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This document is made available under a Creative Commons Attribution 4.0 International (CC BY 4.0) License. Show Swift and the Swift logo are trademarks of Apple Inc. Privacy Policy Cookies 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 and look at the image depicting the array below. Figure 1: Array with index variable 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 by clicking on the Show Code Lens button. 6.2.2. For Loop to Traverse ArraysWe 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. Figure 2: For Loop Traversing Array For example, here is a loop traversing the int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; i++) { 1 array to print every score. Follow the code below in the . int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; 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. 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 . 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 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. The following method has the correct code to subtract amt from all the values in the array values, 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. } // end method 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 int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; i++) { 2 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. 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 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. 6-2-6: Given the following code segment (which is identical to the method above) what will be returned when you execute: int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; i++) { 3; private int[ ] values = {-20, -15, 2, 8, 16, 33}; public static int getIndexOfLastElementSmallerThanTarget(int[ ] values, int compare) { for (int i = values.length - 1; i >=0; i--) {
6-2-7: Given the following code segment (which is not quite identical to the method above) what will be returned when you execute: int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; i++) { 4; int[ ] values = {-20, -15, 2, 8, 16, 33}; public static int getIndexOfLastElementSmallerThanTarget(int[] values, int compare) { for (int i = values.length; i >=0; i--) {
6.2.4. Looping through Part of an ArrayYou 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 ( int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; i++) {
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. 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. 6-2-10: Given the following values of a and the method doubleLast what will the values of a be after you execute: doubleLast()? private int[ ] a = {-20, -15, 2, 8, 16, 33}; public void doubleLast() { for (int i = a.length / 2; i < a.length; i++) {
6-2-11: Given the following values of a and the method mystery what will the values of a be after you execute: mystery()? private int[ ] a = {-20, -15, 2, 8, 16, 33}; public void mystery() { for (int i = 0; i < a.length/2; i+=2) {
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. int temp = 0; int half = a.length / 2; int max = a.length - 1; for (int i = 0; i < half; i++) { } // end for } // end method double total = 0; for (int i = 0; i < a.length && i < 3; i++) { } // end for return total / 3; } // end method When processing all array elements, be careful to start at the first index which is int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; i++) { 6 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 int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; i++) { 7 since ( int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; i++) {
int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; i++) { 9 instead of } // end method int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; i++) { 8), 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. 6-2-14: Which of the following loop headers will cause an ArrayIndexOutOfBounds error while traversing the array scores?
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. 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)? 6-2-17: Given the following code segment, which of the following will cause an infinite loop? Assume that } // end method } // end method for ( int k = 0; k < a.length; k++ ) { while ( a[ k ] < temp ) {
6.2.6. Programming Challenge : SpellCheckerIn this challenge, you will use an array of English words from a dictionary file to see if a given word is spelled correctly. We encourage you to work in pairs for this challenge. Make sure you have done the last coding exercise above which will help you with this challenge. This challenge includes a dictionary file of 10,000 English words which is read into the array dictionary for you. You could use this replit code instead that has an even bigger dictionary of English words 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. If you are interested in how to read in files using Java, there is an optional input files lesson at the end of Unit 7.
Data file: private int[ ] values = {-20, -15, 2, 8, 16, 33}; public static int getIndexOfLastElementSmallerThanTarget(int[ ] values, int compare) { for (int i = values.length - 1; i >=0; i--) { 3 int[] highScores = { 10, 9, 8, 11}; for (int i = 0; i < highScores.length; i++) { 0 Write print10 and spellcheck methods using for loops. Spellchek should take a word as a parameter and return true if it is in the dictionary array. Return false if it is not found. 6.2.8. Summary
6.2.9. Arrays GameTry the game below to practice loops with arrays. Click on Arrays and then check on Loops and click on the elements of the * array that would be printed out by the given code. If you’re stuck, check on Labels to see the indices. We encourage you to work in pairs and see how high a score you can get. |
