Comparing Two ArrayLists

[winter hare]

I have an assignment where our professor wants us to create two different integer arraylists and then find common terms using another method named findCommonElements. Here is what I have as of now. I'm not sure where to go from here.

[midnight ridge]

<@&987246399047479336> please have a look, thanks.

[old drift]

Since it's homework, I assume you are not supposed to use something like Stream.distinct(), and you have to do it the hard way. Iterate through one ArrayList and check if the value exists in the other ArrayList. If it does, add it to a HashSet of common items.

They usually want you to suffer a bit.

[winter hare]

so you're saying create a loop that runs through the integerOne array, and then compares each element to the elements in the second?

[proud kelp]

https://download.java.net/java/early_access/loom/docs/api/java.base/java/util/Collection.html#retainAll(java.util.Collection)

[winter hare]

this is my first programming class so i'm still new to most of this stuff haha

[old drift]

Yes. That would not be the most efficient, but that is the kind of solution they usually want for homework.

[proud kelp]

no if you add all elements of both lists to a set then the set just has no duplicates but all elements of both lists and not just the elements the lists have in common

[old drift]

Right. I got distracted and forgot the problem.

[winter hare]

so it would be something like for(int i:integerOne.length) {
}

then in that loop have an if statement checking if the value at position i exists in integerTwo array?

[hearty pendant]

Its a bit vague that they say not to use any built in methods. Not allowed to just throw one list into a set and check nr 2 against that then, perhaps?

[winter hare]

here are the full written instructions.
Write a program that takes two ArrayLists of integers and finds the common elements between the
two lists. The program should use a method called findCommonElements() that takes two
ArrayList<Integer> as parameters and returns a new ArrayList<Integer> containing the
common elements. You can declare and initialize two ArrayLists of Integers based on your choice.
You must not use any built-in methods to find the common elements.

[proud kelp]

i think that is what they what them to do

[old drift]

The instructions say you can't use built in methods, so you can't use contains(). That means you also need to iterate the other list and check each item, so you would have nested loops and not just one loop.

[hearty pendant]

Sorting and merging might help you make a trivial O(n) solution

[proud kelp]

since it is no ready made solution like retainAll

[proud kelp]

how do you sort in O(n)?

[hearty pendant]

oh true im rarted

[west cradle]

for every element A in list A
     for every element B in list B
         if A equals B
             already_added = false
             for every element COMMON in list of common elements
                 if A equals COMMON
                     already_added = true
             if NOT already_added
                  add A to common elements

@winter hare here is a solution

its technically speaking the slowest one

but its the easiest to implement

[winter hare]

this is the nested for-loop idea?

[west cradle]

all ideas that loop end up being "nested for loop ideas" but yeah

[winter hare]

cool let me give a try.

so i've got the first part with nested loop and I am now at the if statement. so if i used i for the outer loop and j for the inner loop, it would be if(i == j)?

[west cradle]

remember that you are comparing the elements in the lists

not the indexes of those elements

people usually use i and j to track an index

[winter hare]

hm. how would i compare the actual element then?

[west cradle]

for (int i = 0; i < integerOne.size(); i++) {
    int a = integerOne.get(i);
    // ...
}

you need to use .get to get the element from the list

[winter hare]

ah ive seen that get method before.

ok

[west cradle]

so that code above covers this line in the pseudocode

for every element A in list A

in the ... is where you put everything else

[winter hare]

right.

when i go to add the element to the new array, is that done in the if statement? and what variable would i be adding? does it matter if it is a or b?

[west cradle]

if a and b are equal

then no

[winter hare]

i got it! thank you all!