Introduction to Arrays

• This is chapter 11, pages 515-540.
  • You should read that.
• Arrays are very old, and for various reasons don't behave like everything else.
  • Arrays are from C
  • They are NOT part of the standard template library
  • There is an array type in this library but that is not what we are talking about.
• Some ideas
  • Homogeneous: they can contain only one type of data.
  • Contiguous: ie it is a block of memory.
  • Constant time access to all elements.
• Definition: A one dimensional array is a structured collection of components, all of the same type, that is given a single name. Each component (array element) is accessed by an index that indicates the component's position within the collection.
• Array Indexing
  • Arrays start at a base address (A)
  • Element i is located at the $A + i*sizeof(element)$.
• So arrays are a way to allocate a huge amount of memory.
• Declaring an array
  • DataType identifier [constant integer expression];

  • // ok
    int numbers[20];   // declare numbers to be an array of 20 elements each an integer.
    char word[100];   // word can hold up to 100 characters.
    
    // better
    const int MAX_WORDS = 2000;
    ...
    
    string dictionary[MAX_WORDS];
    

  • DataType identifier [constant integer expression] = {item1, item2, ... itemN}; // the integer expression should be equal to N

  • char notes[7] = {'a','b','c','d','e','f','g'};
    

  • DataType identifier [] = {item1, item2, ... itemN};

  • float gradeLowerBounds[] = {0.0, 0.6, 0.7, 0.8, 0.9};
    

• Note
  • The array size (constant integer expression) must be known at complie time.
  • The following code will work with g++ but is not standard

  • int size;
    cout << "Enter the array size: "
    cin >> size;
    
    string dictionary[size];
    

  • In general, I will mark such usage as incorrect.
• When using arrays you generally have three items
  • The array,
  • The maximum size
    • For now, this should be a constant.
  • The size in use.
• When to use an array
  • Only if you need to store the information.
  • Everything you have done up to this point you could read the information and process it as you work.
  • But many problems require you to store information and process it many times.
• Accessing elements
  • ArrayIdentifier[integer expression]
  • It is customary to check to see if the integer expression is between 0 and the maximum size before indexing into the array.
  • We frequently take advantage of c's short circuit evaluation

  • CONST int MAX_SIZE = 100;
    ...
    string dictionary[MAX_SIZE];
    ...
    while (i < MAX_SIZE and word != dictionary[i]) {
    
           

  • Note that if i >= MAX_SIZE, the loop will abort.
• Let's write a program to store all of the unique words in a file.
  • You may assume that there will be no more that 1000 unique words in the file.
  • This is a standard statement until later in the semester when you will learn how to deal with dynamic memory.