1. Lecture 6 Concepts of Programming Languages
Arne Kutzner
Hanyang University / Seoul Korea

2. Concepts of Programming Languages
Topics
Primitive Data Types
Character String Types
User-Defined Ordinal Types
Array Types
Associative Arrays
Record Types
Union Types
Pointer and Reference Types
Concepts of Programming Languages

3. Concepts of Programming Languages
Introduction
A data type defines a collection of data objects and a set of predefined operations on those objects
Design issue for data types: What operations are defined and how are they specified?
Concepts of Programming Languages

4. Concepts of Programming Languages
Primitive Data Types
Primitive data types: Data types that are not defined in terms of other data types
Some primitive data types are some times merely reflections of the hardware
Others require only a little non-hardware support for their implementation (Code for supporting floating-point numbers on processor that does not support these.)
Almost all programming languages provide a set of primitive data types
Concepts of Programming Languages

5. Primitive Data Types: Integer
For representing numbers without fractions (without dot)
Often there are several different integer types with different sizes:
E.g.: Java’s signed integer sizes: byte, short, int, long
Concepts of Programming Languages

6. Primitive Data Types: Floating Point
Model real numbers (but only as approximations)
Many languages support at least two floating-point types (e.g., float and double) of different sizes
There is a standard for floating-point number:
IEEE Floating-Point Standard 754
Concepts of Programming Languages

7. Representation of floating-point values
32 bit single precision float
64 bit double precision float
Concepts of Programming Languages

8. Floating point example
IEEE representation scheme scheme: Example:
exponent
fraction
bit 1
bit 23
exponent is 1
Concepts of Programming Languages

9. Problems with floating point values
The IEEE representation scheme can result in unexpected approximations in the context of decimal numbers: E.g. the decimal value 0.1
Experiment: sum up 1Mio times 0.1 and check the result
Concepts of Programming Languages

10. Primitive Data Types: Complex
Some languages support a complex type, (e.g., Fortran and Python)
Representation: Each value consists of two floats, the real part and the imaginary part
Example: Literal form in Python:
(7 + 3j), where 7 is the real part and 3 is the imaginary part
Concepts of Programming Languages

11. Primitive Data Types: Decimal
Stores a number by using decimal digits (4 bits per digit)
Advantage: accuracy
Disadvantages: limited range, wastes memory
Supported by e.g.:
COBOL C#
Important in the context business applications (money)
Concepts of Programming Languages

12. Primitive Data Types: Boolean
Simplest of all
Range of values: “true” and “false”
Could be implemented as bits, but often as bytes
Concepts of Programming Languages

13. Primitive Data Types: Character
Stored as numeric codings
Most commonly used coding: ASCII
An alternative, 16-bit coding: Unicode
Includes characters from most natural languages
Originally used in Java
C# and JavaScript also support Unicode
Concepts of Programming Languages

14. Character String Types
Values are sequences of characters
Design issues:
Supported as
primitive data type or
build on top of other data-types (e.g. arrays)
Support of “immutable” strings
Pattern-matching as part of the language definition
Concepts of Programming Languages

15. Character String Types Operations
Typical operations:
Assignment and copying
Comparison (=, >, etc.)
Catenation
Substring reference
Pattern matching (support of regular expressions)
Concepts of Programming Languages

16. Character String Type in Certain Languages
C and C++
Not primitive
Use char arrays and a library of functions that provide operations
C++
Not primitive two approaches (C-strings and std::string-class)
Fortran and Python
Primitive type with assignment and several operations
Java
Semi-primitive via the String and StringBuffer classes
Perl, JavaScript, Ruby, and PHP
Primitive type
built-in pattern matching by using regular expressions
Concepts of Programming Languages

17. Concepts of Programming Languages
Ordinal Types
An ordinal type is one in which the range of possible values can be easily associated with the set of positive integers
Examples of primitive types that represent ordinal types
integer, char, boolean
User defined ordinal types
Enumerated types
Subrange types
Concepts of Programming Languages

