1. Chapter 6 Structured Data Types Arrays Records

2. Copyright © 2007 Addison-Wesley. All rights reserved. 1–2 Definitions data type –collection of data objects –a set of predefined operations descriptor : collection of attributes for a variable object : instance of a user-defined (abstract data) type

3. Copyright © 2007 Addison-Wesley. All rights reserved. 1–3 Structured Data Types Built out of other types –usually composed of multiple elements. –homogeneous : all elements have the same type –heterogeneous : elements have different types

4. Copyright © 2007 Addison-Wesley. All rights reserved. 1–4 Structured Data Types Arrays –aggregate of homogeneous data elements indexed by its position Associative arrays –unordered collection of key-value pairs Records –heterogeneous aggregate of data elements indexed by element name

5. Copyright © 2007 Addison-Wesley. All rights reserved. 1–5 Array Operations Whole array operations: –assignment –catenation Elemental operations same as those of base type Indexing : mapping from indexes to elements array_name (index_value_list)  an element

6. Copyright © 2007 Addison-Wesley. All rights reserved. 1–6 Array Design Issues What types are legal for subscripts? Are subscripting expressions in element references range checked? When are subscript ranges bound? When does allocation take place? What is the maximum number of subscripts? Can array objects be initialized? Are any kind of slices allowed?

7. Copyright © 2007 Addison-Wesley. All rights reserved. 1–7 Binding Time Choices Static: compile-time binding of subscript range and memory Fixed stack-dynamic: subscript ranges static, allocated at declaration time (C, C++)‏ Stack-dynamic: run-time binding of subscript range and memory Fixed heap-dynamic: storage binding is dynamic but fixed after allocation (Java, C and C++) Heap-dynamic: binding of subscript ranges and storage allocation is dynamic (Perl and JavaScript)‏

8. Copyright © 2007 Addison-Wesley. All rights reserved. 1–8 Array Initialization Some language allow initialization at the time of storage allocation –C, C++, Java, C# example int list [] = {4, 5, 7, 83} –Character strings in C and C++ char name [] = “freddie”; –Arrays of strings in C and C++ char *names [] = {“Bob”, “Jake”, “Joe”};

9. Copyright © 2007 Addison-Wesley. All rights reserved. 1–9 Memory for arrays For 1D arrays, contiguous block of memory with equal amount of space for each element Two approaches for multi-dimensional arrays –Single block of contiguous memory for all elements Arrays must be rectangular Address of array is starting memory location –Implement as arrays of arrays (Java)‏ Jagged arrays are possible Array variable is a pointer (reference)‏

10. Copyright © 2007 Addison-Wesley. All rights reserved. 1–10 Element Access Access function maps subscript expressions to an address in the array Access function for single-dimensioned arrays: address(list[k]) = address (list[lower_bound])‏ + ((k-lower_bound) * element_size)‏ Two common ways to organize 2D arrays –Row major order (by rows) – used in most languages –Column major order (by columns) – used in Fortran

11. Copyright © 2007 Addison-Wesley. All rights reserved. 1–11 Memory Allocation for 2D Array Row major (by rows) or column major order (by columns) for 2D array Access function maps subscript expressions to an address in the array

12. Copyright © 2007 Addison-Wesley. All rights reserved. 1–12 Row-major access formula Location (a[I,j]) = address of a [row_lb,col_lb] + (((I - row_lb) * n) + (j - col_lb)) *element_size

13. Copyright © 2007 Addison-Wesley. All rights reserved. 1–13 2D Arrays in Java

14. Copyright © 2007 Addison-Wesley. All rights reserved. 1–14 Rectangular and 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 matrix has rows with varying number of elements –Possible when multi-dimensioned arrays actually appear as arrays of arrays

15. Copyright © 2007 Addison-Wesley. All rights reserved. 1–15 Slices A slice is some substructure of an array; it is nothing more than a referencing mechanism Slices are only useful in languages that have array operations –Java allows row slices from 2D arrays –Fortran 95 Integer, Dimension (10) :: Vector Integer, Dimension (3, 3) :: Mat Integer, Dimension (3, 3) :: Cube Vector (3:6) is a four element array

16. Copyright © 2007 Addison-Wesley. All rights reserved. 1–16 Slices Examples in Fortran 95

17. Copyright © 2007 Addison-Wesley. All rights reserved. 1–17 Compile-Time Descriptors Single-dimensioned array Multi-dimensional array

18. Copyright © 2007 Addison-Wesley. All rights reserved. 1–18 Associative Arrays An associative array is an unordered collection of data elements that are indexed by an equal number of values called keys –A hash table has the same behavior Design Issues: 1. What is the form of references to elements? 2. Is the size static or dynamic?

