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 Loops

loops2 Leading Decision prime the loop //START while (condition) //TEST { body of loop – //ACTION group of one or more statements indent one level //RESTART }

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 TRUE body statement Expression

loops 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)

loops5 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;

loops6 Loops Sentinel controlled keep processing data until a special value is entered to indicate that processing should stop Read blood pressures until a special value (like -1) selected by you is read. Count Controlled keep processing data for a specified number of times Read 100 blood pressures. End-of-file Controlled keep processing data as long as there is more data in the file Read all the blood pressures from a file no matter how many are there Flag Controlled keep processing data while a flag condition is true Read blood pressures until a dangerously high BP (200 or more) is read.

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

loops 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

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

loops10 Example cin >> number; while (number < 0) { cout << "Enter positive values only! "; cin >> number; }

loops11 Reading from a file first open file (later) infile points to file (can be any name) infile >> fahrTemp; 1 infile >> cityName >> fahrTemp;

loops12 Reading from a file with a sentinel value infile >> fahrTemp; //read from a filestart while (fahrTemp != 999)//test { celsTemp = (5 * (fahrTemp – 32))/9 ; //action cout << fahrTemp << celsTemp; infile >>fahrTemp; //restart }

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

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

loops15 Example infile >> fahrTemp; //start while (infile) //test { celsTemp = (5 * (fahrTemp – 32))/9;//actions cout << fahrTemp <<celsTemp; infile >> fahrTemp;//restart }

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

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

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

loops21 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

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;

total = 0; cin >> item; while (item != Sentinel) { total = total + item; cin >> item; } Adding a Running Total to a Sentinel- Controlled While Loop

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

loops27 Infinite Loop index = 1; while (index < 5) cout << "Good Morning!“ << endl;

loops28 Never executed while (ans == "yes" ) { …. cout << “Add another number? "; cin >> answer; }

loops29 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)

loops30 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; }

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;

loops32 Trailing Decision Loop do { Body } while (condition); Test at the bottom Statements are executed at least once

loops33 Trailing decision loop Body condition TRUE FALSE

loops34 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" );

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);

loops36 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

loops37 Counted Loop 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

loops38 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;

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.