Problems with Sentinels

Here is part A of the program with the blanks filled.

    // Group "A"
    int sumA = 0;      // the sum of scores for group "A"
    int countA = 0;    // initialize the group A count
    
    while ( (value = scan.nextInt()) != -1 )
    {
      sumA   = sumA + value ; // add to the sum
      countA = countA + 1;    // increment the count
    }

    if ( countA > 0 )
      System.out.println( "Group A average: " + ((double) sumA)/countA ); 
    else
      System.out.println( "Group A  has no students" );

Notice how input of the next integer, and the testing of that integer, is done in the condition of the while statement:

    while ( (value = scan.nextInt()) != -1 )

The part inside the inner-most parentheses is done first. This is

    (value = scan.nextInt())

This reads in an integer and assigns it to value. The entire expression has the value of that integer. If a -1 was read, then the expression has the value -1. Next, the value of the expression is compared to -1:

    while ( expression != -1 )

If a -1 was just read, the relational expression will evaluate to false and the while loop will stop.