chapter 4


1) Enumeration
• An enumeration types is one in which the user enumerates all of the possible values, which are symbolic constants.
• In this all possible values are named constants and are enumerated in the definition. (called enumerated constants )
• In Ada a typical enumeration type is shown as: Type DAYS is {M,T,W,Th,F,Sa,S}
• In C,
- enum colors { red=1 , blue, green, yellow, pink, black}; colors mycolor = blue;
• Advantages : more readable program.
• Disadvantages in C#, Java : - no arithmetic operations are legal on enumeration types.     

subrange

• A subrange type is an ordered contiguous subsequence of an ordinal type.
• These types were introduced by Pascal.
• In Pascal ,
type DaysOfWeek = (Sunday, Monday, Tuesday, Wednesday, Thursday, Friday,
Saturday); DaysOfWorkWeek = Monday..Friday;
• In Ada,
type Days is (mon, tue, wed, thu, fri, sat, sun);
subtype weekdays is Days range mon…….fri;
• Advantages:
• More readable program
• Reliablity

           Array types

• An array is a homogeneous aggregate of data elements in which an individual element is identified by its position in the aggregate.  

1. Static array :• subscript ranges are statically bound and storage allocation is static.
• advantage: efficiency
• eg. Arrays declared in C, C++ using static modifier.
2. Fixed stack-dynamic array : • subscript ranges are statically bound.
• storage allocation is done at elaboration time.
• Ex. Arrays declared in C and C++ functions 
3. Stack dynamic array : • Subscript ranges and the storage allocation are dynamically bound at
elaboration time.
• Once the subscript ranges are bound and the storage is allocated they remain fixed during the lifetime of the variable.
• adv. : flexibilility.
Ex. Supported by Ada 
4. Fixed heap-dynamic array: • Both the subscript ranges and storage binding are done when the user program requests them during execution.
• storage is allocated from the heap.
• Supported by Fortran-95.
• eg. Array in C or C++ using malloc, new resp.
5. Heap dynamic array : • The binding of subscript ranges and storage allocation is dynamic and can change any number of times during array’slifetime.
• Adv. – arrays can grow and shrink during program execution.
• supported by C#, Javascript, Perl 

Record Types
• A record (struct in C) is possibly a heterogeneous aggregate of data elementsin which the individual elements are identified by names.
• Design issues are : – What is the syntactic form of references to fields?
– Are elliptical (indirect) references allowed?

Discriminated VS Free union
• In Fortran the Equivalence statement is used to specify unions.
• In C, C++ ,the unions are called free unions, because
programmers are allowed complete freedom from type checking in their use.
• Type checking of unions requires that each union construct include a type indicator.
• Such an indicator is called a tag or discriminant and a union with a discriminant is called discriminated union.
• Pascal allows the user to specify variables of a variant record type, that will store only one possible type values in the variant.              

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