How can I slice an int array by stride in Java?

Question

I am learning Java.

In Python, I can:

my_list = [1,2,3,4]
my_new_list = my_list [::-1]
print my_new_list                # [4,3,2,1]

Is there any method I can use in Java to do that on int array?

Edit1: Sorry for the bad example. How about my_list [::-2]?

Edit2: What I mean my_list [::-2] is like

my_list = [1,2,3,4]
my_new_list = my_list [::-2]
print my_new_list                # [4,2]

Edit3: Slicing by stride in Python is to "check" the item in a list every step (stride).If the step is negative, Python would check the item from the end of the list to the beginning of the list. For example:

my_list = [1,2,3,4,5,6,7,8,9]
my_list1 = my_list[::2]       # my_list1 becomes [1,3,5,7,9]
my_list2 = my_list[::3]       # my_list2 becomes [1,4,7]
my_list3 = my_list[::4]       # my_list3 becomes [1,5,9]
my_list4 = my_list[::-3]      # my_list4 becomes [9,6,3]

@BoristheSpider Thank you for the method. How about just slicing by stride? like my_list [::-2] — Jay Wang, Dec 29 '15 at 12:20
That is a separate question - please describe your **full** question when posting, not _part_ of it. — Boris the Spider, Dec 29 '15 at 12:22
Further, not all Java developers speak Python - please describe the problem, the input and the expected output. — Boris the Spider, Dec 29 '15 at 12:24
@JayWong and what is your desired output after my_list [::-2]? — Sindhoo Oad, Dec 29 '15 at 12:26
As this is a Q&A site, the primary goal of any question should be future usefulness - sorry to be annoying; but in order for a question to be a good question it should contain all relevant information for a future user reading it... — Boris the Spider, Dec 29 '15 at 12:27
@BoristheSpider Yes, I see. I was too naive and not responsible. — Jay Wang, Dec 29 '15 at 12:29
@OAD The new list would become [4,2], sorry for the missing information. — Jay Wang, Dec 29 '15 at 12:30
@JayWong will you tell us in simple language how this output is coming, like is this coming checking even numbers of alternate values? — Sindhoo Oad, Dec 29 '15 at 12:34
@OAD Yes, I have edited my question. I thought it was a common feature among programming languages, sorry. — Jay Wang, Dec 29 '15 at 12:41

singhakash · Answer 1 · 2015-12-29T12:38:57.140

4

You can convert the array to list then use Collections.reverse() on that list.

Integer a[]={1,2,3,4};
List <Integer> list = Arrays.asList(a);
System.out.println(list);
Collections.reverse(list);
System.out.println(list);

DEMO

edited Dec 29 '15 at 12:38

answered Dec 29 '15 at 12:21

singhakash

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I am sorry for the misleading example, can I do something like my_list [::-2]? – Jay Wang Dec 29 '15 at 12:24
@JayWong can you explain more about the output – singhakash Dec 29 '15 at 12:37

score 1 · Answer 2 · answered Dec 29 '15 at 12:22

You can do it in following way, with the help of Collections#reverse and for using that method we first have to convert array to list and after reversing the list we will again convert it to array. You should better use List instead of array if possible.

public class Test {

    public static void main(String[] args) {
        Integer[] arr = new Integer[] { 1, 2, 3, 4 };
        List<Integer> list = Arrays.asList(arr);
        Collections.reverse(list);
        arr = list.toArray(new Integer[list.size()]);
        System.out.println(Arrays.toString(arr));
    }

}

OUTPUT

[4, 3, 2, 1]

score 1 · Answer 3 · answered Dec 29 '15 at 13:22

You can write it like this in Java 8

int a = { 1, 2, 3, 4 };
int step = -2;

int b = IntStream.range(0, a.length)
            .filter(i -> i%step == 0)
            .map(i -> a[(step < 0) ? a.length-i-1 : i])
            .toArray();

It should work with all example arrays and steps you provided.

score 0 · Answer 4 · answered Dec 29 '15 at 12:27

0

Apache Commons Lang has an utility method for that:

ArrayUtils.reverse(int[] array)

answered Dec 29 '15 at 12:27

Jan

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score 0 · Accepted Answer · answered Dec 29 '15 at 13:21

0

Java does not have something builtin what you want, but you can built your own logic for it, something like this, just put the value in val , for example my_list[::2] 2 in this case val will be 2

public static void main (String[] args) throws java.lang.Exception
{
int ml[] = {1,2,3,4,5,6,7,8,9};
int val = -2;
ArrayList al = new ArrayList();
if(val>0){
for(int i = 0 ; i<ml.length;i+=val){
    al.add(ml[i]);
}
}
else if(val<0){
for(int i = ml.length-1 ;  i>=0; i+=val){
    al.add(ml[i]);
}   

}


for(int i = 0 ; i<al.size();i++){
    System.out.println(al.get(i));
}

}

answered Dec 29 '15 at 13:21

Sindhoo Oad

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@JayWong this code is a mess - I would not use it. You would be better off with a smart `Iterator` design to create a "view" of the underlying data. – Boris the Spider Dec 29 '15 at 13:29
I would appreciate if you provide him a better solution rather by telling him how to do, he is newbie in java. – Sindhoo Oad Dec 30 '15 at 04:05

How can I slice an int array by stride in Java?

5 Answers5