Repetition

1. Repetition Chapter 7 C++ An Introduction to Computing, 3rd ed.

2. Objectives • Expand on intro to repetition structures • Examine for loops • Study while and do loops • Look at various kinds of input loops • Consider how to choose best loop for a situation • First look at algorithm analysis • Introduce code reusability via inheritance C++ An Introduction to Computing, 3rd ed.

3. Intro Example: Punishment of Gauss • Summation problem:For punishment in grade school, Gauss was required to sum numbers 1 – 100. He came up with answer (5050) very quickly. (He did not use a repetition algorithm.) • We will use repetition in a function to sum the values of 1 through any number, n. C++ An Introduction to Computing, 3rd ed.

4. Steps Required • Procedure will require: • Initialize a running total to 0 • Initialize a count to 1 • Loop through: • Add count to the running total • Add 1 to count • Additional objects now seen: C++ An Introduction to Computing, 3rd ed.

5. Operations Operations • Receive an integer (n) • Initialize integers, runningTotal= 0, count = 1 • Add two integers, count and runningTotal, store result • Repeat step iii, for each value of count = 1 through n • Return the integer (runningTotal) C++ An Introduction to Computing, 3rd ed.

6. Note use of for loop to do step 2 of the algorithm Algorithm, Coding, Testing Algorithm for summation: • Initialize runningTotal to 0 • For each value of count in range 1 through n • Add count to runningTotal • Return runningTotal • View function source code, Figure 7.1 • Driver program, Figure 7.2 • Sample runs of Figure 7.2 C++ An Introduction to Computing, 3rd ed.

7. The for Loop • Counter-controlled loops • A set of statements executed once for each value in a specified range C++ An Introduction to Computing, 3rd ed.

8. count = first F count <= last T Statement count++ A Counting Loop • The for loop most commonly used to count • From one value first • To another value last: for (int count = first; count <= last; count++) Statement C++ An Introduction to Computing, 3rd ed.

9. InitializerExpr F LoopCondition T Statement Note: if the LoopCondition is initially false, then the body of the loop will not be executed even once. IncrementExpr The for Loop for (InitializerExpr; LoopCondition; IncrementExpr) Statement • When LoopCondition becomes false, • Control proceeds to the next statement. C++ An Introduction to Computing, 3rd ed.

10. count = first count = first F count <= last count <= last F T T Statement Statement count++ count++ Nested Loops • Consider the statement in a for loop • It can be any kind of statement • Including another for statement C++ An Introduction to Computing, 3rd ed.

11. Nested Loops • Loops can be nested: for (int val1 = 1; val1 <= limit1; val1++) for (int val2 = 1; val2 <= limit2; val2++) cout << val1 << ‘*’ val2 “ = “ << val1 * val2 << endl; • Output (suppose limit1 == 2, limit2 == 3): 1*1 = 1 1*2 = 2 1*3 = 3 2*1 = 2 2*2 = 4 2*3 = 6 See also Figure 7.3 And the sample run C++ An Introduction to Computing, 3rd ed.

12. Noncounting Loops • One of the quirks of the C++ for loop • Its three expressions can be omitted: for (;;) { StatementList } • Such a loop will execute infinitely many times … • Unless statements within StatementList permit execution to leave the loop. C++ An Introduction to Computing, 3rd ed.

13. StatementList1 T Expression F StatementList2 Noncounting Loops • This is the forever loop • a for loop without expressions: for (;;) { StatementList1 if (Expression) break; StatementList2 } • Repetition continues so long as Expression is false! C++ An Introduction to Computing, 3rd ed.

14. T Expression F StatementList2 Pretest Loops • If StatementList1 is omitted from forever loop – results in • A test-at-the-top or pretest loop: for (;;) { if (Expression) break; StatementList2 } StatementList1 C++ An Introduction to Computing, 3rd ed.

15. F Expression T Statement The while Loop • For such situations, C++ provides the more readable while loop, • pattern is: while (Expression) Statement • Statement can be either a single or compound C++ statement. • Repetition continues so long as Expression is true! C++ An Introduction to Computing, 3rd ed.

16. Example: Bouncing Ball Problem • When a ball is dropped, it bouncesto ½ of its previous height. • We seek a program which simulates this • Display number of each bounce and height • Repeat until bounce height is very small Objects C++ An Introduction to Computing, 3rd ed.

17. Operations • Input a real value, the original height • Initialize bounce to zero • Divide height by 2 for rebound height • Increment bounce • Display current bounce number, height • Repeat iii – v as long asheight ≥ SMALL_NUMBER C++ An Introduction to Computing, 3rd ed.

18. Algorithm • Initialize bounce 0 • Prompt for, read value for height • Display original height value with label • Loop: • If height < SMALL_NUMBER, terminate • Replace height with height / 2 • Add 1 to bounce • Display bounce and height • End loop C++ An Introduction to Computing, 3rd ed.

19. Coding and Testing • Note use of while loop, Figure 7.4 • Instead of a pretest forever for loop • Note sample run C++ An Introduction to Computing, 3rd ed.

20. StatementList1 T Expression F Post-test Loops • If StatementList2 is omitted in a forever loop – results in • a test-at-the-bottom orpost-test loop: for (;;) { StatementList1 if (Expression) break; } StatementList2 C++ An Introduction to Computing, 3rd ed.

21. Statement F Expression T The do Loop • For such situations, C++ provides the more readable do loop, • Pattern do Statement while (Expression); • Statement can be either a single or compound C++ statement. • Repetition continues so long as Expression is true! C++ An Introduction to Computing, 3rd ed.