19. Copyright © 2007 Addison-Wesley. All rights reserved. 1–19 Associative Arrays in Perl Names begin with %; l iterals are delimited by parentheses %hi_temps = ("Mon" => 77, "Tue" => 79, “Wed” => 65, …); Subscripting is done using braces and keys $hi_temps{"Wed"} = 83; –Elements can be removed with delete delete $hi_temps{"Tue"};

20. Copyright © 2007 Addison-Wesley. All rights reserved. 1–20 Other Languages Ruby has hashes –ht = {key1=> vlaue1, …} –use ht[key1] to access Python has dictionary type –ht = {key1 : value1, …} –use ht[key1] to access In C++, Java provide library classes In C, must use user-defined type

21. Copyright © 2007 Addison-Wesley. All rights reserved. 1–21 Record Types A possibly heterogeneous aggregate of data elements Individual elements identified by field name Like a class with no methods and only public data. Design issues: –What is the syntactic form of references to the field? –Are elliptical references allowed

22. Copyright © 2007 Addison-Wesley. All rights reserved. 1–22 Definition of Records in Ada Record structures are indicated in an orthogonal way type Emp_Rec_Type is record First: String (1..20); Mid: String (1..10); Last: String (1..20); Hourly_Rate: Float; end record; Emp_Rec: Emp_Rec_Type;

23. Copyright © 2007 Addison-Wesley. All rights reserved. 1–23 structs in C Define a record in C using the struct syntax struct record { int var1; double var2; } Structs can be copied struct record r1, r2 // mem for 2 records r1.var1 = 1; r1.var2 = 2.3; r2 = r1; // copy data from r1 into r2

24. Copyright © 2007 Addison-Wesley. All rights reserved. 1–24 References to Record Fields Most language use dot notation Emp_Rec.Name Fully qualified references must include all record names Elliptical references allow leaving out record names as long as the reference is unambiguous, for example in COBOL FIRST, FIRST OF EMP-NAME, and FIRST of EMP-REC are elliptical references to the employee’s first name

25. Copyright © 2007 Addison-Wesley. All rights reserved. 1–25 Operations on Records Assignment is very common if the types are identical Ada allows record comparison Ada records can be initialized with aggregate literals COBOL provides MOVE CORRESPONDING –Copies a field of the source record to the corresponding field in the target record

26. Copyright © 2007 Addison-Wesley. All rights reserved. 1–26 Records vs. Arrays Straight forward and safe design Use records when collection of data values is heterogeneous Access to array elements is much slower than access to record fields – subscripts are dynamic –field names are static

27. Copyright © 2007 Addison-Wesley. All rights reserved. 1–27 Implementation of Record Type Offset address relative to the beginning of the records is associated with each field

28. Copyright © 2007 Addison-Wesley. All rights reserved. 1–28 Union Types A type whose elements are allowed to store different types at different times during execution Fortran, C, and C++ provide free union – no language support for type checking Type checking requires extra element –Type indicator called a discriminant –Supported by Ada

29. Copyright © 2007 Addison-Wesley. All rights reserved. 1–29 Evaluation of Unions Potentially unsafe construct –Do not allow type checking Java and C# do not support unions –Reflective of growing concerns for safety in programming language

30. Copyright © 2007 Addison-Wesley. All rights reserved. 1–30 Type Equivalence Consider the problem of two structured types: –Are two record types compatible if they are structurally the same but use different field names? –Are two array types compatible if they are the same except that the subscripts are different? (e.g. [1..10] and [0..9])‏ –Are two enumeration types compatible if their components are spelled differently?

31. Copyright © 2007 Addison-Wesley. All rights reserved. 1–31 Two approaches Name type compatibility : two variables have compatible types if they are in either the same declaration or in declarations that use the same type name –Easy to implement but highly restrictive: –Subranges of integer types are not compatible with integer types –Formal parameters must be the same type as their corresponding actual parameters (Pascal)‏ Structure type compatibility means that two variables have compatible types if their types have identical structures –More flexible, but harder to implement