Enhanced For Loops

Objective #1: Use "enhanced" for loops with arrays.

• A special kind of for loop exists which can be used to efficiently iterate through all the elements of an array. This kind of for loop is called an enhanced for loop but it is also referred to as a for each loop.
  
  
• An enhanced for loop can be used to traverse through an array just like a traditional for loop. To traverse an array means to visit each element of the array. Example:
  
  int[] scores = {99, 75, 82};
  
  for (int value : scores)
  {
     System.out.print(value + " ");
  }
  
  When writing an enhanced for loop, you separate the two portions of its first line with a colon. On the left of the colon you must declare a variable with any name of your choice. In this case, the variable was named value is being used but it could just as easily have been named something like element or x. It is not good style and would be very misleading to use the variable named i since it would confuse people reading the code making them think it is a traditional for loop instead of an enhanced for each loop. When i is used as a variable name in traditional for loops, it represents the word index which refers to an index position within an array. In this case, value is not marking the position of an element within the array.
  
  To the right of the colon you must use the variable name of the data structure that is being traversed such as the one-dimensional array scores. Typically, in this AP course, we will iterate through arrays (and ArrayLists that we will study later.)
  
  

  enhanced for loop	traditional for loop
  int[] scores = {99, 75, 82}; int sum = 0; // running total for (int value : scores) {    System.out.print(value + " ");    sum += value; } System.out.println(sum);	int[] scores = {99, 75, 82}; int sum = 0; // running total for (int i = 0; i < scores.length; i++) {    System.out.print(scores[i] + " ");    sum += scores[i]; } System.out.println(sum);

  
  
  Both code segments display the elements of the array scores and compute and display the sum of those elements.
  
  Notice that the length property isn't used in the enhanced for loop code segment since the computer automatically knows the length of the array. The computer guarantee that every element in the array (int values in this example) will eventually be plugged into the variable named value.
  
  Also, a loop control variable like i that is often used in a traditional for loop is unnecessary and undesirable in an enhanced for loop. Behind-the-scenes, something called an iterator (not covered on the AP exam) is used to go from element to element of the array. We will not be studying iterators in this Java course however they are very useful in advanced algorithms. I do not believe an iterator guarantees that you will be traversing elements in the order that they are actually stored in the array (by subscript/index position) from "left to right" but you should assume that it is traversed that way when completing AP exam multiple choice questions.
  
  Here is an example that uses an enhanced for loop to find the maximum value in an array
  
  int[] scores = {99, 75, 82};
  int max = scores[0];
  
  for (int value : scores)
  {
  
     if (value > max)
     {
        max = value;
     }
  
  }
  
  System.out.print("The largest value in the array is " + max);
  
  
• It is a common beginner's mistake to incorrectly code the loop above with something like this
  
  for (int value : scores)
  {
  
     if (scores[value] > max)
     {
        max = scores[value];
     }
  
  }
  
  This would be incorrect because value is the element itself like 99, 75, or 82 and not an index position like 0, 1, or 2. It would make no sense to refer to scores[99] since the length of scores is only 3. In general, you will not use square brackets inside the body of an enhanced for loop.
  
  
• In general, almost anything that can be done with a traditional for loop could be rewritten with an enhanced for loop such as finding a specific key value, counting the number of occurrences of a specific value, and so on. However, there is one shortcoming to an enhanced for loop. You cannot modify a data structure that is being traversed with an enhanced for loop. For example, the task of adding one to each integer in an array with an enhanced for loop would cause an error.
  
  int[] scores = {99, 75, 82};
  
  for (int value : scores)
  {
      value++;
  }
  
  The loop above would not modify scores so that it contains {100, 76, 83}. The array would still contain the values {99, 75, 82}.
  
  To be safe, you should only perform tasks with an enhanced for loop that need to access and "read" elements of an array rather than modifying the elements in any way.
  

Objective #2: Use nested "enhanced" for loops with two-dimensional arrays.

• A one-dimensional array can be used as the loop variable in an enhanced for loop. For example, here's a method that finds the sum of all the elements of a two-dimensional array of integers.
  
  int[][] table = {{5, 7, 5},
                   {9, 4, 3},
                   {8, 1, 4}};
  int sum = 0;
  
  for (int[] currentRow : table)
  {
  
     for (int value: currentRow)
     {
        sum += value;
     }
     
  }
  
  System.out.print("The sum of the values in the array is " + sum);
  
  Note that the control variable of the outer loop, currentRow, is a a one-dimensional array that represents a row of the two-dimensional array. The empty square brackets must be used with the data type int to correctly declare this currentRow variable as a one-dimensional array.