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Loops

Loops. A loop is a repetition control structure. body - statements to be repeated control statement - decides whether another repetition needs to be made leading decision loop - control statement before body trailing decision loop - control statement after body Counted loop - for

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Loops

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  1. Loops • A loop is a repetition control structure. • body - statements to be repeated • control statement - decides whether another repetition needs to be made • leading decision loop - control statement before body • trailing decision loop - control statement after body • Counted loop- for • Logical loop – while or do..while

  2. Leading Decision prime the loop //START while (condition) //TEST { body of loop – //ACTION group of one or more statements indent one level //RESTART } loops

  3. When the expression is tested and found to be false, the loop is exited and control passes to the statement which follows the loop body. while LOOP FALSE Expression TRUE body statement

  4. 1-2-3-4 for While Loops Initial condition //START TEST - while(…) ACTION (steps needed to solve the problem) RESTART (often the same as step 1) loops

  5. Example cout <<"Do you want to play?(y/n)"; //START cin >> ans; while ((ans == 'y') || (ans == 'Y'))// TEST { … //body // ACTION cout <<"Do you want to continue(y/n)?";//RESTART cin >> ans; } cout <<"Thanks for playing! " << endl; loops

  6. Loops loops

  7. A Sentinel-controlled Loop • Read numbers until -1, 999 • Not always easy to determine sentinel value • requires a "priming read" • "priming read" means you read one set of data before the while

  8. 1-2-3-4 Sentinel Value • Initial condition (START) • Get first value • TEST - while(…) • while (val != sentinel) • ACTION • RESTART- often the same as step 1 • Get next value loops

  9. // Sentinel controlled loop total = 0; cout << "Enter the price of the item (-1 to stop ) ";//START cin >> itemPrice; while (itemPrice > 0) // while not sentinel TEST { total = total + itemPrice; // ACTION cout <<"Enter item price(-1 to stop ) "; // RESTART cin >> itemPrice; } cout << total;

  10. Example cin >> number; while (number < 0) { cout << "Enter positive values only! "; cin >> number; } loops

  11. Reading from a file • first open file (later) • infile points to file (can be any name) • infile >> fahrTemp; 1 • infile >> cityName >> fahrTemp; loops

  12. Reading from a file with a sentinel value infile >> fahrTemp; //read from a file start while (fahrTemp != 999) //test { celsTemp = (5 * (fahrTemp – 32))/9 ; //action cout << fahrTemp << celsTemp; infile >>fahrTemp; //restart } loops

  13. End-of-File Controlled Loop • depends on fact that a file goes into fail state when you try to read a data value beyond the end of the file • No trailer record while (there is a record) while (not end of file) while (infile) // this is c • Computer indicates there are no more records by sending a signal to the program • Must read record before entering loop – there may be no records

  14. 1-2-3-4 Reading from file • START : Read first record • TEST- while (infile) • ACTIONS • RESTART - Read next record- often the same as step 1 loops

  15. Example infile >> fahrTemp; //start while (infile) //test { celsTemp = (5 * (fahrTemp – 32))/9;//actions cout << fahrTemp <<celsTemp; infile >> fahrTemp ; //restart } loops

  16. // End-of-file controlled loop //Open file total = 0; infile >> thisBP; // priming read Start while (infile) //test { total = total + thisBP; //action infile >> thisBP; // read another - restart loop } cout << total;

  17. Count-controlled loop • Do something a set number of times • Need counter • initialize • increment • iteration counter - incremented during each iteration of the loop • event counter - incremented each time a particular event occurs

  18. 1-2-3-4 Count • Start: Initialize counter • Test - while (counter < limit) • Actions • Restart: Increment counter loops

  19. Known Count //Print Hello 10 times int count ; count = 0; //start while (count < 10) //test { cout << "Hello "; //action count = count + 1; //restart }

  20. variable count //Print Hello 10 times int count ; cout << "How many times should we print Hello?" cin >> count; 1 while (count > 0) 2 { cout << "Hello "; 3 count = count -1 ; 4 }

  21. Accumulators and Counters • To find the average of a group of numbers-need running total and how many numbers • Counter – storage area in which we count • Initialize: count = 0; • Increment: • count = count + 1 or count++ • Accumulator – storage area for keeping cumulative or running totals • Initialize: total = 0; • Update: • total = total + number • total_wages_paid = total_wages_paid + net_pay loops

