I am having an issue with a fill-in-the-code digital textbook problem. All the code is permanent and cannot be changed, so the problem can only be solved by using the area that states //Write code here.

The problem asks to implement the removeOdd method.

import java.util.Arrays;

public class RemoveTester
{
   public static int removeOdd(int[] values, int size) 
   { 
      //Write code here
   }
   public static void main(String[] args)
   {
      int[] a = { 22, 98, 95, 46, 31, 53, 82, 24, 11, 19 };
      int sizeBefore = 8;
      int sizeAfter = removeOdd(a, sizeBefore);
      System.out.print("a: [ ");
      for (int i = 0; i < sizeAfter; i  ) 
      {
         System.out.print(a[i]   " ");
      }
      System.out.println("]");
      System.out.println("Expected: [ 22 98 46 82 24 ]");      

      int[] b = { 23, 97, 95, 45, 31, 53, 81, 24, 11, 19 };
      sizeBefore = 7;
      sizeAfter = removeOdd(b, sizeBefore);
      System.out.print("b: [ ");
      for (int i = 0; i < sizeAfter; i  ) 
      {
         System.out.print(b[i]   " ");
      }
      System.out.println("]");
      System.out.println("Expected: [ ]");      
   }
}

The way I tried to implement removeOdd is by doing:

int evenCount = 0;
      
      for(int i = 0; i<size; i  ){
         if(values[i]%2==0){
            evenCount  ;
         }
      }
      int[] newValues = new int[evenCount];
      int newCount =0;
      for(int i = 0; i<evenCount; i  ){
         if(values[i]%2==0){
            newValues[newCount] = values[i];
            newCount  ;
         }
      }
      values = newValues;
      return evenCount;

When the program is compiled and ran, main prints the beginning of the original a or b arrays instead of only the even elements in a or b. I cannot find a way to alter the original arrays within the method removeOdd into the new arrays with only their even elements. I can't think of any other way to do this either. Any help would be greatly appreciated!

CodePudding user response：

The other answers give a good description of what the problem is with your overall approach...that is, why your results don't make it back to the calling method.

If your code were otherwise correct, you could use it as is and just copy the result back into the original array at the end. As it is, you had one flaw in your logic. So if you fix that flaw, and then do the copy at the end, you should get the correct result:

public static int removeOdd(int[] values, int size)
{
    int evenCount = 0;

    for(int i = 0; i<size; i  ){
        if(values[i]%2==0){
            evenCount  ;
        }
    }
    int[] newValues = new int[evenCount];
    int newCount =0;
    for(int i = 0; i<size; i  ) {    // <- Need to iterate over the entire input array
        if(values[i]%2==0){
            newValues[newCount] = values[i];
            newCount  ;
        }
    }
    for (int i = 0 ; i < evenCount ; i  )  // <- now copy your result to the original array
        values[i] = newValues[i];
    return evenCount;
}

Rather than creating an extra array to temporarily hold the even values, you can use the same logic to copy into the original array directly:

public static int removeOdd(int[] values, int size)
{
    int newCount =0;
    for(int i = 0; i<size; i  ) {
        if(values[i]%2==0){
            values[newCount] = values[i];
            newCount  ;
        }
    }
    return newCount;
}

CodePudding user response：

Because Java is pass by value and not pass by reference, setting the value of the values argument will not change the value of the a variable.

What you have to do is remove all the odd elements from the array and shift the remaining even elements to the left so that the actual resultant form of a looks like this:

{22, 98, 46, 82, 24, 0, 0, 0, 0, 0}

CodePudding user response：

As @Geoff pointed out Java is pass by value, which means when you pass your array as an argument, what you get inside the method is a reference to the array object which is different from the original reference int[] a you have inside your main method (the caller). Therefore, when you do values = newValues; you are pointing that new reference which used to point to the same object as int[] a to the array object newValues points to, thereby not updating the original array a but actually losing any reference to it.