Thread: Errors with LispList

Write static method which performs the following operation:
length takes a list and returns the number of integers in it. For example, with [7,3,8,12,9,14]
it would return 6.

import java.util.Scanner;
 
class Lab4Q2
{
 
 public static void main(String[] args)
 {
  Scanner in = new Scanner(System.in);
  System.out.print("Enter a list (of integers): ");
  String str = in.nextLine();
  LispList<Integer> b = parseIntLispList(str);
  System.out.print("The list you entered is: ");
  System.out.println(b);
  int l = length(b);
  System.out.println("The number of integers is: "+l);
 
 }
 
 public static int length(LispList<Integer> a){
   int l = a.size();
   return l;
 }
 
}

ahhh missing method

public static LispList<Integer> parseIntLispList(String str)
 {
  String line = str.trim();
  String contents = line.substring(1,line.length()-1).trim();
  if(contents.length()==0)
     return LispList.empty();
  String[] nums = contents.split(",");
  LispList<Integer> list = LispList.empty();
  for(int i=nums.length-1; i>=0; i--)
      {
       String num = nums[i].trim();
       list = list.cons(Integer.parseInt(num));
      }
  return list;
 }

... LispList isn't a standard Java API class. The size method probably doesn't work because who-ever wrote it didn't implement the size method.

Out of curiosity, what is a LispList? As far as I can tell, it doesn't differ from the two implementation methods of lists: linked lists and array lists (based off of how lists operate in the Lisp language).

Not true, that's a very inefficient way to keep track of the size (it's O(n)). Instead, I'd keep track of every time you add/remove something, and then update a variable containing the size of the list accordingly. The memory usage over-all is actually less (don't have to keep track of all the counting variables), but more importantly your running time becomes O(1), a huge difference on large lists that have their length used many times.