1. Chapter 6 – Data Types
CSCE 343

2. Primitive Types Types not defined in terms of other types
double, int, char, etc.
Java signed integer types: byte, short, int, long
Decimal types: fixed accuracy (C#, COBOL)
Boolean type
Advantage readability, reliability
C: if (x=5){…}
Char type
Coding: ASCII (8 bit), Unicode (16 bit)

3. String Types Value: sequence of characters Issues: Typical operations
primitive or char array?
Static or dynamic length?
Typical operations
Assignment, comparison, catenation, substring, pattern matching
C, C++: char arrays, (string class)
SNOBOL4: primitive
Java: String class
Built in operations (.equals(), + , substring())

4. String Length Static: set when the string is created
COBOL, Java, C++ string, Python, Ruby
Limited Dynamic: varying length up to fixed size
char arrays in C using \0.
Dynamic: varying length no maxium
Perl, JavaScript, Java (StringBuilder)

5. Ordinal Types
Type with limited range associated with positive integers:
Java: integer, char, boolean
Enumeration types
Named constants for all values
C#:
enum days {mon, tue, wed, thu, fri, sat, sun};
enum days x = mon;
Aids readability
Aids reliability:
Compiler checks operations, range

6. Subrange Types A sub-range of an ordinal type Aid to readability
subtype Index is Integer range 0..99
Aid to readability
Clear about valid values
Reliability
Range checks

7. Array Types Array: aggregate of homogenous data elements Issues:
Each element is identified by position
Issues:
Legal types for subscripts?
Range checking?
Binding of subscript to ranges.
When does array allocation take place?
Allow multidimensional and/or ragged arrays
Max number of subscripts?
Initialization?
Slicing?

8. Indexes Types: Range checking: Index Type
FORTRAN, C, C++, Java: integer types only.
Pascal: any ordinal type (integer, boolean, char, enumeration)
Range checking:
C, C++, Perl, FORTRAN: No
Java, C#: Yes
Perl: Kind of (no error message, returns undef)
Index Type
static binding

9. Array Categories Must determine binding of subrange
binding memory allocation
stack or heap memory
Static: a[10]
Allocation, subscript range bindings at compile time. (efficient)
Fixed stack-dynamic: a[10]
Subscript ranges are static, allocation is done at declaration execution (space efficient)
Stack dynamic: a[x]
Subscript range and storage is done at run-time (declaration) fixed after allocation
Fixed heap-dynamic:a[x]
Similar to stack dynamic, fixed after allocation (heap)
Heap dynamic
Everything (range, storage) is dynamic (heap)
ArrayList, vector

10. Examples Static: C,C++ static modifier
Fixed stack-dynamic: C, C++ local arrays
Stack dynamic: C++
Fixed heap-dynamic: Java
Heap-dynamic: Perl, JavaScript
foo(){
int a[10];
int b[x];
int c[] = new int[x];
ArrayList d = new ArrayList();

11. Initialization C, C++, Java, C#: Character strings in C, C++:
int list[] = {4,5,7,83};
Character strings in C, C++:
char name[] = “Nick”;
Arrays of strings in C, C++:
char *names[] = {“Bob”, “Jake”, “Jane”};
names[0] is a pointer to ‘B’ (literal char array)
Java initialization of String objects:
String[] names = {“Bob”, “Jake”, “Joe”};
Java 2D arrays:
int[][] a = {{1,2}, {3,4,5}};

12. Array Implementation Contiguous memory Calculating address:
addr(a[i]) = addr(a[0]) + ( i * size )
In general:
addr(a[i]) = addr(a[lb]) + ( (i – lb) * size )

13. 2D Array Implementation
Contiguous memory:
Row-major order (most common)
Column-major order (FORTRAN)
Address functions:
Row major:
addr(a[i][j]) = addr(a[0][0]) + (((i * n) + j ) * size)

14. Array Slicing Extracting parts of arrays: Perl and Python
Python example
a1 = [0, 1, 2, 3, 4, 5, 6]
a2 = a1[3:5]
a3 = a1[:4]
a4 = a1[3:]

15. In-class Find the memory address of char a[20][15]; float b[15][20];
Make the following assumptions
a is allocated memory starting at byte 800 in memory
b is allocated memory starting at byte 1800 in memory
a float is 4 bytes
two dimensional arrays use row-major order
Find the memory address of
a[10][5]
b[5][10]

16. Associative Arrays Like the STL map (key, data value) pairs
Key is index to data value
typical data is stored in an array
array component selected with hash(key)
Python example

17. Record Types Record: aggregate of data elements
Elements (fields) identified by name
Before records parallel arrays were used
C: struct, Ada: record, COBOL: records
Access to elements with dot (.):
x.name
Operations:
Assignment allowed?
Comparison?

18. Implementation of Record Types
Compile time descriptor.
Runtime descriptor not needed, since this information is available at compile time.

19. Union Types Allows different types at different times
C,C++, no type checking (free union)
Ada includes type checking with discriminant.
Evaluation:
Increase flexibility
Potentially unsafe (type checking)
Java, C# do not support unions.
Example code for struct and union

20. Pointer and Reference Types
Pointer: value is memory address or NULL
Indirect addressing
Manage dynamic memory (heap)
Design issues:
What are scope & lifetime of pointer variables?
What is the lifetime of heap-dynamic variable?
Are there restrictions as to what a pointer can point to?
Are pointers used for dynamic storage management and/or indirect addressing
Should the language have pointer types, reference types, or both?

21. Pointer Operations Assignment and dereferencing
int *ptrInt, *array, x=5;
//assignment
ptrInt = &x;
array = malloc(x*sizeof(int));
//dereferencing
x = (*ptrInt)+1;
a[0] = x;

22. Pointer Problems Dangling Pointers int *p; { int x; p = &x;}
//p points to memory that has been reclaimed
*char foo(){char s[20]; return s;} yikes!
Lost heap-dynamic variables (memory leak)
int *ptrArray, a[30];
ptrArray = malloc(1000* sizeof(int));
ptrArray = a;

23. Pointers in C, C++ Point at any variable of correct type
Used for dynamic storage management and addressing
Pointer arithmetic (more interesing?)
Explicit dereferencing and address-of operators
Domain type need not be fixed (void *)
generic pointer
cannot be dereferenced

24. C Reference types Special kind of pointer type
Primary use: formal parameters
See example code
What are advantages and disadvantages of pass-by-reference?
Java extends C++ reference variables to replace pointers entirely
implicitly dereferenced
C# includes both references of Java and pointers of C++

25. Heap Management Two approaches to garbage collection:
Reference counters (eager approach): done incrementally
heap cell store count of references to it
mark-sweep (lazy approach): done when available cell list is empty
heap cell has a “garbage bit”
set all garbage bits on
check all pointes to turn off garbage bit
sweep to recover

26. Exam 1
review guide