1. Array An “average.cpp” program
Read in a list of numbers
Count them and sum them up
Calculate the average
Now - what if we want to know how many of them are above average?
Must be able to go through them all again
Need to be able to store them in memory

2. Arrays Collection of individual data values Two characteristics
An array is ordered - elements numbered.
An array is homogeneous - all values same type
Two properties must be defined
Element type - type of value for elements
Array size - number of elements in array

3. Array Declaration int scores[NumScores]; scores
elementtype arrrayname [ size ];
#define NumScores 10
int scores[NumScores];
scores

4. Array Selection scores[i] = 0; }
To refer to element of array, specify array name and position of element (subscript)
scores[2] = 25;
To set all elements of array
for (i=0; i<NumScores; i++) {
scores[i] = 0; }
To initialize elements of array in declaration
int primes[]={2,3,5,7,11,13,17,19};

5. Changing Numbers Program
Add an array for the values (assume 100)
#define MaxScores 100
int scores[MaxScores];
Store values in array
scores[counter] = value;
Check if value > average
if (scores[i] > average) ...
See aboveavg.cpp

6. 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() {
int list[MaxElements];
GetIntArray(list);
ReverseIntArray(list);
PrintIntArray(list); }

7. Problems with Method Two issues: Number of elements in array??
MaxElements or how many entered before sentinel
GetIntArray and ReverseIntArray must change values of argument arrays in main

8. 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
PrintIntArray(list, n);

9. Function Prototypes void PrintIntArray(int array[], int n);
Could write as
void PrintIntArray(int array[MaxElements], int n);
Better to write
void PrintIntArray(int array[], int n);
Then can pass array of any size
Also
void ReverseIntArray(int array[], int n);

10. GetIntArray Prototype
GetIntArray has different structure
Do not know the effective size before call
GetIntArray determines effective size
Needs to know actual size to limit it
Also give it sentinel value for flexibility
int GetIntArray(int array[], int max,
int sentinel);

11. 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

12. Implementing PrintIntegerArray
void PrintIntArray(int array[], int n) {
int i;
for (i = 0; i < n; i++)
cout << array[i] << endl; }

13. int GetIntArray(int array[], int max, int sentinel){
int n = 0, value;
cout << ” ? ”;
cin >> value;
while (value != sentinel) {
if (n >= max) cout << ”Too many items”;
else array[n] = value;
n++; }
return n;

14. ReverseIntArray void ReverseIntArray(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] }

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

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