1. 1 Chapter 6 Looping Dale/Weems/Headington

2. 2 l Physical order vs. logical order l A loop is a repetition control structure based on a condition. l it causes a single statement or block to be executed repeatedly While loop TRUE – execute the body, FALSE – exit loop Exits the loop only at the top Errors: = vs. == Semicolon at end of While Braces What is a loop?

3. 3 While Statement SYNTAX while ( Expression ) {.. // loop body. } NOTE: Loop body can be a single statement, a null statement, or a block.

4. 4 Two Types of Loops count controlled loops – executes a number of times loop control variable - initialize and test – multiple roles infinite loop – omit incrementation repeat a specified number of times event-controlled loops – termination depends on some event some condition within the loop body changes and this causes the repeating to stop

5. 5 int count ; count = 4; // initialize loop variable while (count > 0) // test expression { cout << count << endl ; // repeated action count -- ; // update loop variable } cout << “Done” << endl ; Count-controlled Loop

6. 6 Infinite loops Other loops: Do while For Looping problems are difficult to solve Easier to read than design Phases Loop entry Iteration – before each iteration – loop test After iteration – to statement following loop

7. 7 myInfile contains 100 blood pressures Use a while loop to read the 100 blood pressures and find their total Count-Controlled Loop Example

8. 8 ifstream myInfile ; int thisBP ; int total ; int count ; count = 0 ; // initialize while ( count < 100 ) // test expression { myInfile >> thisBP ; total = total + thisBP ; count++ ; // update } cout << “The total = “ << total << endl ;

9. 9 Event-controlled Loops l Sentinel controlled – long lists of data keep processing data until a special value which is not a possible data value is entered to indicate that processing should stop l End-of-file controlled keep processing data as long as there is more data in the file l Flag controlled keep processing data until the value of a flag changes in the loop body

10. 10 Examples of Kinds of Loops Count controlled loop Read exactly 100 blood pressures from a file. End-of-file controlled loop Read all the blood pressures from a file no matter how many are there.

11. 11 Examples of Kinds of Loops Sentinel controlled loop Read blood pressures until a special value (like -1) selected by you is read. Flag controlled loop Read blood pressures until a dangerously high BP (200 or more) is read.

12. 12 A Sentinel-controlled Loop l requires a “priming read” – read before first test l “priming read” means you read one set of data before the while l Another read at end of loop

13. 13 // Sentinel controlled loop total = 0; cout << “Enter a blood pressure (-1 to stop ) ”; cin >> thisBP; while (thisBP != -1)// while not sentinel { total = total + thisBP; cout << “Enter a blood pressure (-1 to stop ) ”; cin >> thisBP; } cout << total;

14. 14 End-of-File Controlled Loop l depends on fact that a file goes into fail state when you try to read a data value beyond the end of the file

15. 15 total = 0; myInfile >> thisBP; // priming read while (myInfile) // while last read successful { total = total + thisBP; myInfile >> thisBP; // read another } cout << total; // End-of-file controlled loop

16. 16 //End-of-file at keyboard total = 0; cout << “Enter blood pressure (Ctrl-Z to stop)”; cin >> thisBP; // priming read while (cin) // while last read successful { total = total + thisBP; cout << “Enter blood pressure”; cin >> thisBP; // read another } cout << total;

17. 17 Flag-controlled Loops l you initialize a flag (to true or false) l use meaningful name for the flag l a condition in the loop body changes the value of the flag l test for the flag in the loop test expression

18. 18 countGoodReadings = 0; isSafe = true; // initialize Boolean flag while (isSafe) { cin >> thisBP; if ( thisBP >= 200 ) isSafe = false; // change flag value else countGoodReadings++; } cout << countGoodReadings << endl;

19. 19 Loops often used to l count all data values l count special data values l sum data values l keep track of previous and current values

20. 20 Previous and Current Values l write a program that counts the number of != operators in a program file l read one character in the file at a time l keep track of current and previous characters

21. 21 Keeping Track of Values (x != 3) { cout << endl; } FILE CONTENTS previous current count ( x 0 ! = 1 = ‘ ‘ 1 x ‘ ‘ 0 3 ) 1 ‘ ‘ 3 1 ‘ ‘ ! 0

