Question

There is something wrong with my code.
My goal is to translate the pseudocode into java code
Yes my coding is an assignment, I don’t want any answer just to show me where the problem is

What I have to do is to compute the size of the intersection between two unsorted list of students containing no duplicates.

I will show the pseudocode and my java code corresponding to this pseudocode.

Pseudo code:

inter <-- 0

Array C[m+n]

for i <-- 0 to m-1  do  C[i] <-- A[i]

for i <-- 0 to n-1  do  C[i+m] <-- B[i]

C <-- sort(C, m+n);

pointer <-- 0

while (pointer < m+n-1) do{

if(C[pointer]=C[pointer+1]){

inter <-- inter+1

pointer <-- pointer+2

}

else pointer <-- pointer+1

}

return inter

Java Code:

public static int intersectionSizeMergeAndSort(studentList L1, studentList L2) {
/* Write your code for question 4 here */
  int intersectionSize = 0;
  int[] C = new int[L1.studentID.length+L2.studentID.length];
  for(int i = 0; i<L1.studentID.length; i++){
  C[i] = L1.studentID[i];
  }
  for(int i = 0; i<L2.studentID.length; i++){
  C[i+L1.studentID.length] = L2.studentID[i];
  }
  Arrays.sort(C);
  int pointer = 0;
  while(pointer<((C.length-1))){
    if(C[pointer] == C[pointer+1]){
    intersectionSize = intersectionSize + 1;
    pointer = pointer + 2;
    }
    else {
      pointer = pointer + 1;
  }
 return intersectionSize;

}
return 0;
}

My main method:

public static void main(String args[]) throws Exception {

studentList firstList;
studentList secondList;

// This is how to read lists from files. Useful for debugging.

// firstList=new studentList("COMP250.txt", "COMP250 - Introduction to Computer Science");
// secondList=new studentList("MATH240.txt", "MATH240 - Discrete Mathematics");

// get the time before starting the intersections
long startTime = System.currentTimeMillis();

// repeat the process a certain number of times, to make more accurate average     measurements.
 for (int rep=0;rep<1000;rep++) {

 // This is how to generate lists of random IDs. 
 // For firstList, we generate 16000 IDs
 // For secondList, we generate 16000 IDs

 firstList=new studentList(2 , "COMP250 - Introduction to Computer Science");
 secondList=new studentList(2 , "MATH240 - Discrete Mathematics");


 // run the intersection method
 int intersection=studentList.intersectionSizeMergeAndSort(firstList,secondList);
 System.out.println("The intersection size is: "+intersection);
 }

// get the time after the intersection
long endTime = System.currentTimeMillis();


System.out.println("Running time: "+ (endTime-startTime) + " milliseconds");
 }

}

Note: L1 and L2 have been declared previously
But I am not getting the results I am aiming for.
Can someone point out what is wrong ?

Thank you

Answer 1

Off the top of my head, it appears to me that your return intersectionSize; statement occurs within your while loop, so that you loop never gets beyond the first iteration and does not properly compute the intersectionSize. I would delete that statement and replace your return 0; with return intersectionSize; like so…

public static int intersectionSizeMergeAndSort(studentList L1, studentList L2) {
   /* Write your code for question 4 here */
   int intersectionSize = 0;
   int[] C = new int[L1.studentID.length + L2.studentID.length];
   for (int i = 0; i < L1.studentID.length; i++) {
      C[i] = L1.studentID[i];
   }
   for (int i = 0; i < L2.studentID.length; i++) {
      C[i + L1.studentID.length] = L2.studentID[i];
   }
   Arrays.sort(C);
   int pointer = 0;
   while (pointer < (C.length - 1)) {
       if (C[pointer] == C[pointer + 1]) {
          intersectionSize = intersectionSize + 1;
          pointer = pointer + 2;
       } else {
          pointer = pointer + 1;
       }
    }
    return intersectionSize;
}