CSci 160 Lecture 33

1. CSci 160Lecture 33 Martin van Bommel

2. Passing Arrays to Functions • Write a program to: • Read in list of integers until sentinel 0 is entered • Reverse the elements in the list • Display list in reverse order int main(void) { int list[NElements]; GetIntegerArray(list); ReverseIntegerArray(list); PrintIntegerArray(list); }

3. Problems with Method • Two issues: • Number of elements in array?? • NElements or how many entered before sentinel • GetIntegerArray and ReverseIntegerArray must change values of argument arrays in main • up until now, values of arguments cannot be changed outside function

4. Generalize Number of Elements • Only wish to reverse and print actual number of elements, array contains more elements in declaration (maximum number) int list[MaxElements]; • Call actual number of elements effective size • Print and reverse must then be told size PrintIntegerArray(list, n);

5. Function Prototypes • Could write as void PrintIntegerArray(int array[MaxElements], int n); • Better to write void PrintIntegerArray(int array[], int n); • Then can pass array of any size • Also void ReverseIntegerArray(int array[], int n);

6. GetIntegerArray Prototype • GetIntegerArray has different structure • Don’t know effective size before call • GetIntegerArray determines effective size • Needs to know actual size to limit it • Also give it sentinel value for flexibility int GetIntegerArray(int array[], int max, int sentinel);

7. Mechanics of Array Parameters • When variable used as parameter, only its value is passed • When array name used as parameter, only base address of array sent to function • Function can then operate on the array in place without copying its contents • Storage for parameter array is shared with actual argument array

8. Implementing PrintIntegerArray void PrintIntegerArray(int array[], int n) { int i; for (i = 0; i < n; i++) { printf(”%d\n”, array[i]); } }

9. int GetIntegerArray(int array[], int max, int sentinel) { int n=0, value; printf(” ? ”); scanf(”%d”, &value); while (value != sentinel) { if (n == max) Error(”Too many items”); array[n] = value; n++; printf(” ? ”); scanf(”%d”, &value); } return (n); }

10. ReverseIntegerArray void ReverseIntegerArray(int array[], int n) { int i, limit; limit = n / 2; for (i = 0; i < limit; i++) { Swap values in array[i] and array[n - i - 1] } }

11. Swap Array Elements • Try Swap(array[i], array[n-i-1]); • No, because passing values • Must use Swap(array, i, n-i-1);

12. SwapIntegerElements void SwapIntegerElements(int array[], int p1, int p2) { int tmp; tmp = array[p1]; array[p1] = array[p2]; array[p2] = tmp; }