Arrays and for loops

Arrays

• When we want to store multiple items in a variable, and the number of items is not likely to change, an array is a good way to do this.
• All of the items in an array must be of the same type, and we must declare this when the array variable is declared. The following declares an array of integers with the size as yet unspecified:
  int[] numbers;
• To have Java allocate space for the array, we have to instantiate it with the size specified. We do this using the reserved word new, which we'll see later on is also used whenever we create any new object in a program.
  numbers = new int[25];
• To access an element in an array, we put the index in brackets:
  numbers[11]

  The first item in array has index 0, so the above expression would access the twelfth element in the numbers array.

• To assign a value to a position in an array, we use the usual = assignment operator with the position index again in brackets:
  numbers[8] = 6;
• To initialize an array to a particular set of values, we can put those values between {}:
  int[] numbers = {2, 3, 4, 5, 6};
  Note that in this case, we don't use new (because the expression between the curly brackets actually creates the array) and don't need to specify the size of the array (because this is given by the number of elements in the expression between the curly brackets). Note also that this is only possible when the array is first declared. We can't do something like the following:
  int[] numbers;
  ...
  numbers = {2, 3, 4, 5, 6};

for loops

• When we want to do something over and over again, we write a loop.
• When we know how many times we want to repeat the statement(s), we can use a for loop:
  for (initialization; condition; increment)
     statements;
  First the initialization statement is executed. Then the condition is evaluated. If it is false, then the loop is finished and processing continues after it. If it is true, then the statements in the for body are executed. After the statements in the body are executed, the increment statement is executed. Then the condition is evaluated again. If it is true, the statement in the body is evaluated again. This process repeats until the condition becomes false. Here is an example with an array; for is often used to access and/or assign the elements in an array. This example uses an assignment operator that we haven't seen before, ++. This operator increments the value of an integer variable by 1. So index++; is the same as index = index + 1;
  {
   int[] numbers = new[5];
   int index;
  
   for (index = 0; index < 5; index++)
      numbers[index] = index;
  }
• To access the length of an array, we use the variable length, which each variable automatically has. For example, if the variable animals is bound to an array of animals, we can know how many positions the array has with the expression animals.length.

© 2004. Indiana University and Michael Gasser. www.indiana.edu/~gasser/Q530/Notes/arrays.html 8 Sep 2004