数组和扩展的困境

Question

So I know that the java convention is to use ArrayList<> when it comes to expansions and many other applications. The typical array cannot expand. My java course is elementary so we are still reviewing over arrays right now. As much as I want to use an arraylist I cant. How do I make it to where I store only elements that satisfy the condition in my counter array?

public int[] above100Degrees()
   {
      int[] blazing = new int[temps.length];
      for( int i = 0; i < temps.length; i++ )
      {
         if( temps[i] > 100 )
         {
            blazing[i] = temps[i];
         }
      }
      return blazing;
   }

Output

The temperature above 100 degrees is:   0   0   0   0   0   0   0   103 108 109

Answer 1

Just count how many elements match your filter first , then create the array, then populate it. It means you'll need to go through the array twice, but there are no really nice alternatives unless you want to end up creating multiple arrays. So something like:

public int[] above100Degrees() {
    // First work out how many items match your filter
    int count = 0;
    // Are you allowed to use the enhanced for loop? It's not necessary, but it
    // makes things simpler.
    for (int temp : temps) {
        if (temp > 100) {
            count++;
        }
    }

    // Create an array of the right size...
    int[] ret = new int[count];

    // ... and populate it.
    int index = 0;
    for (int temp : temps) {
        if (temp > 100) {
            ret[index++] = temp;
        }
    }
    return ret;
}

Answer 2

I would use a loop to find how many are above 100 before assigning the array.

public int[] above100Degrees()
{
    int newArrayLength=0;
    for( int i = 0; i < temps.length; i++ )
    {
        if( temps[i] > 100 )
        {
            newArrayLength++;
        }
    }

    int[] blazing = new int[newArrayLength];
    int positionInNewArray = 0;
    for( int i = 0; i < temps.length; i++ )
    {
        if( temps[i] > 100 )
        {
            blazing[positionInNewArray] = temps[i];
            positionInNewArray++;
        }
    }
    return blazing;
}

Answer 3

You could do a ternary operation

resultString = "The temperature above 100 degrees is: ";
for(int i = 0; i < blazing.length; i++){
    resultString += blazing[i] != 0 ? blazing[i] : "";
}

Note: This would require more memory than JonSkeets answer, but could potentially be more efficient. If your expect your array length to get very large, go with JonSkeet's answer. In other words, this won't scale well.

Answer 4

One way is to count things before setting up the array. Another way is to set up the array first and keep track of the count, then create a new array:

public int[] above100Degrees()
   {
      int[] blazing = new int[temps.length];
      int count = 0;
      for( int i = 0; i < temps.length; i++ )
      {
         if( temps[i] > 100 )
         {
            blazing[count++] = temps[i];
         }
      }

      // At this point, `count` is the number of elements you're going to return;
      // and the first `count` elements of `blazing` hold those elements, while the
      // remaining elements of `blazing` are garbage

      int[] result = new int[count];
      for ( int i = 0; i < count; i++ )
           result[i] = blazing[i];

      return result;
   }

This approach would be better if the condition you're testing for takes a lot of time to calculate (as opposed to temps[i] > 100 , which hardly takes any time). You could use Arrays.copy to create the result array, but if you can't use ArrayList you probably can't use Arrays.copy either.

Answer 5

Your code is keeping the cells in blazing array for i <= 100 ; You need to ignore these and start populating from i = 101 .