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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 Leading Decision prime the loop //START while (condition) //TEST { body of loop – //ACTION group of one or more statements indent one level //RESTART } loops 2 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 1-2-3-4 for While Loops 1.Initial condition //START 2.TEST - while(…) 3.ACTION (steps needed to solve the problem) 4.RESTART (often the same as step 1) loops 4 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 5 Loops Sentinel controlled keep processing data until a special value is entered to indicate that processing should stop Count Controlled keep processing data for a specified number of times keep processing data as long as there is more data in the file keep processing data while a flag condition is true End-of-file Controlled Flag Controlled loops Read blood pressures until a special value (like -1) selected by you is read. Read 100 blood pressures. Read all the blood pressures from a file no matter how many are there Read blood pressures until a dangerously high BP (200 or more) is read. 6 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 1-2-3-4 Sentinel Value 1. Initial condition (START) • Get first value 2. TEST - while(…) • while (val != sentinel) 3. ACTION 4. RESTART- often the same as step 1 • Get next value loops 8 // 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; Example cin >> number; while (number < 0) { cout << "Enter positive values only! "; cin >> number; } loops 10 Reading from a file • • • • first open file (later) infile points to file (can be any name) infile >> fahrTemp; 1 infile >> cityName >> fahrTemp; loops 11 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 12 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 1-2-3-4 Reading from file 1. 2. 3. 4. START : Read first record TEST- while (infile) ACTIONS RESTART - Read next recordoften the same as step 1 loops 14 Example infile >> fahrTemp; //start while (infile) //test { celsTemp = (5 * (fahrTemp – 32))/9;//actions cout << fahrTemp <<celsTemp; infile >> fahrTemp; //restart } loops 15 // 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; 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 1-2-3-4 Count 1. 2. 3. 4. Start: Initialize counter Test - while (counter < limit) Actions Restart: Increment counter loops 18 Known Count //Print Hello 10 times int count ; count = 0; while (count < 10) { cout << "Hello "; count = count + 1; } //start //test //action //restart 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 } 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 21 Counter: • initialize : count = 0; • increment : count++; Adding a Counter to a Sentinel-Controlled While Loop int count; count = 0; // initialize cin >> item; while (item != sentinel) { count++; // increment Process(item); cin >> item; } Accumulator: • Initialize: total = 0; • Update: total = total + num; Adding a Running Total to a SentinelControlled While Loop total = 0; cin >> item; while (item != Sentinel) { total = total + item; cin >> item; } 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 Infinite Loop index = 1; while (index < 5) cout << "Good Morning!“ << endl; loops 27 Never executed while (ans == "yes" ) { …. cout << “Add another number? "; cin >> answer; } loops 28 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 29 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 30 Flag Controlled Loop countGoodReadings = 0; isDangerous = false; // initialize Boolean flag while (!isDangerous) //test { cin >> thisBP; //action if ( thisBP >= 200 ) isDangerous = true; // change flag value else countGoodReadings++; } cout << countGoodReadings << endl; Trailing Decision Loop do { Body } while (condition); Test at the bottom Statements are executed at least once loops 32 Trailing decision loop Body condition TRUE FALSE loops 33 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 34 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); 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 36 Counted Loop 1 2 4 for (initial step; cond; expression) body (3) Automatic: 1. initial step: counter = initVal 2. Check cond: counter < finalVal 3. If true execute body, else exit 4. Expression: counter++ Back to 2 loops 37 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 38 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 NESTED LOOPS Exercises: 1. Show screen output: for (outer = 0; outer < 2; outer++) { for (inner = 0; inner <=2; inner++) { cout<< outer<< '\t' << inner << '\n'; } } 2. Show screen output: for (outer = 0; outer < 3; outer++) { for(inner = 2; inner <=4; inner++) { cout<< inner << ' '; } cout<< '\n'; } 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"; } 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.