CS 455 Midterm 1 Solution

Problem 2 Answer shown in underlined bold.

public class LastNonDigit {
   public static void main(String[] args) {
      Scanner in = new Scanner(System.in);
      String word = in.next();
      boolean found = false;
      int position = word.length()-1;

      while (!found && position >= 0) {
         char ch = word.charAt(position);
         if (!Character.isDigit(ch)) {
            found = true;
         }
         else {
            position--;
         }
      }
      if (found) {
         System.out.println("Last non-Digit: " + word.charAt(position));
         System.out.println("Position: " + position);
      }
      else {
         System.out.println("All digits");
      }
   }
}

Part B [4].
Note: spacing added here for readability.

foo:   1 [0, 1, 0]
main1: 0 [0, 1, 0]
foo:   2 [0, 1, 1]
main2: 1 [0, 1, 1]

Problem 4
Solution shown in bold.

1. (SG) data structure(s) used in a SpiralGenerator object
   
2. (both) the types of the parameters for methods of SpiralGenerator
   
3. (SGT) contents of failure messages, such as one reporting that the segment wasn't horizontal or vertical.
   
4. (both) preconditions on SpiralGenerator methods
   
5. (SG) names of SpiralGenerator instance variables
   
6. (both) the class name SpiralGenerator
   
7. (SGT) names of methods of SpiralGeneratorTester
   
8. (SG) the types of local variables in methods of SpiralGenerator
   
9. (SGT) the class name SpiralGeneratorTester

Problem 5
Solution shown in bold.

public class Car {

   private int mpg;
   private double milesDriven;
   private double gasLeft;
  
   public Car(int milesPerGallon) {

      mpg = milesPerGallon;
      milesDriven = 0;
      gasLeft = 0;

   }
   
   public double gasLeft() {

      return gasLeft;

   }
   
   public double milesDriven() {

      return milesDriven;

   }
   
   public void addGas(double gallons) {
 
     gasLeft += gallons;

   }
   
   public double drive(int miles) {

      double gasNeeded = miles / (double) mpg;
      double actualMiles = miles;
      if (gasNeeded > gasLeft) {
         actualMiles = gasLeft * mpg;
         gasLeft = 0.0;
      }
      else { 
         gasLeft -= gasNeeded;
      }
      milesDriven += actualMiles;
      return actualMiles;
   }

}

Alternate solution for drive method:

   public double drive(int miles) {

      double potential = gasLeft * mpg;
      double actualMiles = miles;
      if (actualMiles > potential) {
         actualMiles = potential;
      }
      gasLeft -= (actualMiles / mpg);
      milesDriven += actualMiles;
      return actualMiles;

   }

Solution 1. One-loop solution

   public static int[] rotateLeft(int[] nums, int k) {

      int[] result = new int[nums.length];

      k = k % nums.length;  // equivalent smaller rotation
                            // (optional because of % in loop)

      for (int dest = 0; dest < nums.length; dest++) {
         result[dest] = nums[(dest + k) % nums.length];
      }

      return result;  

   }

   public static int[] rotateLeft2(int[] nums, int k) {

      int[] result = new int[nums.length];

      k = k % nums.length; // convert to a value in [0, nums.length-1]

      // shift last part left
      int dest = 0;
      for (int source = k; source < nums.length; source++) {
         result[dest] = nums[source];
         dest++;
      }

      // shift first part right
      for (int source = 0; source < k; source++) {
         result[dest] = nums[source];
         dest++;
      }

      return result;  

   }