Understanding Control Structures: do/while Loops, Break, Continue, and Switch Statements

1. Overview • do/while loop • break statement • continue statement • switch statement

2. do/while loop do { statement; } while (condition); Step1: Execute statements inside braces Step2: Evaluate condition. Step3: If condition is true repeat Step1 through 3, otherwise continue execution at next statement following the do/while statement Usually use { } even when not necessary Useful when you want the statements to execute at least once. i.e reading input.

3. do while example do { cout << “Enter a number between 1 and 10: “; cin >> number; } while (number <= 0 || number > 10);

4. do/while example What will the output of the following program sequence be? int j = 4; do { j = j - 2; cout << “Hi”; } while (++j > 0); Walk through this example showing how j changes.

5. Loop Exercises 1) How many times does the statement inside the do/while body execute? What gets displayed? int i = 0, x = 2; do { x = x + i; } while (++ i < 3); cout << “x = “ << x << endl; 2) How many times does the cout statement execute? for (int i = 1; i <= 4; i++) { for (int j = 1; j <= 3; j++) { cout << “Hello” << endl; } }

6. continue statement skip the rest of this iteration and continue with the next iteration of the loop int x = 1; while ( x <= 10) { if (5 == x) { x++; continue; } cout << x << endl; x++; } condition (x <= 10) is next statement executed after the continue statement.

7. continue statement • for (int x = 1; x <= 10; x++) { • if (5 == x) • continue; • cout << x << endl; • } • next statements executed after continue are • x ++; • x <= 10;

8. break statement • Breaks out of the loop. Next statement executed after a • break statement is the statement immediately following the loop. • for (int x = 1; x <= 10; x++) { • if (5 == x) • break; • cout << x << endl; • } • Next statement executed after the break statement is the • statement immediately following the for loop.

9. else if example if (‘A’ == lettergrade) gpa = 4.0; else if (‘B’ == lettergrade) gpa = 3.0; else if (‘C’ == lettergrade) gpa = 2.0; else if (‘D’ == lettergrade) gpa = 1.0; else if (‘F’ == lettergrade) gpa = 0.0; else cout << “Illegal letter grade” << endl;

10. switch statement switch (lettergrade) { case ‘A’: gpa = 4.0; break; case ‘B’: gpa = 3.0; break; case ‘C’: gpa = 2.0; break; case ‘D’: gpa = 1.0; break; case ‘F’: gpa = 1.0; break; default: cout << “Illegal letter grade” << endl; }

11. switch statement syntax switch (controlling expression) { case label1: statement(s); break; case label2: statement(s); break; . . . case labeln: statement(s); break; default: statement(s); }

12. switch statement rules • controlling expression must be of type int or char • labels must be constants that match the type of the controlling expression • default statement can be anywhere. More intuitive at bottom. • can have multiple case labels sharing the same code case ‘A’: case ‘a’ : statements; break;

13. else if that cannot be implemented as a switch int score; cout << “Enter score: “; cin >> score; cout << “Your grade is “; if (score < 68) cout << “NR” << endl; else if (score < 78) cout << “C” << endl; else if (score < 88) cout << “B” << endl; else cout << “A” << endl;

14. common programming errors • Forgetting the break statement. Statements for next case get executed by mistake. • Forgetting the space between the word case and the label. Will not be flagged by compiler but won’t execute the way you intend.

15. Grandma’s Attic for (char letter = 'a'; letter <= ‘i'; letter++) { cout << "I'm going to Grandma's attic and I'm going to get "; switch (letter) { case 'i': cout << "an igloo, "; case 'h': cout << "a hat, "; case 'g': cout << "a goat, "; case 'f': cout << "a fox, "; case 'e': cout << "an elephant, "; case 'd': cout << "a dinosaur, "; case 'c': cout << "a cookie, "; case 'b': cout << "a ball and "; case 'a': cout << "an apple" << endl; default: break; } }