22. Example: Counting Digits • Humans looking at a number can easily count digits 63 3291058 • Computers must do more than "scan and count" • We seek a function which • Receives an integer value from the caller • Counts digits in the integer • Returns the count to the caller C++ An Introduction to Computing, 3rd ed.

23. Objects and Operations Operations: • Integer division (intValue/ 10) • Integer addition (add 1 to numDigits) • Repetition of i and ii as long as intValue is not 0 C++ An Introduction to Computing, 3rd ed.

24. Algorithm • Initialize numDigits to 0 • Loop • Increment numDigits • Divide intValue by 10, store result in intValue • If intValue is 0, terminate repetition • Return numDigits • Note – we could use a forever for loopfor ( ; ; ) { numDigits++; intValue /= 0; if (intValue == 0) break; } C++ An Introduction to Computing, 3rd ed.

25. Coding and Testing • Note that the source code, Figure 7.5 uses a do loop instead • Observe the driver program, Figure 7.6 • Sample Runs C++ An Introduction to Computing, 3rd ed.

26. Input Loops • The forever loop is ideal for • Reading a list of values • Where the end is marked by a sentinel(i.e., an invalid value). • Pattern for (;;) { Prompt for value Read value if (value is the sentinel) break; Process value } C++ An Introduction to Computing, 3rd ed.

27. Forever loop Example • Read and average a list of test scores: double ReadAndAverage() { double score, sum = 0.0; int count = 0; for (;;) { cout << “Enter a test score (-1 to quit): “; cin >> score; if (score < 0) break; // test for sentinel count++; sum += score; } if (count > 0) return sum / count; else { cerr << “\n* no scores to average!\n” << endl; exit(1); } } C++ An Introduction to Computing, 3rd ed.

28. Use of while Loop • See also program to process collection of failure times and find mean time to failure. • Figure 7.7 • Sample Run • Note: the forever loop in Figure 7.7 could have been a while loopcout << "Enter a failure time (-1 to quit) :";cin >> failureTime;while (failureTime >= 0) { failureTimeSum += failureTime; numComponents++; cout << "Enter a failure time (-1 to quit) : "; cin >> failureTime;} C++ An Introduction to Computing, 3rd ed.

29. End-of-File as Sentinel Value • Recall that cin is an object of type istream • Status flag values can be accessed • The eof flag value can be used as a sentinel • Forever for loop used in Figure 7.8for ( ; ; ) { cin.get(ch); if (cin.eof()) break; cout << '"\t' >> int (ch) << endl; } C++ An Introduction to Computing, 3rd ed.

30. End-of-File as Sentinel Value • Problems • Platform independence in using the eof as a flag • Results of the good flag being set – if stream is cleared, possible to read aftereof mark! • Note program in Figure 7.9 which illustrates this C++ An Introduction to Computing, 3rd ed.

31. Input Loops The Counting Approach • Consider a program which prompts for, receives as input the number of incoming items • Then design the for loop to count that many items • Note such a for loop in Figure 7.10 C++ An Introduction to Computing, 3rd ed.

32. Input Loops The Counting Approach • Problem • Number of incoming data items may be difficult to determine • Possible to query user before/after each iteration • Note use of do loop in this approach, Figure 7.11 • Also possible to create a bool function to do the query • Call the query function in the while( condition) C++ An Introduction to Computing, 3rd ed.

33. Choosing a Loop • Use the for loop for counting problems. • Design algorithms for non-counting problems using a general Loop statement, and see where it is appropriate for repetition to terminate: • If at the loop’s beginning, use the while loop • If at its end, use the do loop • If in its middle, use the forever loop. C++ An Introduction to Computing, 3rd ed.

34. Intro to Algorithm Analysis • Consider once again, Gauss's quick solution (not using repetition) … • Sum of numbers 1 … 5 C++ An Introduction to Computing, 3rd ed.

35. Intro to Algorithm Analysis • So in general: • Thus possible to write a non looping version of our sum functionint sum (int n){ return n * (n + 1) / 2;} • A better algorithm • Takes many less operations C++ An Introduction to Computing, 3rd ed.

36. OBJECTive Thinking:Code Reuse through Inheritance • Consider our Name class • What if we need a class which also requires titles C++ An Introduction to Computing, 3rd ed.

37. Inheritance • When a new class is a specialization of an existing class • We use the C++ inheritance mechanism • It drives the new class from the existing class • Illustration #include "Name.h"class TitledName : public Name{ // attributes and operations}; C++ An Introduction to Computing, 3rd ed.

38. Inheritance • TitleName class derived from Name • Figure 7.13 • Driver program with sample run, Figure 7.14 C++ An Introduction to Computing, 3rd ed.

39. Inheritance • Public variables and methods from parent class available to child class objects C++ An Introduction to Computing, 3rd ed.

40. Inheritance • Note from TitledName definitions: • Constructors and initialization • Accessor and mutator methods • I/O methods • General Principle:Constructors and methods of a class should only access instance variables defined within the same class. Inherited instance variables should be accessed through inherited constructors or methods C++ An Introduction to Computing, 3rd ed.

41. Object Oriented Design • Identify objects in problem • Build class to represent if needed • As needed use inheritance to consolidate common attributes, operations • Identify operations in problem • If operation not predefined, write function to perform that operation • As appropriate identify class which should provide the operation, define operation as a method • Organize objects, operations into algorithm that solves your problem C++ An Introduction to Computing, 3rd ed.