7.2. Traversing Arrays with For Loops

7.2.1. Index Variables

In the last lesson, we mentioned that you can use a variable for the index of an array. You can even do math with that index and have an arithmetic expression inside the [], like below.

// highScores array declaration
int[] highScores = { 10, 9, 8, 8};
// use a variable for the index
int index = 3;
// modify array value at index
highScores[index] = 11;
// print array value at index
System.out.println(  highScores[index] );
System.out.println(  highScores[index - 1] );

What does the code above print out? You can follow the code in this visualizer and look at the image depicting the array below.

../_images/arraywithindex.png

Figure 1: Array with index variable

coding exercise Coding Exercise

What do you think the following code will print out? First trace through it on paper keeping track of the array and the index variable. Then, run it to see if you were right. You can also follow it in the visualizer by clicking on the Show Code Lens button.

7.2.2. For Loop to Traverse Arrays

We can use iteration with a for loop to visit each element of an array. This is called traversing the array. Just start the index at 0 and loop while the index is less than the length of the array. Note that the variable i (short for index) is often used in loops as the loop counter variable and is used here to access each element of an array with its index.

../_images/arrayForLoop.png

Figure 2: For Loop Traversing Array

For example, here is a loop traversing the highScores array to print every score. Follow the code below in the visualizer.

int[] highScores = { 10, 9, 8, 11};
for (int i = 0; i < highScores.length; i++)
{
    System.out.println(  highScores[i] );
}

Note

Using a variable as the index is a powerful data abstraction feature because it allows us to use loops with arrays where the loop counter variable is the index of the array! This allows our code to generalize to work for the whole array.

coding exercise Coding Exercise

What do you think the following code will print out? First trace through it on paper keeping track of the array and the index variable. Then, run it to see if you were right. Try the Code Lens button. Then, try adding your name and a friend’s name to the array names and run the code again. Did the code work without changing the loop?

The following code demonstrates a loop that changes the values in an array. In this code, the array is passed as an argument to the static methods in the class. Arrays in Java are objects. The array variables are references to an address in memory. Since arrays can be very large, we do not want to copy them when we pass them into methods. When an array is passed as an argument to a method, the name of the array refers to its address in memory. Therefore, any changes to the array in the method will affect the original array. You can also try the code in the Java visualizer.

coding exercise Coding Exercise

What does the following code print out? Trace through it keeping track of the array values and the output. Then run it to see if you’re right. Notice that in this code, the array is passed as an argument to the methods. You can also try the code in the Java visualizer with the Code Lens button.

Note

Arrays in Java are objects. When arrays are passed in as arguments to methods, any changes to the array in the method will affect the original array, since the array name is a reference value refering to the address of the array in memory.

exercise Check Your Understanding

The following method has the correct code to subtract amt from all the values in the array <b>values</b>, but the code is mixed up. Drag the blocks from the left into the correct order on the right. You will be told if any of the blocks are in the wrong order.

        public static void subAll(int[] values, int amt)
{
---
     for (int i = 0;
---
        i < values.length;
---
        i++)
---
   {
---
      values[i] = values[i] - amt;
---
   } // end for loop
---
} // end method
        

7.2.3. Looping From Back to Front

You don’t have to loop through an array from the front to the back. You can loop by starting at the back of the array and move toward the front during each time through the loop. In the example below, the method getIndexOfLastElementSmallerThanTarget returns the index of the last element in the array that is smaller than the given argument. The return statement inside the loop stops the execution of the loop and the method and returns the index that is found immediately back to the main method. It returns -1 if there is no number in the array that is smaller than the given number.

coding exercise Coding Exercise

What does the following code print out? Notice that the array and the target are passed in as arguments to the getIndexOfLastElementSmallerThanTarget method. Trace through it keeping track of the array values and the output. Then run it to see if you’re right. You can also try the code in the Java visualizer with the Code Lens button. Can you add another method that finds the index of the last element greater than the target instead of smaller than the target and have main print out a test of it? Call this method getIndexOfLastElementGreaterThanTarget and give it 2 arguments and a return value like the method below.

exercise Check Your Understanding

7.2.4. Looping through Part of an Array

You don’t have to loop through all of the elements of an array. You can loop through just some of the elements of an array using a for loop. The following code doubles the first five elements in an array. Notice that it uses a complex conditional (&&) on line 14 to make sure that the loop doesn’t go beyond the length of the array, because if you had an array that had less than 5 elements, you wouldn’t want the code to try to double the 5th element which doesn’t exist! Notice that in this code, the array is a private instance variable of the class ArrayWorker. It is created in the constructor and changed or accessed by the methods.

What will the following code print out? Can you write a similar method called tripleFirstFour() that triples the first 4 elements of the array? Make sure you test it in main.

coding exercise Coding Exercise

You can even start in the middle and loop through the rest of the array.

