1. Primitive Data Types: Numbers Strings Ordinal Types Pointers
Chapter 6
Primitive Data Types:
Numbers
Strings
Ordinal Types
Pointers

2. Data Types Data types provide a context for operations
the + operator works differently for different types of numbers (and for strings)‏
Types also limit the operations that can be performed to those that make sense
dividing strings or characters doesn't make sense
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3. Type Systems provide
A mechanism for defining types and associating them with language constructs
named constants, variables, record fields, parameters, …
A set of rules for
type equivalence
type compatibility
type inference
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4. Ways to think about types
Denotational
a type is a set of values
Constructive
types are built-in or created from existing types
Abstraction-based
a type is an interface consisting of a set of operations
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5. Definitions
data type
collection of data objects
a set of predefined operations
descriptor : collection of attributes for a variable
data object : instance of a user-defined (abstract data) type
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6. Ways of classifying types
Simple vs. compound
Built-in vs. user-defined
boolean, character, integer, floating point
enumeration types, classes, records/structures
discrete (enumerable) or not
integer, boolean, character, enumeration, subrange
real, compound
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7. Data Types Primitive Structured
not defined in terms of other data types
defined in the language
often reflect the hardware
Structured
built out of other types
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8. Integer Types Usually use hardware representation
May have several ranges
Java’s signed integer sizes: byte, short, int, long
C/C++ have unsigned versions of the same types
Scripting languages often just have one integer type
Python has an integer type and a long integer which can get as big as it needs to.
python integer is same as C long (?= 32 bits)‏
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9. Representing Integers
Can convert positive integers to base 2
How do you handle negative numbers with only 0s and 1s?
Sign bit
Ones complement
Twos complement - this is the one that is used
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10. Representing negative integers
Sign bit
Ones complement
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11. Twos Complement
To get the binary representation, take the complement and add 1
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12. Floating Point Types Model real numbers
only an approximation due to round-off error
For scientific use support at least two floating-point types (e.g., float and double; sometimes more
Usually based on hardware
IEEE Floating-Point Standard 754
32 and 64 bit standards
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13. Representing Real Numbers
We can convert the decimal number to base 2 just as we did for integers
How do we represent the decimal point?
fixed number of bits for the whole and fractional parts severely limits the range of values we can represent
Use a representation similar to scientific notation
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14. IEEE Floating Point Representation
Normalize the number
one non-zero bit before decimal point
Use one bit to represent the sign (1 for negative)‏
Use a fixed number of bits for the exponent which is offset to allow for negative exponents
Exponent = exponent + offset
(-1)sign 1.Fraction x 2Exponent
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15. Floating Point Types C, C++ and Java have two floating point types
double
Most scripting languages have one floating point type
Python's floating point type is equivalent to a C double
Some scripting languages only have one kind of number which is a floating point type
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16. Fixed Point Types (Decimal)‏
For business applications (money) round-off errors are not acceptable
Essential to COBOL
.NET languages have a decimal data type
Store a fixed number of decimal digits
Operations generally have to be defined in software
Advantage: accuracy
Disadvantages: limited range, wastes memory
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17. C# decimal Type 128-bit representation Range: 1.0x10-28 to 7.9x1028
Precision: representation is exact to 28 or 29 decimal places (depending on size of number)‏
no roundoff error
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18. Other Primitive Data Types:
Boolean
Range of values: two elements, one for “true” and one for “false”
Could be implemented as bits, but often as bytes
Character
Stored as numeric codings
Most commonly used coding: ASCII
An alternative, 16-bit coding: Unicode
Complex (Fortran, Scheme, Python)‏
Rational (Scheme)‏
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19. Character Strings Values are sequences of characters Operations:
Assignment and copying
Comparison (=, >, etc.)
Catenation
Substring reference
Pattern matching
Design issues:
Is it a primitive type or just a special kind of array?
Should the length of strings be static or dynamic?
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20. Character String Implementations
C and C++
Not primitive
Use char arrays and a library of functions that provide operations
SNOBOL4 (a string manipulation language)‏
Primitive
Many operations, including elaborate pattern matching
Java
String class
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21. String Length Options Static: COBOL, Java’s String class
Limited Dynamic Length: C and C++
a special character is used to indicate the end of a string’s characters
Dynamic (no maximum): SNOBOL4, Perl, JavaScript
Ada supports all three string length options
Aid to writability
As a primitive type with static length, they are inexpensive to provide--why not have them?
Dynamic length is nice, but is it worth the expense?
