1. Arrays

2. Why array?
Lots of times we want to store a large number of data of same type
Ex, 30 students’ midterm exam score

3. Why array
Arrays allow you to group individual elements together into a single data structure, with a single name
It’s hard to do any serious programming without relying heavily on arrays
These elements are stored in contiguous memory locations, and are thus easy for the compiler to find if you tell it the index of the element you want – “give me the 4th element”
It is easy to sort them
The code are compact

4. Declaring Array Compiler need to know: Type Name Size
Example: int score[16];
const in size=16;
double score[size];
char name[20];

5. Accessing array
We can access individual elements of the array using brackets after the array name
Examples:
If homeScores is an array of int, an individual element of the array is of type int, and can be used anywhere an int is used
int currentScore = homeScores[3];
Note: this reads 4th element (indices start at 0)
homeScores[0] = 0;
total = inputNums[0] + inputNums[1];
The I/O functions know how to work with arrays
cin >> inputNums[4];
If the inputNumber is double type, compiler knows that one element of inputNums is of type double, so it will go read a double

6. Caution You must be very careful with your array accesses
Causes some nasty bugs
When you run off the end of the array, you may change another variable, and the bug appears to be in the other variable
int myArray[20];
int myNum = 0; …
myArray[20] = 5; // you probably just changed the
// value of myNum
Array index start from 0.

7. Initializing array Initializing arrays Methods of initializing arrays
We like to initialize all of our variables when we declare them
Likewise, it is good to initialize arrays
Global scope:  Automatically initialized to zero (or equivalent)
Local scope:  Not automatically initialized
Methods of initializing arrays
You can initialize when you declare them
int primes[5] = {3, 5, 7, 11, 13}; //full initialization
You can even leave off the array size
int nums[ ] = {0, 1, 2, 3, 4}; //implicit iniitalization
If you provide less values than elements, it will pad with zeros: int nums[8] = {0, 1, 2}; makes it {0,1,2,0,0,0,0,0} //partital intialization
You can use a loop to initialize the array
for (i = 0; i < arraySize; i++) {
myArray[i] = -1; }

8. Exercise you declare an array: int myArray[10] = {1, 2, 3, 4}
What is the value of x after:
x = myArray[1]
x = myArray[3]
x = myArray[0]
x = myArray[7]
x = myArray[10]
Which of these are legal?
myArray[4] = myArray[3];
myArray[3] = 0;
0 = myArray[0];
int x = myArray;
float f = myArray[9];

9. Example use of arrays int i= 0; int numValues; int values[100];
double total = 0.0;
cout<<“please enter a number that is less than 100:”<<endl;
cin>> numValues;
cout<<“please enter “<<numValues<< “integers”<<endl;
cout<<“I can average them for you”<<endl;
for (i=0; i< numValues; i++) {
cin >> values[i]; }
total += values[i];
double average = total / numValues;
cout << “ The average value of all the numbers you input is“ << average << endl;

10. Constant arrays
const int studentID[3] = {1234, 4321, 2341};
The values of the array are constant and cannot be changed
Which of the following would be legal?
int x = studentID[2];
studentID[1] = studentID[2];
studentID[0]++;
cout << studentID[2];

11. Character arrays Demo of a C string without null char myName[10];
Character arrays work just like any other array
The type just happens to be char
Before the string class came along, character arrays were the only way to do string processing
You initialize a character array like a string
char myProfession[9] = “engineer”
Note that “engineer” only has 8 letters, but the array is size 9 – this is because old C-style strings always are terminated with a special character - \0 (ascii value=0)
You can do I/O like it was a string
cout << myProfession; // will print “engineer”
cin >> myProfession; // will read a string into the array
Demo of a C string without null

12. Character array example
char color[6] = “red”;
char color[6] = “green”;
char color [6] = “tan”
r e d \0 \0 \0
g r e e n \0
t a n \0 \0 \0

13. Exercise You can also treat character arrays just like any array
int word[5] = “ball”;
word[0] = ‘f’; // “ what you got ? ”
word[1] = ‘i’; // “ what you got?”
word[3] = ‘e’; // “ what you got?”
Demo of program: arrayinit.cpp in puTTy

14. Multi-Dimensional Arrays
C++ allows you to represent multi-dimensional data with multi-dimensional arrays
Typically we don’t go beyond 3 dimensions, more because of human understanding limitation than computer limitation
Declaring a multi-dimensional array
int matrix [10][10];
Ten rows of ten elements each
Actually the compiler just allocates 100 contiguous elements in memory, but computes proper offsets

15. 1 dimensional array: array[5]

16. Multi-Dimensional Arrays
Using a multi-dimension array
We use it just like 1-dimensional, but must specify both row and col
int x = nums[2][5];
nums[4][7] = 5;
Initializing a multi-dimensional array
int nums[3][3] = {1,2,3,4,5,6,7,8,9};
First row is 1,2,3; second row is 4,5,6; third is 7,8,9
Again, if insufficient values are provided, zeros will be added

17. Multi-Dimensional Arrays Example
What does the following code do?
Demo of flipping a square matrix
for (i = 1; i < size; i++) {
for (j =0; j < i; j++) {
temp = square[i][j];
square[i][j] = square[j][i];
square[j][i] = temp; }