Module #5: Algorithms, Integers, Matrices

1. Module #5:Algorithms, Integers, Matrices Rosen 5th ed., §2.1 ~29 slides, ~1 lecture (c)2001-2002, Michael P. Frank

2. Chapter 2: Algorithms, the Integers, and Matrices • §2.1: Algorithms (Formal procedures) • §2.2: The Growth of Functions • §2.3: Complexity of algorithms • §2.4: The Integers & Division • §2.7: Matrices. (c)2001-2002, Michael P. Frank

3. §2.1: Algorithms • An algorithm is a finite set of precise instructions for performing a computation or for solving a problem.. • A computer program is an implementation of an algorithm using the languages that a computer can understand and execute. (c)2001-2002, Michael P. Frank

4. Example Algorithm • Bold word is keyword • italic is variable or constant you defined • Inside { } is comment Proceduremax( a1,a2,…,an: integers) max:=a1 fori :=2 ton if max<ai then max:=ai {max is the largest element} (c)2001-2002, Michael P. Frank

5. Executing the Max algorithm • Let {a1,a2,a3,a4,a5}={7,12,3,15,8}. Find its maximum… • Set max = a1 = 7. • Look at next element: a2 = 12. • Is a2>max? Yes, so change max to 12. • Look at next element: a2 = 3. • Is 3>12? No, leave max alone…. • Is 15>12? Yes, max=15… (c)2001-2002, Michael P. Frank

6. procedurename(argument: type) variable:=expression informal statement beginstatementsend {comment} ifcondition then statement [else statement] for variable:=initial value to final valuestatement whileconditionstatement procname(arguments) Not defined in book: returnexpression Declaration STATEMENTS Our Pseudocode Language: §A2 (c)2001-2002, Michael P. Frank

7. procedure procname(arg: type) • Declares that the following text defines a procedure named procname that takes inputs (arguments) named arg which are data objects of the type type. • Example:proceduremaximum(L: list of integers) [statements defining maximum…] (c)2001-2002, Michael P. Frank

8. variable:=expression • An assignment statement evaluates the expression expression, then reassigns the variable variable to the value that results. • Example:v:= 3x+7 (If x is 2, changes v to 13.) • In pseudocode (but not real code), the expression might be informal: • x:= the largest integer in the list L (c)2001-2002, Michael P. Frank

9. Informal statement • Sometimes we may write a statement as an informal English imperative, if the meaning is still clear and precise: “swap x and y” • Keep in mind that real programming languages never allow this. • When we ask for an algorithm to do so-and-so, writing “Do so-and-so” isn’t enough! • Break down algorithm into detailed steps. (c)2001-2002, Michael P. Frank

10. Groups a sequence of statements together:beginstatement 1statement 2 …statement nend Allows sequence to be used like a single statement. Might be used: After a procedure declaration. In an if statement after then or else. In the body of a for or while loop. beginstatementsend (c)2001-2002, Michael P. Frank

11. {comment} • Not executed (does nothing). • Natural-language text explaining some aspect of the procedure to human readers. • Also called a remark in some real programming languages. • Example: • {Note that v is the largest integer seen so far.} (c)2001-2002, Michael P. Frank

12. ifconditionthenstatement • Evaluate the propositional expression condition. • If the resulting truth value is true, then execute the statement statement; otherwise, just skip on ahead to the next statement. • Variant: ifcondthenstmt1elsestmt2Like before, but iff truth value is false, executes stmt2. (c)2001-2002, Michael P. Frank

13. whileconditionstatement • Evaluate the propositional expression condition. • If the resulting value is true, then execute statement. • Continue repeating the above two actions over and over until finally the condition evaluates to false; then go on to the next statement. (c)2001-2002, Michael P. Frank

14. whileconditionstatement • Also equivalent to infinite nested ifs, like so: if conditionbeginstatement if conditionbeginstatement …(continue infinite nested if’s)endend (c)2001-2002, Michael P. Frank

15. forvar:=initialtofinalstmt • Initial is an integer expression. • Final is another integer expression. • Repeatedly execute stmt, first with variable var :=initial, then with var :=initial+1, then with var :=initial+2, etc., then finally with var :=final. • What happens if stmt changes the value that initial or final evaluates to? (c)2001-2002, Michael P. Frank

