7.8. Code Practice with Arrays

Fix the following code so that it prints every other value in the array arr1 starting with the value at index 0.

public class Test1
{
    public static void main(String[] args)
    {
        int arr1 = {1, 3, 7, 9, 15, 17};
        for (int index = 0; index <= arr1.length; index += 2)
        {
            System.out.print(index + ", ");
        }
    }
}

====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "1, 7, 15, ";
        boolean passed = getResults(expect, output, "Expected output from main");
        assertTrue(passed);
    }

@Test
    public void testCodeContains1()
    {
        boolean passed = checkCodeContains("correct array data type", "int[] arr1");
        assertTrue(passed);
    }

@Test
    public void testCodeContains2()
    {
        boolean passed =
                checkCodeContains(
                        "correct for loop condition",
                        "for (int index = 0; index < arr1.length; index+=2)");
        assertTrue(passed);
    }

@Test
    public void testCodeContains3()
    {
        boolean passed =
                checkCodeContains(
                        "access values in array instead of index",
                        "System.out.print(arr1[index] + \", \")");
        assertTrue(passed);
    }
}

Change line 5 to add the [] on the declaration of arr1 to show that it is an array of integer values. Change line 6 to index < arr1.length so that you don’t go out of bounds (the last valid index is the length minus one). Change line 8 to print arr1[index].

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Fix the following to print the values in the array a1 starting with the value at the last index and then backwards to the value at the first index.

public class Test1
{
    public static void main(String[] args)
    {
        int[] a1 = {1, 3, 7, 9, 15};
        for (int i = a1.length; i > 0; i--)
        {
            System.out.print(arr[i] + ", ");
        }
    }
}

====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "15, 9, 7, 3, 1, ";
        boolean passed = getResults(expect, output, "Expected output from main");
        assertTrue(passed);
    }

@Test
    public void testCodeContains1()
    {
        boolean passed = checkCodeContains("correct starting index", "int i = a1.length-1;");
        assertTrue(passed);
    }

@Test
    public void testCodeContains2()
    {
        boolean passed = checkCodeContains("correct ending index", "i >= 0;");
        assertTrue(passed);
    }

@Test
    public void testCodeContains3()
    {
        boolean passed =
                checkCodeContains(
                        "correct array variable name", "System.out.print(a1[i] + \", \");");
        assertTrue(passed);
    }
}

Change line 6 to a1.length - 1 since the last valid index is one less than the length of the array and i >= 0 since the first valid index is 0. Change line 7 to a1.

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Rewrite the following code so that it prints all the values in an array arr1 using a for-each loop instead of a for loop.

In a for-each loop you specify the type of the values in the array, a name for the current value, and then a : and then the name of the array. The first time through the loop the value will be the one at index 0. The next time the one at index 1 and so on until you reach the last value in the array.

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Finish the following code so that it prints out all of the odd values in the array a1. Hint: use % to check for odd values.

If the remainder of the value divided by 2 is 1 then it is odd so print it out followed by a space (to keep the values separated).

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Finish the following method getSum to return the sum of all values in the passed array.

public class Test1
{

public static int getSum(int[] arr) {}

public static void main(String[] args)
    {
        int[] a1 = {1, 2, 5, 3};
        System.out.println(
                "It should print 11 " + " and your answer is: " + getSum(a1));
    }
}

====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "It should print 11  and your answer is: 11";
        boolean passed = getResults(expect, output, "Expected output from main");
        assertTrue(passed);
    }

@Test
    public void testMethod()
    {
        int[] nums = {10, 20, 30, 40, 50};
        Object[] args = {nums};

// name of method, arguments are (nums, 30)
        String output = getMethodOutput("getSum", args);
        String expect = "150";

boolean passed = getResults(expect, output, "getSum({10, 20, 30, 40, 50})");
        assertTrue(passed);
    }
}

Declare a variable to hold the sum and initialize it to zero. Loop through all the values in the array using a for-each loop and add each value to the sum. Return the sum.

