2. 1 ELEC 206 Chapter 3 Control Structures

3. 5-Step Problem Solving Methodology 1. State the problem clearly. 2. Describe the input and output. 3. Work a hand example. 4. Develop a solution. 5. Test your solution. 2

4. Control structures  Algorithm Development  Conditional Expressions  Selection Statements  Repetition Statements  Structuring Input Loops 3

5. Algorithm Development  An algorithm is a sequence of steps for solving a problem.  Engineering problem solutions to real world problems require complex algorithms.  Development of a good algorithm increases the quality and maintainability of a solution, and reduces the overall time required to implement a correct solution. 4

6. Top-Down Design  Top-down design begins with a "big picture" description of a problem solution in sequential steps.  The sequential steps are refined until the steps are detailed enough to translate to language statements.  The refined steps, or algorithm, can be described using pseudo code or flowcharts. 5

7. Evaluation of Alternative Solutions  Most problems have more than one solution.  There may not be a single best solution, but some solutions are better than others.  Elements that contribute to a good solution: –correctness –reliability –readability –maintainability –execution speed –memory considerations –user interface 6

8. Structured Programming  A structured program is written using simple control structures, including: –Sequence – steps are performed one after another. –Selection – one set of statements is executed if a given condition is true, a different set of statements, or no statements at all, is executed if the condition is false. –Repetition –A set of statements is executed repeatedly as long as a given condition is true. 7

9. 8 Structured Programming  Sequence  Selection  Repetition ? truefalse ? true false ? => conditional expression

10. Conditional Expressions  A conditional expression is a Boolean expression that evaluates to true or false.  Selection structures and repetition structures rely on conditional expressions.  Relational operators and logical operators are used to form conditional expressions. 9

11. 10 Relational Operators ==equality ==equality !=non equality !=non equality <less than <less than >greater than >greater than <=less than equal to <=less than equal to >=greater than equal to >=greater than equal to

12. 11 Logical Operators !not !not &&and &&and ||or ||or

13. Logical Operators ABA&&BA||B!A!B 000011 010110 100101 111100 12 Truth table for conditional expressions 0 = false 1=true

14. 13 Operator Precedence 1. >= 2. == != 3. && 4. ||

15. 14  (-6 =10)  (3.0 >= 2.0) || (3.0 >= 4.0)  (3.0 >= 2.0) && (3.0 >= 4.0) true && true results in true Practice! - evaluate true || false results in true true && false results in false

16. 15 Selection Statements  The C++ programming language supports the implementation of selection with: –if statements –switch statements

17. 16 The if statement if(expression) statement; /*single statement executed if expression is true */ if expression is true */ // statement block is executed if expression is true. if(expression) { statement1; statement2; … statement n; }

18. 17 The if statement - examples if (x>0) ++k; if(x>0) { x=sqrt(x); ++k; }

19. 18 The if - else statement if(expression) statement; else statement; statement; if(expression) { statement block } else { statement block }

20. 19 The nested if-else if(x > y) if(y < z) k++; else m++; else j++;

21. 20 Practice! int x=9, y=7, z=2, k=0, m=0, j=0; if(x > y) if(y >z && y>k) k++; else m++; else j++; What are the values of j, k and m?

22. 21 The switch statement switch(expression) { case constant: statement(s); break; case constant: statement(s); break; /* default is optional*/ default: statement(s); }

23. 22 The switch statement  Expression must be of type integer or character.  The keyword case must be followed by a constant.  break statement is required unless you want all subsequent statements to be executed.

24. 23 switch statement example char ch; int ecount=0, vowels=0, other=0; cin.get(ch); while(!cin.eof()) { switch(ch) {case ‘e’:ecount++; case ‘a’: case ‘i’: case ‘o’: case ‘u’:vowels++; break; default:other++; }//end switch cin.get(ch); }//end while cout << ecount << ‘,’ << vowels << ‘,’ << other << endl;

25. 24 Practice! Convert these nested if/else statements to a switch statement : if (rank==1 || rank==2) cout << "Lower division \n"; else { if (rank==3 || rank==4) cout << "Upper division \n"; else { if (rank==5) cout << "Graduate student \n"; else cout << "Invalid rank \n"; }

26. REPETITION STATEMENTS while statement do while statement for statement Structuring input loops 25

27. 26 Repetition Statements  The C++ programming language supports the implementation of repetition with: –while statements –do/while statements –for statements

28. 27 The while statement while (expression) statement; { statement block } ? true false

29. 28 The do/while statement dostatement; while (expression) do{ statement block } while (expression) ? true false

30. 29 Practice ! #include using namespace std; int main() { int n=4; while(n>0) { cout << n << endl; --n; } cout << “value of n outside while is “ << n << endl; return 0; } Program Trace: Output?

31. 30 The for statement initalize ? increment/ decrement true statement(s) false

32. 31 The for statement for(initialization; expression; increment/decrement) statement; for(initialization; expression; increment/decrement) { statement; }

33. 32 The for statement - examples //sum integers from //1 to 10 inclusive #include using namespace std; int main() { int sum=0; for(int i=1;i<11;++i) { sum = sum + i; } cout << sum << endl; return 0; } Alternate solution: //sum integers from //1 to 10 #include using namespace std; int main() { int sum=0; for(int i=1;i<=10;i++) sum = sum + i; cout << sum << endl; return 0; }

34. The for statement - example //sum odd integers from //1 to n inclusive #include<iostream> using namespace std; int main() { int sum=0, n; int sum=0, n; cout << "enter non-negative integer: "; cout << "enter non-negative integer: "; cin >> n; cin >> n; for(int i=1;i<=n;i+=2) for(int i=1;i<=n;i+=2) sum = sum + i sum = sum + i cout << sum << endl; cout << sum << endl; return 0; return 0;} 33

35. The for statement - example //sum odd integers from //1 to n inclusive //Alternate Solution #include<iostream> using namespace std; int main() { int sum=0, n; int sum=0, n; cout << "enter non-negative integer: "; cout << "enter non-negative integer: "; cin >> n; cin >> n; for(int i=1;i<=n;++i) for(int i=1;i<=n;++i) { if(i%2) sum = sum + i; if(i%2) sum = sum + i; } cout << sum << endl; cout << sum << endl; return 0; return 0;} 34

36. 35 Practice!  Write a program solution to print all integer values between 1 and n that are multiples of 5 (ie evenly divisible by 5).  Compare your solution with another person's solution.

37. 36 The break statement  break; –terminates loop –execution continues with the first statement following the loop Example: What is the output? for(int i-0; i<=10; ++i) { if(i%2) break; cout << i << endl; }

38. 37 The continue statement  continue; –forces next iteration of the loop, skipping any remaining statements in the loop Example: What is the output? for(int i-0; i<=10; ++i) { if(i%2) continue; cout << i << endl; }

39. 38 Practice ! //This while loop calculates n! int nfact=1, n; cout << "enter positive integer "; cin >> n; while(n > 1) { nfact = nfact*n; n--; } cout << n << "! = " << nfact << endl; //What is the output for n=5? //Write an alternate solution.

40. Structuring Input Loops 39  Repetition is useful when inputting data from standard input or from a file.  Common repetition structures:  counter-controlled  sentinel-controlled  end-of-data controlled

41. Counter-controlled Repetition Structure i 0 while i < = counter input data value //Do something with data value increment i end while 40

42. Sentinel-controlled Repetition Structure input data value while data value ! = sentinel value //Do something with data value input next data value end while 41

43. eof()-controlled Repetition Structure input data value while end-of-file is not true //Do something with input data input next data value end while 42