Introduction to Algorithms and programming-II

1. Introduction to Algorithms and programming-II Chapter 2 Control Structures II Department of Computer Science Academic Year: 2017-2018 Semester: Two Dr. Maytham Mustafa Hammood

2. C++ Programming: From Problem Analysis to Program Design, Fourth Edition • Chapter 5: Control Structures II (Repetition) C++ Programming: From Problem Analysis to Program Design, Fourth Edition

3. Objectives In this chapter you will: • Learn about repetition (looping) control structures • Explore how to construct and use count-controlled, sentinel-controlled, flag-controlled, and EOF-controlled repetition structures • Examine break and continue statements • Discover how to form and use nested control structures C++ Programming: From Problem Analysis to Program Design, Fourth Edition

4. Why Is Repetition Needed? • Repetition allows you to efficiently use variables • Can input, add, and average multiple numbers using a limited number of variables For example, to add five numbers: • Declare a variable for each number, input the numbers and add the variables together • Create a loop that reads a number into a variable and adds it to a variable that contains the sum of the numbers C++ Programming: From Problem Analysis to Program Design, Fourth Edition

5. The while Loop • The general form of the while statement is: while(expression) statement • while is a reserved word • Statement can be simple or compound • Expression acts as a decision maker and is usually a logical expression • Statement is called the body of the loop • The parentheses are part of the syntax C++ Programming: From Problem Analysis to Program Design, Fourth Edition

6. The while Looping (continued) • Infinite loop: continues to execute endlessly, can be avoided by including statements in the loop body that assure exit condition will eventually be false C++ Programming: From Problem Analysis to Program Design, Fourth Edition

7. The while Looping (continued) C++ Programming: From Problem Analysis to Program Design, Fourth Edition

8. Designing while Loops C++ Programming: From Problem Analysis to Program Design, Fourth Edition

9. Case 1: Counter-Controlled while Loops • If you know exactly how many pieces of data need to be read, the while loop becomes a counter-controlled loop C++ Programming: From Problem Analysis to Program Design, Fourth Edition

10. Example: write a program that prompts the user to input 5 integer items then find summation and a average for those items. #include <iostream> using namespace std; const int limit=5; //variable to store the number of items int main() { int number; //variable to store the number int sum; //variable to store the sum int counter; //loop control variable sum = 0; counter = 0; while (counter < limit) { cout << “ Enter number for processing:“; cin >> number; sum = sum + number; counter++; } C++ Programming: From Problem Analysis to Program Design, Fourth Edition

11. Example of Counter-Controlled while (continued) cout << “The sum of the " << limit << " numbers = " << sum << endl; if (counter !=0 ) cout << “The average = “ << sum / counter << endl; else cout << “No input." << endl; return 0; } C++ Programming: From Problem Analysis to Program Design, Fourth Edition

12. Case 2: Sentinel-Controlled while Loops • Sentinel variable is tested in the condition and loop ends when sentinel is encountered C++ Programming: From Problem Analysis to Program Design, Fourth Edition

13. Example: write a program that prompts the user to input set of integer items ending with item that is equal to -999 then find summation and a average for those items. #include <iostream> using namespace std; const int SENTINEL = -999; //variable to store the number of items int main() { int number; //variable to store the number int sum = 0; //variable to store the sum int count = 0; //variable to store the total numbers read cout << “ Enter numbers ending with “ << SENTINEL << endl; cin >> number; while (number != SENTINEL) { sum = sum + number; count++; cin >> number; } C++ Programming: From Problem Analysis to Program Design, Fourth Edition

14. Example of Sentinel-Controlled while (continued) cout << “The sum of the " << count << " numbers is " << sum <<endl; if (count != 0) cout << “The average = “ << sum / count << endl; else cout << “No input." << endl; return 0; } C++ Programming: From Problem Analysis to Program Design, Fourth Edition

15. Case 3: Flag-Controlled while Loops • A flag-controlled while loop uses a bool variable to control the loop • The flag-controlled while loop takes the form: C++ Programming: From Problem Analysis to Program Design, Fourth Edition

16. EOF- Controlled while Loops • Use an EOF (End Of File)-controlled while loop • The logical value returned by cin can determine if the program has ended input • The syntax is: cin >> variable; while (cin) { . cin >> variable; . } C++ Programming: From Problem Analysis to Program Design, Fourth Edition

17. Example: write a program that prompts the user to input set of integer items ending with item that is negative then find summation positive items. #include <iostream> using namespace std; int main() { int num, sum; bool isNegative; sum = 0; isNegative = false; cout << "Enter numbers: "; cin >> num; C++ Programming: From Problem Analysis to Program Design, Fourth Edition