16. forvar:=initial to finalstmt • for can be exactly defined in terms of while, like so: beginvar:=initialwhilevar finalbeginstmtvar:=var + 1endend (c)2001-2002, Michael P. Frank

17. procedure(argument) • A procedure call statement invokes the named procedure, giving it as its input the value of the argument expression. • Various real programming languages refer to procedures as functions (since the procedure call notation works similarly to function application f(x)), or as subroutines, subprograms, or methods. (c)2001-2002, Michael P. Frank

18. Another example task • Problem of searching an ordered list. • Given a list L of n elements that are sorted into a definite order (e.g., numeric, alphabetical), • And given a particular element x, • Determine whether x appears in the list, • and if so, return its index (position) in the list. • Problem occurs often in many contexts. • Let’s find an efficient algorithm! (c)2001-2002, Michael P. Frank

19. Search alg. #1: Linear Search procedurelinear search(x: integer, a1, a2, …, an: distinct integers)i:= 1while (i n  x  ai)i:=i + 1ifi n then location:=ielselocation:= 0return location {index (0-if not found!)} (c)2001-2002, Michael P. Frank

20. Search alg. #2: Binary Search • Basic idea: On each step, look at the middle element of the remaining list to eliminate half of it, and quickly zero in on the desired element. <x <x <x >x (c)2001-2002, Michael P. Frank

21. Search alg. #2: Binary Search procedurebinary search(x:integer, a1, a2, …, an: increasing integers)i:= 1 {left endpoint of search interval}j:=n{right endpoint of search interval}while i<j begin{while interval has >1 item}m:=(i+j)/2 {midpoint}ifx>amtheni := m+1 else j := mendifx = aithenlocation:=ielselocation:= 0returnlocation (c)2001-2002, Michael P. Frank

22. Practice exercises • 2.1.3: Devise an algorithm that finds the sum of all the integers in a list. [2 min] • proceduresum(a1, a2, …, an: integers)s:= 0 {sum of elems so far}fori:= 1 ton {go thru all elems}s:=s + ai {add current item}{at this point s is the sum of all items}returns (c)2001-2002, Michael P. Frank

23. Sorting: Bubble Sort • Please Refer Example 4 for real case (It’s very important!) procedurebubblesort(a1, a2, …, an:real number with n2)for i:= 1 ton - 1forj:= 1 ton - iifaj>aj+1then swap aj andaj+1 {a1, a2, …, an is in increasing order} (c)2001-2002, Michael P. Frank

24. Sorting: Insertion Sort procedureinsertionsort(a1, a2, …, an: real number with n2)for j:= 2 ton begini:= 1 whileaj>aii:=i + 1 m:=aj for k:= 0 to j – i – 1 aj-k :=aj-k-1 ai:=m end {a1, a2, …, an are sorted} (c)2001-2002, Michael P. Frank

25. Review §2.1: Algorithms • Characteristics of algorithms. • Pseudocode. • Examples: Max algorithm, linear search & binary search algorithms. • Intuitively we see that binary search is much faster than linear search, but how do we analyze the efficiency of algorithms formally? • Use methods of algorithmic complexity, which utilize the order-of-growth concepts from §1.8. (c)2001-2002, Michael P. Frank

26. Review: max algorithm proceduremax(a1, a2, …, an: integers) v:=a1 {largest element so far} fori:= 2 ton {go thru rest of elems} ifai > vthen v:=ai {found bigger?} {at this point v’s value is the same as the largest integer in the list} returnv (c)2001-2002, Michael P. Frank

27. Review: Linear Search procedurelinear search(x: integer, a1, a2, …, an: distinct integers)i:= 1while (i n  x  ai)i:=i + 1ifi n then location:=ielselocation:= 0return location {index or 0 if not found} (c)2001-2002, Michael P. Frank

28. Review: Binary Search • Basic idea: On each step, look at the middle element of the remaining list to eliminate half of it, and quickly zero in on the desired element. <x <x <x >x (c)2001-2002, Michael P. Frank

29. Review: Binary Search procedurebinary search(x:integer, a1, a2, …, an: distinct integers)i:= 1 {left endpoint of search interval}j:=n {right endpoint of search interval}while i<j begin {while interval has >1 item}m:=(i+j)/2 {midpoint}ifx>amtheni := m+1 else j := mendifx = aithenlocation:=ielselocation:= 0returnlocation (c)2001-2002, Michael P. Frank