Java Community | Help. Code. Learn.•8mo ago

Cannot understand this behaviour of java when adding a list to a list of lists.

https://leetcode.com/problems/subsets/submissions/1236349921 this is the leetcode ques i was solving as you can see the ans list which is supposed to be a subset of the set is being formed and is printed in the stdout just before it was added to the fin global string which is ultimately returned as the answer. on further debugging its adding the current string n number of times where n is the array index of the power set array so in the last iteration its adding a null set n(8) times. But when i pass a clone of the list in function call, the program works as intended Any explanation?

66 Replies

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webbdays•8mo ago

code?

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lucifer7303OP•8mo ago

Its in the link Its a leetcode submission with the code, the stdout and the return value

Kyo-chan•8mo ago

No, it says "submission does not exist"

lucifer7303OP•8mo ago

https://leetcode.com/problems/subsets/submissions/1236349921 sorry, it mustve expired, updated the link

Kyo-chan•8mo ago

Same result. I'm not a leetcode expert, but maybe it doesn't allow you to share code

lucifer7303OP•8mo ago

wait lemme share a screenshot

Kyo-chan•8mo ago

yeah, no Computers are capable of copying text. Do that instead

john•8mo ago

javaclass Solution {
   List<List<Integer>> fin=new ArrayList<>();
    void helper(List<Integer> ans,int[] nums,int i){
        // System.out.println(ans.toString());
        if(i>=nums.length){
            System.out.print(ans.toString()+" ");
            // System.out.println(fin.toString());
            fin.add(ans);
            return;
        }
        ans.add(nums[i]);
        helper(ans,nums,i+1);
        ans.removeLast();
        helper(ans,nums,i+1);

    }
    public List<List<Integer>> subsets(int[] nums) {
    //   List<List<Integer>> fin=new ArrayList<>();
      helper(new ArrayList<>(),nums,0);
      return fin;  
    }
}

javaclass Solution {
   List<List<Integer>> fin=new ArrayList<>();
    void helper(List<Integer> ans,int[] nums,int i){
        // System.out.println(ans.toString());
        if(i>=nums.length){
            System.out.print(ans.toString()+" ");
            // System.out.println(fin.toString());
            fin.add(ans);
            return;
        }
        ans.add(nums[i]);
        helper(ans,nums,i+1);
        ans.removeLast();
        helper(ans,nums,i+1);

    }
    public List<List<Integer>> subsets(int[] nums) {
    //   List<List<Integer>> fin=new ArrayList<>();
      helper(new ArrayList<>(),nums,0);
      return fin;  
    }
}

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lucifer7303OP•8mo ago

i remember there being a rule for not posting the whole code somewhere in the server, maybe im confused with the rules of another server the subset() function is called by the main leetcode driver code

Kyo-chan•8mo ago

It's not so much a rule as a limitation of discord, and it's not like a screenshot would do anything better

lucifer7303OP•8mo ago

lucifer7303OP•8mo ago

the input,output(return of the subset() function) and the stdout i tried taking the fin ans list as a parameter for the helper function but that doesnt help as you can see the ans list is being formed and printed but when i do fin.add(ans) it just adds it n number of times, with n being its (index+1) in the fin list

Kyo-chan•8mo ago

Obviously add() is called more than once

lucifer7303OP•8mo ago

what? doesnt it add at the end of the list

Kyo-chan•8mo ago

You said that when you call add(), it adds n number of times

lucifer7303OP•8mo ago

im adding a list at the end of a list of lists

Kyo-chan•8mo ago

No, it's just that add() was called n times

lucifer7303OP•8mo ago

but how the prob and confusing part is when i pass the clone of the ans list then this code works perfectly

Kyo-chan•8mo ago

Well add() is part of your finishing logic of the recursion I'm not sure why that's weird. You're supposed to do exactly that

john•8mo ago

javajavaclass Solution {
   List<List<Integer>> fin=new ArrayList<>();
    void helper(List<Integer> ans,int[] nums,int i){
        // System.out.println(ans.toString());
        if(i>=nums.length){
            //System.out.print(ans.toString()+" ");
            // System.out.println(fin.toString());
            fin.add(ans);
            return;
        }
        ans.add(nums[i]);
        helper(new ArrayList(ans),nums,i+1);
        ans.removeLast();
        helper(new ArrayList(ans),nums,i+1);

    }
    public List<List<Integer>> subsets(int[] nums) {
    //   List<List<Integer>> fin=new ArrayList<>();
      helper(new ArrayList<>(),nums,0);
      return fin;  
    }
}

javajavaclass Solution {
   List<List<Integer>> fin=new ArrayList<>();
    void helper(List<Integer> ans,int[] nums,int i){
        // System.out.println(ans.toString());
        if(i>=nums.length){
            //System.out.print(ans.toString()+" ");
            // System.out.println(fin.toString());
            fin.add(ans);
            return;
        }
        ans.add(nums[i]);
        helper(new ArrayList(ans),nums,i+1);
        ans.removeLast();
        helper(new ArrayList(ans),nums,i+1);

    }
    public List<List<Integer>> subsets(int[] nums) {
    //   List<List<Integer>> fin=new ArrayList<>();
      helper(new ArrayList<>(),nums,0);
      return fin;  
    }
}