18. Example of Flag-Controlled while (continued) while (cin && !isNegative) { if (num < 0) //if num is negative, terminate the loop after this iteration { cout << "Negative number found in the data." << endl; isNegative = true; } else { sum = sum + num; cin >> num; } } cout << endl; cout << "The sum is: " << sum << endl; return 0; } C++ Programming: From Problem Analysis to Program Design, Fourth Edition

19. More on Expressions in while Statements • The expression in a while statement can be complex For example: while ((noOfGuesses < 5) && (!isGuessed)) { … } C++ Programming: From Problem Analysis to Program Design, Fourth Edition

20. do…while Looping (Repetition) Structure • General form of a do...while: • The statement executes first, and then the expression is evaluated • To avoid an infinite loop, body must contain a statement that makes the expression false • The statement can be simple or compound • Loop always iterates at least once C++ Programming: From Problem Analysis to Program Design, Fourth Edition

21. do…while Looping (Repetition) Structure (continued) C++ Programming: From Problem Analysis to Program Design, Fourth Edition

22. do…while Looping (Repetition) Structure (continued) C++ Programming: From Problem Analysis to Program Design, Fourth Edition

24. Divisibility Test by 3 and 9 C++ Programming: From Problem Analysis to Program Design, Fourth Edition

25. The for Loop • The general form of the for statement is: for (initial statement; loop condition; update statement) statement; • The initial statement, loop condition, and update statementare called for loop control statements C++ Programming: From Problem Analysis to Program Design, Fourth Edition

26. C++ Programming: From Problem Analysis to Program Design, Fourth Edition

27. The for Loop (continued) • The for loop executes as follows: • initial statement is the first to be executed and is executed only once • loop condition is evaluated • If loop condition evaluates to true • Execute for loop statement • Execute update statement • Repeat previous step until the loop condition evaluates to false • The for loop executes indefinitely if the loop condition is always true C++ Programming: From Problem Analysis to Program Design, Fourth Edition

28. The for Loop (continued) • Fractional values can be used for loop control variables • A semicolon at the end of the for statement is a semantic error • In this case, the action of the for loop is empty C++ Programming: From Problem Analysis to Program Design, Fourth Edition

29. The for Loop (continued) • In a for statement, all three statements (initial statement, loop condition, and update statement) can be omitted • The following is a legal for loop: for(;;) cout<<"Hello"<<endl; C++ Programming: From Problem Analysis to Program Design, Fourth Edition

30. for Looping (Repetition) Structure (continued) C++ Programming: From Problem Analysis to Program Design, Fourth Edition

31. Nested Control Structures • To create the following pattern: * ** *** **** ***** • We can use the following code: for (i = 1; i <= 5 ; i++) { for (j = 1; j <= i; j++) cout << "*"; cout << endl; } C++ Programming: From Problem Analysis to Program Design, Fourth Edition

32. Nested Control Structures (continued) • What is the result if we replace the first for statement with the following? for (i = 5; i >= 1; i--) • Answer: ***** **** *** ** * C++ Programming: From Problem Analysis to Program Design, Fourth Edition

33. Choosing the Right Looping Structure • All three loops have their place in C++ • If you know or can determine in advance the number of repetitions needed, the for loop is the correct choice • If you do not know and cannot determine in advance the number of repetitions needed, and it could be zero, use a while loop • If you do not know and cannot determine in advance the number of repetitions needed, and it is at least one, use a do...while loop C++ Programming: From Problem Analysis to Program Design, Fourth Edition

34. break & continue Statements • break and continue alter the flow of control • The break statement is used for two purposes: • To exit early from a loop • To skip the remainder of the switch structure • After the break statement executes, the program continues with the first statement after the structure C++ Programming: From Problem Analysis to Program Design, Fourth Edition

35. Example: #include <iostream>using namespace std;int main(){ int num, sum; sum = 0; cout << "Enter numbers: "; cin >> num; while (cin) { if (num < 0) //if number is negative, terminate the loop { cout << "Negtive number found in the data." << endl; break; } sum = sum + num; cin >> num; } cout << endl; cout << "The sum is: " << sum << endl; return 0; } C++ Programming: From Problem Analysis to Program Design, Fourth Edition

36. break & continue Statements (continued) • continue is used in while, for, and do-while structures • When executed in a loop • It skips remaining statements and proceeds with the next iteration of the loop C++ Programming: From Problem Analysis to Program Design, Fourth Edition

37. Example:#include <iostream>using namespace std;int main(){ int num, sum; sum = 0; cout << "Enter numbers: "; cin >> num; while (cin) { if (num < 0) { cout << "Negtive number found in the data." << endl; cin >> num; continue; } sum = sum + num; cin >> num; } cout << endl; cout << "The sum is: " << sum << endl; return 0; } C++ Programming: From Problem Analysis to Program Design, Fourth Edition