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22. String Implementation
Static length: compile-time descriptor
Limited dynamic length: may need run-time descriptor
not in C and C++
Dynamic length: needs run-time descriptor;
allocation/deallocation is main implementation issue
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23. User-Defined Ordinal Types
ordinal type : range of possible values corresponds to set of positive integers
Primitive ordinal types
integer
char
boolean
User-defined ordinal types
enumeration types
subrange types
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24. Enumeration Types
All possible values, which are named constants, are provided in the definition
C example
enum days {mon, tue, wed, thu, fri, sat, sun};
Design issues
duplication of names
coercion rules
Aid to readability, e.g., no need to code a color as a number
Aid to reliability, e.g., compiler can check:
operations (don’t allow colors to be added)
No enumeration variable can be assigned a value outside its defined range
Ada, C#, and Java 5.0 provide better support for enumeration than C++ because enumeration type variables in these languages are not coerced into integer types
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25. enums in C (and C++)‏ To define an enumerated type in C
enum weekday {Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday};
enum weekday today = Tuesday;
Use typedef to give the type a name
typedef enum weekday {Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday} weekday;
weekday today = Tuesday;
By default, values are consecutive starting from 0.
You can explicitly assign values
Enum months {January=1, February, …};
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26. Enumerations in Java 1.5
An enum is a new class which extends java.lang.Enum and implements Comparable
Get type safety and compile-time checking
Implicitly public, static and final
Can use either == or equals to compare
toString and valueOf are overridden to make input and output easier
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27. Java enum Example Defining an enum type Declaring an enum variable
enum Season {WINTER, SPRING, SUMMER, FALL};
Declaring an enum variable
Season season = Season.WINTER;
toString() and name() give you the string representation of the name
// println calls toString(), prints WINTER
System.out.println( season);
// name()returns WINTER
String name = season.name();
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28. Java enum Example valueOf lets you convert a String to an enum
Season s = valueOf(“SPRING”);
the ordinal() method returns the position of a particular value in the list of names
int n = s.ordinal() // returns 1
the static member values() gives you an iterable list of the enum values
for (Season s : Seasons.values()) {…}
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29. Subrange Types A contiguous subsequence of an ordinal type
Example: is a subrange of integer type
Ada’s design
type Days is (mon, tue, wed, thu, fri, sat, sun);
subtype Weekdays is Days range mon..fri;
subtype Index is Integer range ;
Day1: Days;
Day2: Weekday;
Day2 := Day1;
Aid to readability
Make it clear to the readers that variables of subrange can store only certain range of values
Reliability
Assigning a value to a subrange variable that is outside the specified range is detected as an error
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30. Implementation of User-Defined Ordinal Types
Enumeration types are implemented as integers
Subrange types are implemented like the parent types
code inserted (by the compiler) to restrict assignments to subrange variables
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31. Pointer and Reference Types
A pointer is a variable whose value is an address
range of values that consists of memory addresses plus a special value, nil (or null or NULL)‏
Provide the power of indirect addressing
Provide a way to manage dynamic memory
A pointer can be used to access a location in the area where storage is dynamically created (usually called a heap)‏
Generally represented as a single number
What are the scope of and lifetime of a pointer variable?
What is the lifetime of a heap-dynamic variable?
Are pointers restricted as to the type of value to which they can point?
Are pointers used for dynamic storage management, indirect addressing, or both?
Should the language support pointer types, reference types, or both?
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32. Pointer Operations
Two fundamental operations: assignment and dereferencing
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
Dereferencing can be explicit or implicit
C++ uses an explicit operation via *
j = *ptr
sets j to the value located at ptr
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33. Pointer Operations Illustrated
Dereferencing a pointer
j = *ptr
assignment
ptr = &j
allocation
ptr = (int*)malloc(
sizeof( int))‏
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34. Pointer Problems Dangling pointers (dangerous)‏ Garbage
A pointer points to a heap-dynamic variable that has been de- allocated
Garbage
An allocated heap- dynamic variable that is no longer accessible to the user program
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35. Pointers in C and C++ Extremely flexible but must be used with care
Pointers can point at any variable regardless of when it was allocated
Used for dynamic storage management and addressing
Pointer arithmetic is possible
Explicit dereferencing and address-of operators
void * gives a generic pointer
can point to any type
cannot be de-referenced but can be assigned to a pointer of a different type
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36. 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 refer to objects (including arrays) only
C# includes both the references of Java and the pointers of C++
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37. Evaluation of Pointers
Dangling pointers and dangling objects are problems as is heap management
Pointers are like goto's--they widen the range of cells that can be accessed by a variable
Pointers or references are necessary for dynamic data structures--so we can't design a language without them
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