18. Concepts of Programming Languages
Enumeration Types
Possible values are named constants
The named constants are provided in the context of data type definition
Examples: C#: enum days {mon, tue, wed, thu, fri, sat, sun}; Ada: type Days is (mon, tue, wed, thu, fri, sat, sun);
Represent user defined ordinal types
Concepts of Programming Languages

19. Enumeration Types (cont.)
Design Issue: are enumeration values coerced to integer?
In C, C++ coerced to integer types
In Ada, C#, Java 5.0 no coercion to integer types
Purposes:
Aid to readability, e.g., no need to code a color as a number
Aid to reliability, due to improved type safeness. (But only if not coerced to integers)
Concepts of Programming Languages

20. Concepts of Programming Languages
Subrange Types
Ordered contiguous subsequence of an ordinal type
Ada examples: subtype Weekdays is Days range mon..fri; subtype Index is Integer range ;
Concepts of Programming Languages

21. Purposes of subrange types
Improves readability
Informs readers that variables can only store certain range of values
Improves reliability
Allows convenient subrange checking / subrange monitoring
Concepts of Programming Languages

22. Implementation of User-Defined Ordinal Types
Enumeration types are normally implemented as integers
Subrange types are implemented like the parent types with code inserted (by the compiler) for range checking
Concepts of Programming Languages

23. Concepts of Programming Languages
Array Types
An array is an aggregate of homogeneous data elements in which an individual element is identified by its position in the aggregate, relative to the first element.
Concepts of Programming Languages

24. Concepts of Programming Languages
Array Indexing
Indexing (or subscripting) is a mapping from indices to elements
array_name (index_value_list)  an element
Index Syntax
FORTRAN, PL/I, Ada use parentheses
Most other languages use brackets
Concepts of Programming Languages

25. Concepts of Programming Languages
Array Indexing (cont.)
Types of array indices:
Integer only: Java, C, C++, Fortran
Ordinal types in general: Ada
Index Range Checking
By default: Java, C#
Not at all: C, Fortran
On demand: C++, Ada
Concepts of Programming Languages

26. Concepts of Programming Languages
Integration of Arrays
Direct via the language definition Examples: C, Fortran, Perl
Indirect via an appropriate class definition Examples: Java, C#
In this case arrays represent objects.
Languages can support both concepts simultaneously Example: C++
Concepts of Programming Languages

27. Concepts of Programming Languages
Arrays Memory Aspects
Location of array:
Static (allocation during compile time)
Stack (allocation during run time)
Heap (allocation during run time)
Size management
Static array sizes (known during compile time)
Dynamic management during runtime (automatic expansion if required)
Concepts of Programming Languages

28. Multidimensional arrays Rectangular versus Jagged Arrays
A rectangular array is a multi-dimensioned array in which all of the rows have the same number of elements and all columns have the same number of elements
A jagged array has rows with varying number of elements
Possible when multi-dimensioned arrays actually appear as arrays of arrays
Concepts of Programming Languages

29. Addressing schemes for rectangular arrays
Two common ways:
Row major order (by rows) – used in most languages
column major order (by columns) – used in Fortran
Concepts of Programming Languages

30. Row-order addressing scheme
Formula: Location (a[I,j]) = address of a [row_lb,col_lb] + (((I - row_lb) * n) + (j - col_lb)) * element_size
Concepts of Programming Languages

31. Concepts of Programming Languages
Array Initialization
Many languages allow an array initialization in the context of the array definition. Example for C, C++, Java, C#: int list [] = {4, 5, 7, 83};
Concepts of Programming Languages

32. Concepts of Programming Languages
Heterogeneous Arrays
A heterogeneous array is one in which the elements need not be of the same type
Supported by Perl, Python, JavaScript, and Ruby
Concepts of Programming Languages

