Data Structures and Algorithms Lecture 4 Msury Mahunnah

Data Structures and Algorithms Lecture 4 Msury Mahunnah Department of Computer Science The Institute of Finance Management

ADT n ADT stands for Abstract Data Types. n A logical view of the data objects together with specifications of the operations required to create and manipulate them. n Pseudo-code - describes an algorithm. n ADT - describes a data structure.

What is a data type? n A set of objects, each called an instance of the data type. Some objects are sufficiently important to be provided with a special name. n A set of operations. Operations can be realized via operators, functions, procedures, methods, and special syntax (depending on the implementing language) n Each object must have some representation (not necessarily known to the user of the data type) n Each operation must have some implementation (also not necessarily known to the user of the data type)

What is a representation? n A specific encoding of an instance n This encoding MUST be known to implementors of the data type but NEED NOT be known to users of the data type n Terminology: "we implement data types using data structures“

Two varieties of data types n Opaque data types in which the representation is not known to the user. n Transparent data types in which the representation is profitably known to the user: - i. e. the encoding is directly accessible and/or modifiable by the user. n Which one you think is better? n What are the means provided by C++ for creating opaque data types?

Why are opaque data types better? n Representation can be changed without affecting user n Forces the program designer to consider the operations more carefully n Encapsulates the operations n Allows less restrictive designs which are easier to extend and modify n Design always done with the expectation that the data type will be placed in a library of types available to all.

How to design a data type Step 1: Specification n Make a list of the operations (just their names) you think you will need. Review and refine the list. n Decide on any constants which may be required. n Describe the parameters of the operations in detail. n Describe the semantics of the operations (what they do) as precisely as possible.

How to design a data type Step 2: Application n Develop a real or imaginary application to test the specification. n Missing or incomplete operations are found as a side-effect of trying to use the specification.

How to design a data type Step 3: Implementation n Decide on a suitable representation. n Implement the operations. n Test, debug, and revise.

Example - ADT Integer Operation Create Description Defines an identifier with an undefined value C/C++ int id 1; Assigns the value of one integer id 1 = id 2; identifier or value to another integer identifier is. Equal Returns true if the values associated with two integer identifiers are the same id 1 == id 2;

Example - ADT Integer Operation Less Than Description Defines an identifier with an undefined value C/C++ id 1 < id 2; Negative Returns the negative of the integer value - id 1; Sum Returns the sum of two integer values id 1 + id 2;

Operation Signatures n n n Create: identifier Integer Assign: Integer Identifier Is. Equal: (Integer, Integer) Boolean Less. Than: (Integer, Integer) Boolean Negative: Integer Sum: (Integer, Integer) Integer

More examples n We’ll see more examples throughout the course n Array n Stack n Queue

Arrays n Array: a set of pairs (index and value) data structure For each index, there is a value associated with that index. representation (possible) implemented by using consecutive memory.

Arrays in C++ int list[5], *plist[5]; list[5]: five integers list[0], list[1], list[2], list[3], list[4] *plist[5]: five pointers to integers plist[0], plist[1], plist[2], plist[3], plist[4] implementation of 1 -D array list[0] base address = list[1] + sizeof(int) list[2] + 2*sizeof(int) list[3] + 3*sizeof(int) list[4] + 4*size(int)

Arrays in C++ (cont’d) Compare int *list 1 and int list 2[5] in C++. Same: list 1 and list 2 are pointers. Difference: list 2 reserves five locations. Notations: list 2 - a pointer to list 2[0] (list 2 + i) - a pointer to list 2[i] (&list 2[i]) *(list 2 + i) - list 2[i]

Example #include <iostream> void print 1(int *ptr, int rows) { int i; cout << "Address Contents" << endl; for (i=0; i < rows; i++) cout << ptr+i << " " << *(ptr+i) << endl; } void main(){ int one[] = {0, 1, 2, 3, 4}; //Goal: print out address and value print 1(one, 5); } Address Contents 0 xbffffbb 4 0 0 xbffffbb 8 1 0 xbffffbbc 2 0 xbffffbc 0 3 0 xbffffbc 4 4

The Array ADT Objects: A set of pairs <index, value> where for each value of index there is a value from the set item. Index is a finite ordered set of one or more dimensions, for example, {0, … , n-1} for one dimension, {(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)} for two dimensions, etc. Methods: for all A Array, i index, x item, j, size integer

Array Create(j, list) : : = return an array of j dimensions where list is a j-tuple whose kth element is the size of the kth dimension. Items are undefined.

Item Retrieve(A, i) : : = if (i index) return the item associated with index value i in array A else return error

Array Store(A, i, x) : : = if (i in index) return an array that is identical to array A except the new pair <i, x> has been inserted else return error