Introduction

Arrays are multi-valued variables of a particular type.

• Declaring an array
  type identifier[size];
  int grade[4];
• Provides size memory locations to store values of the declared type
• Can declare arrays andsingle-valued variables within the same declaration statement
  int lcv, grade[4];
• Access elements of the array using an index or a subscript
• Use array elements as you would single-valued variables
• Indices start at 0 and go thru size-1
• To assign the "first" element of the grade array a 75:
  grade[0] = 75;
• Common Mistake:Accessing element N
  To access the last element of the grade array, use grade[3]
  The valid indices are 0, 1, 2, and 3 for the four element array
• Indices can be represented by anything of type int
  float avgs[5];
  int i;
  i=1;
  avgs[0] = 82.6;
  avgs[i] = 91.4;
  avgs[i+3] = 65.6;

  82.6

  91.4

  65.6

• Can access using a for loop:
  const int SIZE=20;
  int i, grade[SIZE];
  for (i =0; i< SIZE; i++)
     cin >> grade[i];
• The above code took advantage of using a constant
• Array Initialization can be done within the declaration statement using { }
• To declare an array of 3 floating point numbers and intitalize:
  float nums[3] = { 1.0, 2.4, 5.6 };
• When initializing all values, you can omit the size
  float nums[] = { 1.0, 2.4, 5.6 };
• You can initialize all to 0.0 with this:
  int grades[20] = {0};
• BUT this only works with 0
  int grades[20] = {4}; // Only initializes the first to 4, rest are 0

• Initializing Arrays

  for (i=0; i < SIZE; i++)
    arr[i] = rand() % 100;
  

• Printing Arrays

  for (i=0; i < SIZE; i++)
    cout << arr[i] << " ";
  

• Copying Arrays
  array1 = array2; // NOT permitted in C++

  for (i=0; i < SIZE; i++)
    array1[i] = aray2[i];
  

  Summing elements of an array

  total = 0;
  for (i=0; i < SIZE; i++)
    total = total + arr[i];
  

  Finding largest element of an array

  double max = arr[0];
  for (i=1; i < SIZE; i++)
  {
    if (arr[i] > max)
        max = arr[i];
  }
  

  Shifting elements of an array

  double temp = arr[0];
  for (i=1; i < SIZE; i++)
  {
     arr[i - 1] =arr[i] ;
  }
  arr[i - 1] = temp;
  

• Array elements can be passed to functions just like a single-valued variable
  Given the prototype: void myFunction(float);
  If the main program contains: float n, arr[3];
  myFunction can be called with: myFunction(n);
  or with: myFunction(arr[0]);
• Entire Arrays can be passed to functions;
  void getnums(int [], const int); //prototype
  void getnums(int arr[], const int SIZE) //header
  {
     int i;
     cout << "Enter " << SIZE << "numbers: ";
     for (i=0; i<SIZE; i++)
        cin >> arr [i] ;
  }
• If no value is given within the [ ] in the header, then the size of the argument is used. This permits great flexibility: the function can be used for different sized arrays.
• No & is used for pass by reference. Pass by reference is the default.
• To achieve pass-by-value with an array, use const
  void printnums(const int [], const int); //prototype
  void printnums(const int arr[], const int SIZE) //header
  {
     int i;
     cout << "The numbers are " ;
     for (i=0; i<SIZE; i++)
        cout << arr [i] ;
  }
• Functions can return arrays, but pointers are needed (later this semester)

Multi-dimensional Arrays

• Declaring
int a[2][3];
float b [4][2][3];
• Initializing
int a[2][3] = {{ 1,2,3}, {4,5,6}};
• If not enough values, rest of row initialized to 0
• If too many, used in next row
• Used for array of strings
char names [4][ ] = {"Sue, "Mary", "Sally", "George");
• Access using for loop
int arayeg[2] [3] = { {1, 2, 3}, {4, 5, 6}};
for (i = 0; i < 2; i++)
for (j = 0; j < 3; j++)
cout<< arrayeg[i][j] << endl;

• A vector is a dynamic array available through the STL - standard template library
• It holds values of any type
• You do not need to set a size when you declare it - i.e., optional
• You can add elements to it and C++ automatically ads spaces as needed.
• Like an array [ ] are used to access elements

1. Include the vector library
   #include<vector>
2. Declare a vector to hold a type:
   vector<int> myVector;
3. Add an element to the vector usig the push_back method
   myVector.push_back (43);
   myVector.push_back (81);
4. Find the value of an element of the vector using the at method
   x=myVector.at(0); // x is assigned 43
5. Find the size of a vector;
   x=myVector.size(); // x is assigned 2
6. Remove the last element of a function from the vector
   myVector.pop_back();
   x=myVector.size(); // x is now 1

• You can declare a vector with a size:
  vector<int> myVector (100);
• You can intialize all elments to 0:
  vector<int> myVector (100, 0);
• You can initialize a vector to a list of values:
  vector<int> myVector {5, 4, 3, 2, 1};
  
  This automatically gives it an initial size
• To remove all the elements of the vector use clear()
  myVector.clear();