33. Concepts of Programming Languages
Slices
A slice is some substructure of an array
more or less a referencing mechanism
Examples: Newer editions of Fortran (e.g. Fortran 95)
Concepts of Programming Languages

34. Concepts of Programming Languages
Slices (cont.)
Concepts of Programming Languages

35. Concepts of Programming Languages
Associative Arrays
An associative array is an array where the index (subscript) is not of ordinal type.
E.g. Indices of type string
Design issue:
- What is the form of references to elements?
Concepts of Programming Languages

36. Associative Arrays in Perl
Names begin with %; literals are delimited by parentheses
%hi_temps = ("Mon" => 77, "Tue" => 79, "Wed" => 65);
Subscripting is done using braces and keys
$hi_temps{"Wed"} = 83;
Concepts of Programming Languages

37. Record Types / Structures
A record is a possibly heterogeneous aggregate of data elements in which the individual elements are identified by names
Design issues:
Syntactic form of references to fields
Support of elliptical references
Concepts of Programming Languages

38. Concepts of Programming Languages
C++ Example
Definition of Datatype Emp_Rec_Type: struct Emp_Rec_Type { std::string First; std::string Mid; std::string Last;
float Hourly_Rate; }; Data definition: Emp_Rec_Type Emp_Rec;
Concepts of Programming Languages

39. Concepts of Programming Languages
Ada Example
Definition of Datatype Emp_Rec_Type: type Emp_Rec_Type is record First: String; Mid: String; Last: String; Hourly_Rate: Float; end record; Data definition: Emp_Rec: Emp_Rec_Type;
Concepts of Programming Languages

40. COBOL Example COBOL uses level numbers for creating nested records
Definition of record EMP-REC:
01 EMP-REC.
02 EMP-NAME.
05 FIRST PIC X(20).
05 MID PIC X(10).
05 LAST PIC X(20).
02 HOURLY-RATE PIC 99V99.
string consisting of 20 characters
Level numbers
Concepts of Programming Languages

41. References to Record/Structure fields
Record field references
Dot notation (almost all languages): record.field_name Example: Emp_Rec.Mid
Exception COBOL field_name OF record Example: FIRST OF EMP-NAME OF EMP-REC
Concepts of Programming Languages

42. Fully qualified versus elliptical references
Fully qualified references must include all components
Elliptical references allow leaving out record names as long as the reference is unambiguous,
Example in COBOL: FIRST, FIRST OF EMP-NAME, and FIRST of EMP-REC are elliptical references to the employee’s first name
Concepts of Programming Languages

43. Concepts of Programming Languages
Comparison to Arrays
Records are used when the collection of data values is heterogeneous
Access to array elements is a bit slower than access to record fields, because subscripts are dynamic (field names are static)
Concepts of Programming Languages

44. Concepts of Programming Languages
Unions Types
A union is a type whose variables are allowed to store different type values at different times during execution
Design issues
Type-checking on union level
Embedding with respect to records/structures
Concepts of Programming Languages

45. Discriminated vs. Free Unions
Fortran, C, and C++ provide union constructs without support of type checking for unions
This type of union is called free union
Unions with type-checking (during runtime) are called tagged unions or discriminated unions
Requires additional type indicator
Supported e.g by Ada
Concepts of Programming Languages

46. Ada Example for record with union
type Shape is (Circle, Triangle, Rectangle);
type Colors is (Red, Green, Blue);
type Figure (Form: Shape) is record
Filled: Boolean;
Color: Colors;
case Form is
when Circle => Diameter: Float;
when Triangle =>
Leftside, Rightside: Integer;
Angle: Float;
when Rectangle => Side1, Side2: Integer;
end case;
end record;
Concepts of Programming Languages

47. Example graphically illustrated
Concepts of Programming Languages

48. Concepts of Programming Languages
Unions - Discussion
Free unions are unsafe
Do not support type checking during runtime
Java and C# do not support unions
Reflective of growing concerns for safety in programming language
Descriminated unions are safer than free unions but you have to pay a price
Type check during runtime consumes additional computational resources
Concepts of Programming Languages

