• However, declaring all of these variables and assigning them values is a very tedious process.
• Now, imagine there were 100 test scores... Or, an unknown number of test scores...
• We need another system!
• Fortunately, C++ has techniques we can use to organize lists of data

Defining Arrays

• An array is a collection of data items all of the same type
• You declare an array like this:

  dataType variableName[size];
  

• Where:
  • dataType: the data type of all the array items
  • variableName: the name you make up for the array
  • size: the number of data items the array can hold
• For example, the following is the declaration of an array named scores that holds 10 values of type int:

  int scores[10]

• Arrays like this can never change size and the array size must be set when the program is compiled
• When defining an array, you must guess the maximum number of elements you need to store:

  const int MAX_SCORES = 10;
  int scores[MAX_SCORES];
  

• The programmer must keep track of the capacity
• We use a constant to hold the capacity of the array as shown above
• This allows us to know the size anywhere in our code
• If we need to change the size, we change only a single constant and recompile
• When a program executes this statement, it creates 10 contiguous slots in memory like this:

  scores =

• Each of the memory slots can hold one data value

Initializing Array Items

• We specify which slot of an array to access with the [] operator:

  scores[4] = 98;

• The indexes of arrays are numbered starting at 0
  
• We can assign a value to an array element any time after it is declared:

  const int MAX_SCORES = 5;
  int scores[MAX_SCORES];
  scores[0] = 90;
  scores[1] = 95;
  scores[2] = 87;
  scores[3] = 89;
  scores[4] = 98;
  

• We can also initialize array elements in the declaration statement:
  • Called static initialization
  • We use a comma-separated list inside curly-braces
• For example:

  int scores[] = { 90, 95, 87, 89, 98 };
  

• This produces the same array as in the previous example
• The compiler computes the size automatically by counting the items in the list
• If we want a larger array with only the first few elements initialized, we can use:

  int scores[MAX_SCORES] = {90, 95, 87};
  

• Note that if we do not assign a value to an array element, its value is not known.

Accessing Array Items

• To access the slots in an array, we must specify which slot to use with the [] operator
• For instance:

  scores[4] = 98;

• The number inside the brackets is called an index or subscript
• In C++, the slots of an array are numbered starting at 0, as shown below:

  scores =

  98

  [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]

• Thus, assignment to the slot with an index of 4 is put into the fifth slot

Using Slots

• We declared our example array with a data type of int:

  int scores[10];
  

• Because scores is an array containing int values, we can use a slot, such as scores[4], just like any variable of type int:

  scores[4]++;
  cout << scores[4] << endl;
  

• This includes using a slot as an argument to a function with a parameter of the same type:

  void myFun(int singleScore);
  ...
  myFun(scores[4]);
  

Using Arrays to Collect Data Items

• Note that the index of an array can be any integer value
• Thus, we can use an integer variable for the index
• We can use an integer variable with a loop to read data into the array
  
• Also, we can display the contents of an array using a loop
• The following program shows an example of collecting and displaying data items

#include <iostream>
using namespace std;

int main() {
    const int MAX_SCORES = 10;
    int scores[MAX_SCORES];

    cout << "Enter " << MAX_SCORES << " scores:\n";
    for (int i = 0; i < MAX_SCORES; i++) {
        cin >> scores[i];
    }

    cout << "You entered:\n";
    for (int i = 0; i < MAX_SCORES; i++) {
        cout << scores[i] << endl;
    }

    return 0;
}

Activity 20.1: My First Array (10pts)

• Copy the following program into a text editor, save it as myarrays.cpp, and then compile and run the starter program to make sure you copied it correctly.

  #include <iostream>
  using namespace std;
  
  int main() {
      // Enter your code here
  
      return 0;
  }
  

• Inside main(), add a statement to define an array named names that is suitable for holding a list of three (3) names.
• Compile your code to make sure it has correct syntax.

  If you have problems, ask a classmate or the instructor for help as needed.

• Assign values to each of the array elements like:

  names[0] = "Abel Ableson";
  

• Add a for-loop to display all three array values.
• Compile your code to make sure it has correct syntax.

  If you have problems, ask a classmate or the instructor for help as needed.

• When you are finished, upload your source code to Catalyst.