22. 22 int count; char previous; char current; count = 0 ; inFile.get (previous); // priming reads inFile.get(current); while (inFile) { if ( (current == ‘ = ‘) && (previous == ‘!’) ) count++; previous = current;// update inFile.get(current); // read another }

23. 23 initialize outer loop while ( outer loop condition ) {... initialize inner loop while ( inner loop condition ) { inner loop processing and update }... } Pattern of a Nested Loop

24. 24 Patient Data A file contains blood pressure data for different people. Each line has a patient ID, the number of readings for that patient, followed by the actual readings. ID howManyReadings 4567 5180 140 150 170 120 23182170 210 52323150 151 151

25. 25 4567152 2318190 5232151.. There were 432 patients in file. Read the data and display a chart Patient ID BP Average

26. 26 Algorithm Uses Nested Loops l initialize patientCount to 0 l read first ID and howMany from file l while not end-of-file n increment patientCount n display ID n use a count-controlled loop to read and sum up this patient’s howMany BP’s n calculate and display average for patient n read next ID and howMany from file l display patientCount

27. 27 To design a nested loop l begin with outer loop l when you get to where the inner loop appears, make it a separate module and come back to its design later

28. 28 #include #include using namespace std; int main ( ) { int patientCount; // declarations int thisID; int howMany; int thisBP; int totalForPatient; int count; float average; ifstream myInfile;

29. 29 myInfile.open(“A:\\BP.dat”); if (!myInfile ) // opening failed { cout > thisID >> howMany; // priming read

30. 30 while ( myInfile ) // last read successful { patientCount++; cout << thisID; totalForPatient = 0; // initialize inner loop count = 0; while ( count < howMany) { myInfile >> thisBP; count ++; totalForPatient = totalForPatient + thisBP; } average = totalForPatient / float(howMany); cout << int (average +.5) << endl; // round myInfile >> thisID >> howMany; // another read }

31. 31 cout << “There were “ << patientCount << “patients on file.” << endl; cout << “Program terminated.\n”; return 0; }

32. 32 Information About 20 Books in Diskfile “A:\\myIn.dat” 3.98 P 7.41 H 8.79 P. Price of book Hardback or Paperback? WRITE A PROGRAM TO FIND TOTAL VALUE OF ALL BOOKS

33. 33 #include // for cout #include // for file I/O using namespace std; int main (void) { float price ; // declarations char kind ; ifstream myInfile ; float total = 0.0 ; int count = 1; Program to Read Info about 20 Books From a Disk File

34. 34 Rest of Program myInfile.open(“A:\\myIn.dat”) ; // count-controlled processing loop while ( count <= 20 ) { myInfile >> price >> kind ; total = total + price ; count ++ ; } cout << “Total is: “ << total << endl ; myInfile.close( ) ; return 0 ; }

35. 35 Trace of Program Variables count price kind total 0.0 1 3.98 ‘P’ 3.98 2 7.41 ‘H’ 11.39 3 8.79 ‘P’ 20.18 4 etc. 20 21 so loop terminates

36. 36 Complexity l is a measure of the amount of work involved in executing an algorithm relative to the size of the problem

37. 37 Polynomial Times N N 0 N 1 N 2 N 3 constant linear quadratic cubic 11 1 1 1 101 10 100 1,000 1001 100 10,000 1,000,000 1,0001 1,000 1,000,000 1,000,000,000 10,0001 10,000 100,000,000 1,000,000,000,000

38. 38 Loop Testing and Debugging test data should test all sections of program l beware of infinite loops -- program doesn’t stop l check loop termination condition, and watch for “off-by-1” problem l use get function for loops controlled by detection of ‘\n’ character l use algorithm walk-through to verify pre- and postconditions l trace execution of loop by hand with code walk-through l use a debugger to run program in “slow motion” or use debug output statements

39. 39 Program #2 - pg. 233 l Input integers larger than 1 - loop for entry l calculate sum of squares - 1 through that integer - inner loop l output results n declare variables n initialize variables to 0 n priming read-input n while loop - while >1 –count = 1 - loop control variable - input - outer variable l while count <= integer l calculate square l add to totaling variable l add to counting variable –output results n input next number n return 0

40. 40 l Assignment #12 n pg. 231 - prog warm-up exercises 1,4 n pg. 233 - prog problems #2 l Assignment #13 n pg. 269-274 - exam prep exercises - #2-7,9-14.