49. Pointer and Reference Types
A pointer type variable has a range of values that consists of memory addresses and a special value, nil/null/NULL
Provides the power of indirect addressing / memory access
Provides a way for dynamic memory management
Heap allocation / access
Concepts of Programming Languages

50. Concepts of Programming Languages
Issues with Pointers
Existence of untyped pointer variables (something like void * in C)
Existence of reference types (e.g. C++)
Explicit or implicit dereferencing
C/C++: explicit via *
Java: implicit (all non primitive types are references)
In the case of heap-management via pointers:
Allocation/Deallocation scheme
By hand using some new and delete
Automatic allocation and automatic deallocation (automatic garbage collection)
Concepts of Programming Languages

51. Concepts of Programming Languages
Pointer Operations
Two fundamental operations
Assignment is used to set a pointer variable’s value to some useful address
Dereferencing yields the value stored at the location represented by the pointer’s value
Concepts of Programming Languages

52. Pointer Assignment Illustrated
The assignment operation j = *ptr
Concepts of Programming Languages

53. Problems with Pointers and heaps
Dangling pointers
A pointer points to a heap-dynamic variable that has been deallocated
Lost heap-dynamic variable / lost heap space
There is an allocated heap-area that is no longer referred by any pointer
Resulting effect of “losing memory” is called memory leakage
Concepts of Programming Languages

54. Concepts of Programming Languages
Pointers in C and C++
Extremely flexible but must be used with care
Pointers can point at any variable regardless of when or where it was allocated
Used for dynamic storage management and addressing
Pointer arithmetic is possible
Explicit dereferencing and address-of operators
Untype pointers (void *)
void * can point to any type..
Concepts of Programming Languages

55. Concepts of Programming Languages
Reference Types
C++ includes a special kind of pointer type called a reference type that is used primarily for formal parameters
Advantages of both pass-by-reference and pass-by-value
Java extends C++’s reference variables and allows them to replace pointers entirely
References are references to objects, rather than being addresses
C# includes both the references of Java and the pointers of C++
Concepts of Programming Languages

56. Dangling Pointer Problem
Tombstone: extra heap cell that is a pointer to the heap-dynamic variable
The actual pointer variable points only at tombstones
When heap-dynamic variable de-allocated, tombstone remains but set to nil
Costly in time and space
Locks-and-keys: Pointer values are represented as (key, address) pairs
Heap-dynamic variables are represented as variable plus cell for integer lock value
When heap-dynamic variable allocated, lock value is created and placed in lock cell and key cell of pointer
Concepts of Programming Languages

57. Automatic Heap Management
Complex run-time process
Single-size cells vs. variable-size cells
Two approaches to reclaim garbage
Reference counters (eager approach): reclamation is gradual
Mark-sweep (lazy approach): reclamation occurs in memory tight situations
Concepts of Programming Languages

58. Concepts of Programming Languages
Reference Counter
Reference counters: Maintain a counter in every cell that store the number of pointers currently pointing at the cell
Advantage: intrinsically incremental, so significant delays in the application execution are avoided
Disadvantage: complications for cells connected circularly
Concepts of Programming Languages

59. Concepts of Programming Languages
Mark-Sweep
The run-time system allocates storage cells as requested and disconnects pointers from cells as necessary. Method:
Every heap cell has an extra bit used by collection algorithm
All cells initially set to garbage
All pointers traced into heap, and reachable cells marked as not garbage
All garbage cells returned to list of available cells
Disadvantage: When done, it causes significant delays during application execution.
Concepts of Programming Languages

60. Concepts of Programming Languages
Marking Algorithm
Concepts of Programming Languages

61. Concepts of Programming Languages
Variable-Size Cells
All the difficulties of single-size cells plus more
Required by most programming languages
If mark-sweep is used, the marking process becomes more difficult
Concepts of Programming Languages