Remove elements from a List at a specific index-CodePudding

I am trying to program a method that deletes the first, second and third element of every group of 4 elements. It seems not working at all. Could anyone please help?

public static void reduziereKommentare(List<String> zeilen) {
        if (!zeilen.isEmpty()) {
            if (zeilen.size() % 4 != 0) {
                throw new RuntimeException("Illegal size "   zeilen.size()   " of list, must be divisible by 4.");
            }
            for (int i = 1; i <= zeilen.size() % 4; i  ) {
                zeilen.remove(i);
                zeilen.remove(i   1);
                zeilen.remove(i   2);
            }
        }
        System.out.println(zeilen);
    }

CodePudding user response：

As said in the comments, removing an element impacts the indexing. Whenever I need to do something like this, I either use an Iterator, or loop backwards.:

for (int i = zeilen.size() - 4; i >= 0; i -= 4) {
    zeilen.remove(i   2);
    zeilen.remove(i   1);
    zeilen.remove(i);
}

Note that I subtract 4 from i each iteration, so I go back a full block of four each time.

Also note that I remove the largest indexed elements first. If I use i, i 1 and i 2 inside the loop, I again run into the same issue. I could also have used i 3 times, but this makes it more clear.

CodePudding user response：

My take...does not require the size precondition check but you may want to still catch that if it represents an error of broader scope than this method.

Given this test code...

    // Test code
    List<String> myList = new ArrayList<>();
    for (int i = 0; i < 20; i  ) {
        myList.add(String.valueOf(i));
    }

the 'zeilen' loop can be implemented as ...

    // "before" diagnostics
    System.out.println(zeilen);

    // The 'zeilen' loop
    for (int i = 0, limit = zeilen.size(); i < limit; i  ) {
        if ((i 1) % 4 > 0) zeilen.remove(i/4);
    }

    // "after" diagnostics
    System.out.println(zeilen);

and produces

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
[3, 7, 11, 15, 19]

Works with any length list leaving every '4th' element in list.

A few more test cases :

Given                   Results in
[]                      []
[0,1]                   []
[0,1,2,3]               [3]
[0,1,2,3,4]             [3]
[0,1,2,3,4,5,6,7]       [3,7]
[0,1,2,3,4,5,6,7,8]     [3,7]

CodePudding user response：

Would it not be easier to simply add every fourth item to a new list and return that? This would also eliminate any repetitive copying that could be involved when removing elements from a list. And the target list can be appropriately sized to start.

public static List<String> reduziereKommentare(List<String> zeilen) {
    Objects.requireNonNull(zeilen);
    List<String> zeilen1= new ArrayList<>(zeilen.size()/4);

    for(int i = 3; i < zeilen.size(); i =4) {
            zeilen1.add(zeilen.get(i));
    }

    return zeilen1;
}

You could also use a stream.


zeilen = IntStream.iterate(3, i ->i < zeilen.size(), i->i =4)
                .mapToObj(zeilen::get).toList();

Notes:

whether the list is empty or the size is not divisible by 4, this will work. It will just ignore the extra elements.
assigning the result to the original variable will result in the old list being garbage collected.
I only check for a null argument since that would cause an exception. Of course, if alerting the user of the size is important just add the other check(s) back in.