Thread: copy values from arraylist to another

Please I need help :/
How can I copy values from a ArrayList to another.
if I have a ArrayList containing 10 values and I want to copy the 2nd 4rd values and so on.

Review the ArrayList API for methods that should be helpful, then post what you've tried when you need help.

How can I copy values from a ArrayList to another. if I have a ArrayList containing 10 values and I want to copy the 2nd 4rd values and so on.

Here's my code:

ArrayList tab = new ArrayList ();

ArrayList tab2 = new ArrayList ();

{

int i=1;

int j=0;

while ( i < tab.size () )

{

tab.get(i);

i=i+2;

tab2.add(i);

j=j+1;

}

}

Ok, first of all, your loop doesn't make sense.

while ( i < tab.size () )

Tab doesn't contain any object so it has a size of 0, but you say while (1 < 0) do something? The code between the brackets will never be reached.



Just so you know:

i = i + 2

is the same as

i += 2

The first element of an array has an index of 0
This means the second element has an index of 1, the forth element has an index of 3, etc
So you'll probably want to give your i a start value of 0 instead of 1



What do you use that j variable for? You never use it?


How I would do it:

ArrayList<Integer> tab = new ArrayList<>();
    for (int i = 0; i < 10; i++)
        tab.add(i);
 
ArrayList<Integer> tab2 = new ArrayList<>();
    for (int i = 0; i < 10; i += 2)
        tab2.add(tab.get(i + 1));

The upper block puts 10 integer values in the tab arraylist, ranging from 0 to 9
The other block then adds the values of tab at index 1, 3, 5, 7 and 9 to tab2, these are respectively the second, forth, sixth, eight and tenth integer in the tab arraylist

Duplicate topic. This one has more effort.

Please read this topic to learn how to post code correctly and other useful topics for newcomers.

in the second arraylist shouldn't it be get(i) not i+1 ?
and for int i =1?

for (int i = 0; i < 10; i += 2)

The variable i will respectively go trough the following values:
0, 2, 4, 6, 8

tab2.add(tab.get(i + 1));

i + 1 then gives these values:
1, 3, 5, 7, 9

And this are the indexes that are required for what you want to do (:

I'm also still new to Java and my teacher told me that you should try to always give your integer variable in such a for-next loop a value of zero. Good programming practice I guess!