CSE 501N Fall ‘09 08: Control Flow

1. CSE 501NFall ‘0908: Control Flow 24 September 2009 Nicholas Leidenfrost

2. Lecture Outline • Lab 2 questions • Control Flow and Loops • The break Statement • The continue Statement • The return Statement • Array Exercises

3. Control Flow • Recall that control flow (a.k.a. flow of control) is the order in which statements in a program are executed • Certain types of statements allow us to alter the default flow of control (which is linear) • The ifstatement allows us to conditionally execute a particular statement • The elseclause of an if statement allows us to specify an “otherwise” condition • The switchstatement allows us to conditionally execute many cases based on equality • Loops (for, while, do … while) allow us to repetitively execute a statement

4. Control Flow • Sometimes it is necessary to circumvent the execution of statements, and it is undesirable or impossible to do so correctly with an if/elsestatement • Particularly with reference to loops • When desired control flow is logically difficult to achieve due to multiple / nested conditional statements • When we want to exit a loop prematurely

5. Control Flow: break • We have seen how to use breakin the switchstatement: int myCount; int oneCount, twoCount, threeCount; // ... switch (myCount) { case 1: oneCount++; case 2: twoCount++; case 3: threeCount++; // …more cases }

6. Control Flow: break • We have seen how to use breakin the switchstatement: int myCount; int oneCount, twoCount; // ... switch (myCount) { case 1: oneCount++; break; case 2: twoCount++; break; // …more cases }

7. Control Flow: break • What if we want to exit a loop prematurely? public boolean hasValue (int[] searchIn, int searchFor) { boolean found = false; for (int i=0; i<searchIn.length; i++) { if (searchIn[i] == searchFor) found = true; } return found; } Equal to 2 at this point int nums[] = { 1, 2, 3, 4 }; int searchValue; // ... boolean valueFound = hasValue(nums, searchValue);

8. Control Flow: break • What if we want to exit a loop prematurely? public boolean hasValue (int[] searchIn, int searchFor) { boolean found = false; for (int i=0; i<searchIn.length; i++) { if (searchIn[i] == searchFor) { found = true; break; } } return found; } int nums[] = { 1, 2, 3, 4 }; int searchValue; // ... boolean valueFound = hasValue(nums, searchValue);

9. break:Nested loops • What if we want to exit multiple loops prematurely? public boolean hasValue (int[][] searchIn, int searchFor) { boolean found = false; for (int i=0; i<searchIn.length; i++) { for (int j=0; j<searchIn[i].length; j++) if (searchIn[i] == searchFor) { found = true; break; } } } return found; } public boolean hasValue (int[][] searchIn, int searchFor) { boolean found = false; for (int i=0; i<searchIn.length; i++) { for (int j=0; j<searchIn[i].length; j++) if (searchIn[i] == searchFor) { found = true; break; } } if (found) break; } return found; } int nums[][] = { { 1, 2, 3, 4 }, { 5, 6, 7, 8 } }; boolean found = hasValue(nums, 3);

10. Control Flow: return • What if we want to exit a loop prematurely? public boolean hasValue (int[] searchIn, int searchFor) { for (int i=0; i<searchIn.length; i++) { if (searchIn[i] == searchFor) return true; } return false; } int nums[] = { 1, 2, 3, 4 }; int searchValue; // ... boolean valueFound = hasValue(nums, searchValue);

11. Control Flow: continue • What if we want to skip one or more iterations? • The key word continueis used to jump to the start of the next iteration public voidprintGPAs (Student[] students) { for (Student aStudent : students) { if (aStudent == null) continue; System.out.println(aStudent.getGPA()); } } Student myClass[] = { null, null, new Student() }; printGPAs(myClass);

12. Control Flow: break • Syntax Implications • What is the problem in this code? int myCount; int oneCount, twoCount; // ... switch (myCount) { case 1: oneCount++; break; System.out.println(“here”); case 2: twoCount++; break; // …more cases }

13. Control Flow Statements • Syntax Implications public int getIndex (int[] searchIn, int searchFor) { int index = -1; for (int i=0; i<searchIn.length; i++) { if (searchIn[i] != searchFor) { continue; System.out.println(“Not at: “ + i); } else { index = i; break; System.out.println(“Fount at: “ + i); } } return index; } Unreachable! Unreachable!

14. Array Exercises • Inserting into the middle of an (contiguous) array • Removing from the middle of an (contiguous) array • Reversing an array • Making an array contiguous

15. Conclusion • Questions? • Lab 2 due on Tuesday • I will be in lab now