Enumerated Types

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Enumerated Types
An enumerated type is one in which all of the
possible values are enumerated in the definition.
Each of the values becomes associated with a
symbolic constant.
Enumerated types enhance readability, reliability.
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Enumerated Types
An enumerated type is created in C/C as
follows enum suit clubs, hearts, spades,
diamonds
This creates a user defined type enum suit. The
identifiers are clubs, hearts, spades, Each
identifier represents a constant integer,
starting at zero.
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Enumerated Types
An enumerated type is created in C/C as
follows enum suit club, diamond, heart,
spade (Default values are club0, diamond1,
heart2, spade3)
A variable of enum suit is created as enum suit
card1,card2 card1 club card2 spade
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Enumerated Types

• Design Issues
• Is a literal constant (the name) allowed to
• appear in more than one type definition?
• If so, how can type checking be performed?

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Enumerated Types
Pascal, C/C Literal constants must be unique
within a referencing environment
Ada Literal constants may be overloaded
Java No enumerated types
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Enumerated Types
Ada type LETTERS is (A, B, C, D, E,
F, G, H, I) type VOWELS is (A, E,
I, O, U) for LETTER in
VOWELS(A)..VOWELS(I) loop
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Arrays
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Array Types
An array is a homogeneous aggregate of data
elements in which each individual element is
identified by its relative offset from the first
element.
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Array Types

• Design Issues
• types for subscripts
• binding of subscript ranges
• when is memory actually allocated?
• limits on array dimensionality
• special operations
  (initialization, assignment, slicing, output)

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Array Types subscripts
References to array elements are made with a
specification of the array name and an element
selector, also known as a subscript.
Pascal sum sum ai FORTRAN sum sum
a(i)
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Array Types subscripts
References to array elements are made with a
specification of the array name and an element
selector, also known as a subscript.
subscript types - Pascal, Ada integers,
boolean, character and enumeration subscript
bounds - Pascal user defined C fixed
at 0 to n-1 FORTRAN user defined with
default 1 to n
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Array Types memory allocation

• static
• fixed stack-dynamic
• stack-dynamic
• heap-dynamic

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Array Types memory allocation

• static subscript range is statically
  bound static storage allocation
• fixed stack-dynamic subscript range is
  statically bound storage allocation is done at
  run time

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Array Types memory allocation

• stack-dynamic subscript range is
  dynamic storage allocation is done at run time
• heap-dynamic subscript range is
  dynamic dynamic storage allocation at run time

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Array Types dimensionality

• FORTRAN 77 - limited to seven dimensions
• C - one dimension (but arrays can be elements
  of arrays)
• Pascal - unlimited

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Array Types operations

• Operations which operate on an array as a unit
  (rather than one element at a time)
• APL - basic arithmetic (,-,,/), element
  reversal, row reversal, column reversal,
  transpose, inversion, inner product
• Ada - assignment, catenation, limited slicing
• C - assignment
• FORTRAN 77 - no array operations, slicing
• (but some simplified syntax for input and
  output)

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Implementation of Array Types
Implementing arrays requires more compile time
effort than implementing simple types
The code to allow run-time access of array
elements must be generated at compile time
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Implementation of Array Types
Assume we have an array, list, with a lower bound
of 1 and we wish to access its k th
element listk
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Implementation of Array Types
The computation of the address of listk can be
computed as follows address(listk)
address(list1) (k-1)element_size This
simplifies to address(listk)
address(list1) - element_size
kelement_size
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Implementation of Array Types
The generalized formula for the computation of
the address of listk, for an array with an
arbitrary lower bound can be computed as
follows address(listk)
address(listlower_bound) - lower_bound
element_size kelement_size
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Implementation of Array Types
address(listk)
address(listlower_bound) - lower_bound
element_size kelement_size
list100 list101 list107 list108
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Implementation of Array Types
The compile time descriptor for a
single-dimensioned array has the following form
Array Element type Index type Index lower
bound Index upper bound Address
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Implementation of Multidimensional Arrays
Objects of data types with two or more dimensions
must be mapped into the single-dimensioned memory.

• Two choices for storage schemes
• row major order
• Elements in any row are stored
• contiguously in memory
• column major order
• Elements in any column are stored contiguously
  in memory

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Implementation of Multidimensional Arrays
3 4 7 6 2 5 1 3 8
A11 3 A12 4 A13 7 A21
6 A22 2 A23 5 A31 1
A32 3 A33 8
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A11 3 A12 4 A13 7 A21
6 A22 2 A23 5 A31 1
A32 3 A33 8
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Implementation of Array Types
The formula for the computation of the address of
listi,j, for an array with a lower bound of 1,
can be computed as follows
address(listi,j)
address(list1,1) - ((n1)
element_size) ((inj)element_size)
where list has n elements per row
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The compile time descriptor for a
multi-dimensioned array has the following form