Does this work for arrays that have an even number of elements? Does it work for arrays that have an odd number of elements? Modify the main code below to test with both arrays with an even number of items and an odd number.

7.2.5. Parameter passing and arrays - pass by value

Wait a minute! In Unit 5 we discussed Java parameter passing and pass by value, in which the actual argument value is copied into the formal parameter variable. We saw how assigning a new value to a formal parameter variable inside the method does not alter the value stored in the actual argument variable used in the method call.

How come the doubleFirstFive, doubleLastHalf, and doubleLast methods in the last 3 programs were able to modify the contents of an array that was passed into a method through a formal parameter? It is because arrays are objects and the value that gets passed into the method is a reference to the array, meaning a copy of the array’s memory location. Notice the methods do not reassign the formal parameter variable that references the array, but instead use array indexing through the [] operator to assign values into one or more array cells. When you use array indexing to assign a value within a method, you will be assigning to the same array object that was used for the method call.

exercise Check Your Understanding

The following program has the correct code to reverse the elements in an array, a, but the code is mixed up. Drag the blocks from the left into the correct order on the right. You will be told if any of the blocks are in the wrong order.</p>

        public static void reverse(int[] a)
{
---
  int temp = 0;
  int half = a.length / 2;
  int max = a.length - 1;
---
  for (int i = 0; i < half; i++)
  {
---
     temp = a[i];
---
     a[i] = a[max - i];
---
     a[max - i] = temp;
---
  } // end for
---
} // end method
        

The following program has the correct code to return the average of the first 3 items in the array a, but the code is mixed up. Drag the blocks from the left into the correct order on the right. You will be told if any of the blocks are in the wrong order or are indented incorrectly.</p>

        public static double avg3(int[] a)
{
---
  double total = 0;
---
  for (int i = 0; i < a.length && i < 3; i++)
  {
---
    total = total + a[i];
---
  } // end for
  return total / 3;
---
} // end method
        

7.2.6. Common Errors When Looping Through an Array

When processing all array elements, be careful to start at the first index which is 0 and end at the last index. Usually loops are written so that the index starts at 0 and continues while the index is less than arrayName.length since (arrayName.length - 1) is the index for the last element in the array. Make sure you do not use <= instead of <! If the index is less than 0 or greater than (arrayName.length - 1), an ArrayIndexOutOfBoundsException will be thrown. Off by one errors, where you go off the array by 1 element, are easy to make when traversing an array which result in an ArrayIndexOutOfBoundsException being thrown.

exercise Check Your Understanding

coding exercise Coding Exercise

The following code has an ArrayIndexOutOfBoundsException. It has 2 common off-by-one errors in the loop. Can you fix it and make the loop print out all the scores?

Be careful not to jump out of loop too early when you are looking for a value in an array. The method below uses return statements to stop the execution of the method and return a value to the method that called this method. However, you must be careful not to stop the loop too soon.

coding exercise Coding Exercise

What is wrong with the code below? The first time through the loop it will start with the element at index 0 and check if the item at the array index equals the passed target string. If they have the same characters in the same order it will return 0, otherwise it will return -1. But, it has only processed one element of the array. How would you fix the code to work correctly (process all array elements before returning)?

7.2.7. groupwork Programming Challenge : SpellChecker

Make sure you have done the last coding exercise above which will help you with this challenge.

The following Active Code uses a dictionary array of the most common 100 English words. We can use it as a spelling checker! For a more realistic application, you may want to use this repl.it code instead that has a huge dictionary of English words read in from a file and lets you do input with your spell checker. If you use repl, copy in the link for your repl in the Active Code window below to turn it in.

  1. Write a print10() method that takes an array as a parameter and prints out the first 10 words of the array.

  2. Write a spellcheck() method that takes a word and an array as parameters and returns true if the word is in the array. It should return false if it is not found. Test your code below by changing the word sent to the spellcheck() method in main. This algorithm is called a linear search where we step through the array one element at a time (here the dictionary one word at a time) looking for a certain element.

  3. Optional Challenge: Write a method printStartsWith(String firstLetters, String[] dictionary) that prints out the words in the dictionary array that start with the characters in the word firstLetters. You could use the Java String startsWith() method here if you’d like to, or use indexOf() to see if the firstLetters is at index 0 of the string. This is not autograded.

7.2.8. Summary

  • Iteration (loops) can be used to access all the elements in an array, traversing the array.

  • Traversing an array with an indexed for loop or while loop requires elements to be accessed using their indices.

  • Since the index for an array starts at 0 and end at the number of elements − 1, “off by one” errors are easy to make when traversing an array, resulting in an ArrayIndexOutOfBoundsException being thrown.

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