• Array Values and Initialization : a sequence of values for array elements
Example: int coin[] = { 1, 5, 10, 25, 50, 100 };
RECORDS: NAMED FIELDS
• A record type is a template
• A Variable declaration allocates storage
• Operations on Records
Example : z. re := x. re + y. re
• A comparison of Arrays and Records
arrays
records
--------------------------------------------------------------------------------------------component types
component selectors
homogeneous
expressions evaluated
at run time
heterogeneous
names known at
compile time
UNIONS AND VARIANT RECORDS
• Union : a special case of a variant record with an empty
common part
• Variant record : a part common to all records of that type and
a variant part
• figure 4.7 (next slide)
UNIONS AND VARIANT RECORDS(cont’d)
UNIONS AND VARIANT RECORDS(cont’d)
• Layout of Variant Records
1. Fixed Part
2. Tag Field
3. Variant Part
• Variant Records compromise type safety
SETS
• Set Values:in Pascal, All elements must be of the same simple
type
• Set Types: Type set of S represents subsets of S
Example: var A : set of [1..3]
A can denote one of the following sets:
[ ],[1],[2],[3],[1,2],[1,3],[2,3],[1,2,3]
• A set of n elements: implemented as a bit vector of length n
• The basic operation on set is a membership test
POINTERS: EFFICIENCY AND DYNAMIC ALLOCATION
• More efficient to move or copy a pointer to the data structure
• Dynamically Implemented to use in data structure
• First-class citizens
• Pointer type must be declared before used
• Dereferencing operation : indirect access
Example Pascal: dynamic allocation on the heap,
dereferencing, assignment,
equality testing, deallocation
• Serve as links between cells (linked lists)
• Type and layout of storage are known statically before a
program runs
POINTERS: EFFICIENCY AND DYNAMIC
ALLOCATION(cont’d)
• The pointer operations affect access to storage
• Dangling pointer is a pointer to storage that is being used for
another purpose (deallocate the storage, but the reference to
the pointer is still there)
• Pascal restricts pointer operations (only new and dispose); C
stresses flexibility
• Rearranged inexpensively
TWO STRING TABLES
• From a distance, types in Pascal and C are very similar, both have arrays,
records and pointers. Differences in the treatment of pointers, lead to
differences in style. The variable-length string application is famous and
is used to show the differences (section 4.8)
• Pascal
• TeX
• troff
• Word
• all the words are in a pool (buffer), the words are index through an array
called start which contains the beginning position of the word in the pool
starting at 0 so start[0] = 0 and pool[start[0]] is the ’T’
Indirect access through an array
Integers as pointers - disadvantage compiler cannot check that it is a
character
TWO STRING TABLES
• From Yacc - Yet another Compiler Compiler
• start is the actual pointer address in pool not just an index
• C
1. Indirect access through pointers
2. Array name : pointer to a zeroth element - operations on pointers in C
allow successive characters in a string to be accessed
3. Dereferencing and yielding operators
Example:
p = &x;
*p = *p + 1; is same as x = x + 1;
TYPES AND ERROR CHECKING
• Types extend from values to expressions, the type of an
expression x + y can be inferred from the types x and y
• Types of variable bindings
1. Static or early bindings
2. Dynamic or late bindings
Pascal has static bindings of types and dynamic
bindings of values to variables. Lisp has dynamic binding of
both values and types
• Type systems : a set of rules
• Basic rule of type checking
1. Overloading : Multiple Meanings
2. Coercion : conversion from one type to another
3. Polymorphism : parameterized type
TYPES AND ERROR CHECKING(cont’d)
• Type names and type equivalence
Example :
x, y : array [0..9] of integer
z : array [0..9] of integer
In Pascal, x and y have the same type, but z does not
* Structural Equivalence
1. A type name is structurally equivalent to itself
2. Two types are formed by applying the same type constructor
3. After a type declaration, type n = T
TYPES AND ERROR CHECKING(cont’d)
* Forms of Name Equivalence
1. Pure name equivalence
2. Transitive name equivalence, type S = integer; T = S; U = integer;
3. Type expression equivalence
* Type Equivalence - sometimes left ambiguous in Pascal, sometimes
given rules like Modula-2
* Circular Types : linked data structures give rise to recursive or circular
types
struct x {
int p;
struct x *next;
};
TYPES AND ERROR CHECKING(cont’d)
• Static and Dynamic Checking
1. Type error occurs if an operation is improperly applied
2. Programs are checked statically
3. Dynamic checking is done during program execution
4. Strong type ensures freedom from type errors