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Finish the following method to return the sum of all of the non-negative values in the passed array.

public class Test1
{

public static int getSumNonNeg(int[] arr) {}

public static void main(String[] args)
    {
        int[] a1 = {1, 2, 5, 3, -1, -20};
        System.out.println(
                "The code should print 11 "
                        + "and your answer is: "
                        + getSumNonNeg(a1));
    }
}

====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "The code should print 11 and your answer is: 11";
        boolean passed = getResults(expect, output, "Expected output from main");
        assertTrue(passed);
    }

@Test
    public void testMethod()
    {
        int[] nums = {10, -20, 30, -40, 50};
        Object[] args = {nums};

// name of method, arguments are (nums, 30)
        String output = getMethodOutput("getSumNonNeg", args);
        String expect = "90";

boolean passed = getResults(expect, output, "getSum({10, -20, 30, -40, 50})");
        assertTrue(passed);
    }
}

Declare a variable to hold the sum and initialize it to zero. Loop through all the values in the array. If the current value is non negative (greater than or equal to 0) then add it to the sum. Return the sum.

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Finish the following code to print the strings at the odd indices in the array.

Use a for loop and start the index at 1 and increment it by 2 each time through the loop. Print the value at the index.

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Finish the method getSumChars below to return the total number of characters in the array of strings strArr.

public class Test1
{

public static int getSumChars(String[] strArr) {}

public static void main(String[] args)
    {
        String[] strArr = {"hi", "bye", "hola"};
        System.out.println(getSumChars(strArr));
    }
}

====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "9";
        boolean passed = getResults(expect, output, "Expected output from main");
        assertTrue(passed);
    }

@Test
    public void testCodeContains1()
    {
        boolean passed = checkCodeContains("adding length of each string", ".length()");
        assertTrue(passed);
    }

@Test
    public void testCodecontains()
    {
        boolean passed = checkCodeContains("for loop", "for");
        assertTrue(passed);
    }

@Test
    public void testMethod()
    {
        String[] strs = {"a", "aa", "aaa"};
        Object[] args = {strs};

// name of method, arguments are (nums, 30)
        String output = getMethodOutput("getSumChars", args);
        String expect = "6";

boolean passed = getResults(expect, output, "getSumChars({\"a\",\"aa\",\"aaa\"})");
        assertTrue(passed);
    }
}

Declare the sum and initialize it to 0. Use a for-each loop to loop through each string in the array. Add the length of the current string to the sum. Return the sum.

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Finish the method findMin so that it finds and returns the minimum value in the array.

public class Test1
{

public static int findMin(int[] arr)
    {

}

public static void main(String[] args)
    {
        int[] arr = {20, -3, 18, 55, 4};
        System.out.println(findMin(arr));
    }
}

====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "-3";
        boolean passed = getResults(expect, output, "Expected output from main");
        assertTrue(passed);
    }

@Test
    public void testCodeContains2()
    {
        boolean passed = checkCodeContains("for loop", "for");
        assertTrue(passed);
    }

@Test
    public void testMethod()
    {
        int[] nums = {10, 20, 5, 40, 50};
        Object[] args = {nums};

// name of method, arguments are (nums, 30)
        String output = getMethodOutput("findMin", args);
        String expect = "5";

boolean passed = getResults(expect, output, "findMin({10, 20, 5, 40, 50})");
        assertTrue(passed);
    }
}

Declare a variable to hold the minimum value found and initialize it to the first value in the array. Loop from 1 to the length of the array minus one and get the value at that index. If the value is less than the minimum found so far reset the minimum found so far to the value. Return the minimum.

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Finish the method getAverage to calculate and return the average of all of the values in the array.

public class Test1
{

public static double getAverage(int[] arr) {}

public static void main(String[] args)
    {
        int[] arr = {20, 3, 18, 55, 4};
        System.out.println(getAverage(arr));
        ;
    }
}

====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "20.0";
        boolean passed = getResults(expect, output, "Expected output from main");
        assertTrue(passed);
    }

@Test
    public void testCodeContains2()
    {
        boolean passed = checkCodeContains("for loop", "for");
        assertTrue(passed);
    }

@Test
    public void testMethod()
    {
        int[] nums = {10, 20, 30, 40, 50};
        Object[] args = {nums};

// name of method, arguments are (nums, 30)
        String output = getMethodOutput("getAverage", args);
        String expect = "30.0";

boolean passed = getResults(expect, output, "getAverage({10, 20, 30, 40, 50})");
        assertTrue(passed);
    }
}

Declare a variable to hold the total and it should be of type double so that the average is a double. Initialize it to 0. Loop through all the values in the array and add each to the total. Return the total divided by the length of the array.

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Before you keep reading...

Before you keep reading...

7.8. Code Practice with Arrays¶

7.8.1. More Practice¶