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lucifer7303OP•8mo ago

this little difference in the function call of helper fixes the code but why cant i pass the original list it worked in c++ when a friend of mine did it

Kyo-chan•8mo ago

.... Because you're constantly modifying it

lucifer7303OP•8mo ago

he didnt pass its clone

Kyo-chan•8mo ago

I imagine he used a list class directly rather than a pointer to a list class

lucifer7303OP•8mo ago

iirc no, i copied the code almost the same to replicate the code

Kyo-chan•8mo ago

So what? They're different languages, and notably C++ does have stuff like pointers

lucifer7303OP•8mo ago

im weak at pointers so maybe i didnt understand the actual happening

Kyo-chan•8mo ago

C++ with typically copy objects constantly rather than point to them, unless you tell it otherwise

lucifer7303OP•8mo ago

but the thing is the ans list is being formed correctly in the stdout just before adding it to fin im talking about the stdout here its not like the ans list(the first parameter) isnt being formed correctly

Kyo-chan•8mo ago

Because stdout copies the current content of the list and prints it right here right now. It doesn't matter that you change the list later

lucifer7303OP•8mo ago

so its a problem with my oops basics? can you explain it in a lil simpler terms why passing clone of the ans list works but not when i pass the object itself

webbdays•8mo ago

go through how recursion is going on.

lucifer7303OP•8mo ago

i dry ran it, the logic is okay its a problem with passing by value or reference i think rather i should say passing a clone or the object itself

Kyo-chan•8mo ago

You're constantly modifying that list, making it so when you're done with it it is empty. So if you add that list directly, in the end you obtain empty lists, and if you add clones of it, in the end you obtain a bunch of lists that each are what the list was when you cloned it

lucifer7303OP•8mo ago

there are two things i dont understand: why is fin list being reset every function call, isnt it a global variable for that object and why ans is added n number of times

Kyo-chan•8mo ago

fin is not reset. You screenshot shows that it is growing

lucifer7303OP•8mo ago

lets put aside the n addtition of ans list, but shouldnt the fin list be [[1,2],[1,3],[1,3],[1],[1],[1]...]

Kyo-chan•8mo ago

I don't understand your question, you have duplicates

lucifer7303OP•8mo ago

the fin list have duplicates, right? and the fin list is a global variable so its value should be constant among various function calls so why is it that the ans list is added to an empty fin list shouldnt it be added to the modified fin list which was modified in a previous function

Kyo-chan•8mo ago

What?

lucifer7303OP•8mo ago

look at the second line

Kyo-chan•8mo ago

How can a variable that you keep modifying be constant among various function calls?

lucifer7303OP•8mo ago

it formed fin=[[1,2]] and fin is a global variable so the next time i print fin in a function which is called AFTER the execution of previous funnction, it should be [[1,2]] but here its empty adn the ans list is added n times

Kyo-chan•8mo ago

So it's empty and it contains something n times You really need to try and reevaluate how you want to explain the situation For most people, something empty contains nothing

lucifer7303OP•8mo ago

yh i dont think im gonna understand this over text gotta consult a professor in my college

Kyo-chan•8mo ago

I still encourage you not to explain to them that it confuses you that your result that is empty contains n thing

lucifer7303OP•8mo ago

maybe i should dry run again no i mean wait lemme dry run and ill reply btw what i meant was why was fin reset everytime ans is being added to it like a function added [1,2] to the fin list but when the next function adds [1] to it it becomes [1],[1],[1] and not [1,2],[1],[1],[1] im adding duplicates including the problem of the addition of ans n times

Kyo-chan•8mo ago

fin is not reset. It's just you're modifying the object you put inside it

lucifer7303OP•8mo ago

i know it shouldnt have duplicates

webbdays•8mo ago

run a debugger locally if you know how.

lucifer7303OP•8mo ago

oh okay i effing got it my dumb a######## the addition and everything is absolutely fine but since in the end the same list is ended up as being empty and since its the same object added everytime in the fin list all of them are modified to empty right?

Kyo-chan•8mo ago

yup We've all been through that one. It takes time and thinking to get to the realization

lucifer7303OP•8mo ago

thanks a lot kyo-chan👉 👈

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lucifer7303OP•8mo ago

but how do i tackle this prob like isnt making a clone everytime O(n) isnt there a better way and how does c++ do it?? like arent vector objects too?

Kyo-chan•8mo ago

C++ makes clones without you telling it to

lucifer7303OP•8mo ago

oh so when i do not pass the object without & it passes a clone,without any mention well thats just plain stupid if thats the case or maybe not, my basics are wonky in pointers. not c++'s fault

Kyo-chan•8mo ago

When you get used to languages that don't do that you tend to start thinking that C++ is idiotic for doing that. But at least it's consistent between objects and native types

lucifer7303OP•8mo ago

so the only way of doing this ques by this approach is to pass the clone? but that adds to the complexity of the question like doing the cloning n times is O(n^2)

Kyo-chan•8mo ago

If you were allowed not to count the data sent to output as part of the complexity you could reduce, but there is no magic to how the answer expects a number of distinct sets

lucifer7303OP•8mo ago

dont know if asking dsa ques is allowed in this channel i mean by a programming perspective theres no workaround to this prob,right you gotta make a clone

Unknown User•8mo ago

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Cannot understand this behaviour of java when adding a list to a list of lists.