  22. Counter: • initialize : count = 0; • increment : count++;

  23. Adding a Counter to a Sentinel-Controlled While Loop

  24. Accumulator: Initialize: total = 0; Update: total = total + num;

  25. Adding a Running Total to a Sentinel-Controlled While Loop

  26. int thisBP, total, count ; //Open file count = 0; // initialize 1 total = 0; infile >>thisBP ; while ( count < 100 && infile)2 { total = total + thisBP ; 3 count= count + 1 infile >> thisBP; 4 } cout << "The total = " << total << endl; if (count != 0) cout << "The average is " << (float)total/count ; 26

  27. Infinite Loop index = 1; while (index < 5) cout << "Good Morning!“ << endl; loops

  28. Never executed while (ans == "yes" ) { …. cout << “Add another number? "; cin >> answer; } loops

  29. Don't forget to prime the loop! • Initialize initial condition by reading in or setting value • cin >> ans while (ans == 'y') • index = 0 while (index < 10) • infile >> name >> ssNum >>phone; //read record while (infile) loops

  30. int count; float total, avg, num; total = 0; count = 0; infile >> num; while (infile) { total = total + num; count = count + 1; infile >> num; } if (count == 0) cout << "No numbers entered“ << endl; else { avg = total/count; cout << "The average is “ << avg << endl; } loops

  31. Flag Controlled Loop

  32. Trailing Decision Loop do { Body } while (condition); Test at the bottom Statements are executed at least once loops

  33. Trailing decision loop Body condition TRUE FALSE loops

  34. Example do { cout << "Enter two numbers“; cin >> num1 >>num2; cout <<num1 << " + " << num2 << " = " << num1+num2 << endl; cout << "Do you want to enter two numbers again? "; cin >> ans; } while (ans == "yes" ); loops

  35. do { DisplayMenu(); cin >> choice; switch (choice) { case 1: PlayBeginner(); break; case 2: PlayAdvBeginner(); break case 3: PlayIntermediate(); break case 4: break; default: cout << “ Invalid option” << endl; break; } } while (choice != 4);

  36. Counted loop • Fixed number of iterations • Use a variable as a counter which starts at a specified number and increments the variable each time the loop is processed • Repeats until the counter is greater than an ending number • Beginning value, ending value, increment value loops

  37. Counted Loop 1 2 4 for (initial step; cond; expression) body (3) Automatic: • initial step: counter = initVal • Check cond: counter < finalVal 3. If true execute body, else exit 4. Expression: counter++ Back to 2 loops

  38. Examples • for (count = 1; count <= 10; count++) cout <<"Hello“ << endl; • for (count = 1; count <= 10; count++) cout << count << endl; • for (num = 10; num > 0; num--) cout >>num cout << "Blast off“ << endl; loops

  39. Nested Loops • placing of one loop inside the body of another loop is called nesting. • When you "nest" two loops, the outer loop takes control of the number of complete repetitions of the inner loop.   • All types of loops may be nested, the most commonly nested loops are for loops. • When working with nested loops, the outer loop changes only after the inner loop is completely finished

  40. NESTED LOOPS

  41. Exercises: • Show screen output:for (outer = 0; outer < 2; outer++){     for (inner = 0; inner <=2; inner++)    {        cout<< outer<< '\t' << inner << '\n';     }} • Show screen output:for (outer = 0; outer < 3; outer++){     for(inner = 2; inner <=4; inner++)    {          cout<< inner << ' ';     }     cout<< '\n';}

  42. 3. The following set of nested loops is NOT working. Can you find what is wrong??for (ctr1 = 1; ctr1 <=10; ctr1++);{      for(ctr2 = 1; ctr2 <=5; ctr2++)     {           number = ctr1 * ctr2;           cout<< number << '\n';     }} 4.  Show the output for the following program fragment:for (ctr1 = 8; ctr1 > 5; ctr1--){     for(ctr2 = 1; ctr2 < 3; ctr2++)     {           cout<<ctr1<<" "<<ctr2<<" ";      }      cout<<"\n";} 5.  Show the output:for(j=0; j<=5; j++){     for(k=1; k<=j; k++)     {          cout<<"&";     }     cout<<"\n";} 

  43. 6.  Write a program using nested loops to produce the following design:      *     **     ***     ****     *****     ****** 7.  Write a program using nested loops to produce the following design:      A     AB     ABC     ABCD     ABCDE     ABCDEF 8.  Write a program using nested loops to produce a rectangle of *'s with 6 rows and  20 *'s per row.

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