How do I alternate chars from two arrays?

Question

I need help with figuring out how to combine two char arrays and then have the elements alternate in a new array. The given arrays can be different lengths, for example: char1=[B,D] char2 = [c,R,5] the output should be char3= [B,c,D,r,5]

char[] ch3 = new char[ch1.length + ch2.length];
int count = 0;
int count2 = 0;
for(int i = 0; i < ch3.length; i++){
    if(i < ch1.length) {
        if(i%2 == 0 || count2 == ch2.length) {
            ch3[i] = ch1[count];
            count++;
        }
    }
    if(i <ch2.length) {
        if(i%2 != 0 || count == ch1.length) {
            ch3[i] = ch2[count2];
            count2++;
        }
    }
}
return ch3;

It should be ``i < ch3.length`` rather than ``i <= ch3.length``. — NomadMaker, Oct 09 '20 at 21:50
Help us help you - shar some sample input for `ch1` and `ch2` and the result you'd like to get for it — Mureinik, Oct 09 '20 at 21:51
Does this answer your question? [Alternating characters of two different inputs](https://stackoverflow.com/questions/25954495/alternating-characters-of-two-different-inputs) — Sneftel, Oct 09 '20 at 21:52
so the input could be different sized arrays, and then the output should be 1st element from array 1 (ch1) then 1st element from array 2(ch2) and alternating so on — Kides99, Oct 09 '20 at 21:53

Oleg Cherednik · Answer 1 · 2020-10-09T22:21:34.187

public static void main(String... args) {
    int[] one = { 1, 2, 3 };
    int[] two = { 44, 55, 66, 77, 88 };
    System.out.println(Arrays.toString(alternate(one, two)));
}

public static int[] alternate(int[] one, int[] two) {
    // result array length is sum of lengths of given arrays.
    int[] res = new int[one.length + two.length];

    // i - result array iterator
    // j - one array iterator
    // k - two array iterarot
    for (int i = 0, j = 0, k = 0; i < res.length; i++) {
        // for even position we put next element from one array
        if (i % 2 == 0)
            // if one array has more elements - add it
            // if no more elements in one array, add next element from two array
            if (j < one.length)
                res[i] = one[j++];
            else
                res[i] = two[k++];
        // for odd position we put next element from two array
        else
            // if two array has more elements - add it
            // if no more elements in two array, add next element from one array
            if (k < two.length)
                res[i] = two[k++];
            else
                res[i] = one[j++];
    }

    return res;
}

Output:

[1, 44, 2, 55, 3, 66, 77, 88]

this code did work for my problem, do you mind to help me understand what your code is doing? I am a beginner with Java and I am not sure what the ? means @oleg.cherednik — Kides99, Oct 09 '20 at 22:11
@Kides99 See: [What is the Java ?: operator called and what does it do?](https://stackoverflow.com/q/798545/5221149) — Andreas, Oct 09 '20 at 22:18

Andreas · Accepted Answer · 2020-10-09T22:37:39.797

Similar solution to answer by oleg.cherednik, but without the need for slow % remainder operator.

public static int[] alternate(int[] a1, int[] a2) {
    int[] a3 = new int[a1.length + a2.length];
    for (int i1 = 0, i2 = 0, i3 = 0; i3 < a3.length; ) {
        if (i1 < a1.length)
            a3[i3++] = a1[i1++];
        if (i2 < a2.length)
            a3[i3++] = a2[i2++];
    }
    return a3;
}

Tests

System.out.println(Arrays.toString(alternate(
        new int[] { 1, 2, 3 },
        new int[] { 44, 55, 66, 77, 88 })));
System.out.println(Arrays.toString(alternate(
        new int[] { 44, 55, 66, 77, 88 },
        new int[] { 1, 2, 3 })));

Output

[1, 44, 2, 55, 3, 66, 77, 88]
[44, 1, 55, 2, 66, 3, 77, 88]

UPDATE

This algorithm can easily be enhanced to merge many arrays:

public static int[] alternate(int[]... arrays) {
    int[] result = new int[Stream.of(arrays).mapToInt(a -> a.length).sum()];
    int[] ai = new int[arrays.length];
    for (int j = 0; j < result.length; )
        for (int i = 0; i < arrays.length; i++)
            if (ai[i] < arrays[i].length)
                result[j++] = arrays[i][ai[i]++];
    return result;
}

Test

System.out.println(Arrays.toString(alternate(
        new int[] { 1, 2, 3 },
        new int[] { 44, 55, 66, 77, 88 },
        new int[] { 12, 34 },
        new int[] { 5, 6, 7, 8 })));

Output

[1, 44, 12, 5, 2, 55, 34, 6, 3, 66, 7, 77, 8, 88]

UPDATE 2

It can also be done for collections:

@SafeVarargs
public static <T> List<T> alternate(Iterable<? extends T>... inputs) {
    List<T> result = new ArrayList<>();
    @SuppressWarnings("unchecked")
    Iterator<? extends T>[] iter = new Iterator[inputs.length];
    for (int i = 0; i < inputs.length; i++)
        iter[i] = inputs[i].iterator();
    for (int prevSize = -1; prevSize < result.size(); ) {
        prevSize = result.size();
        for (int i = 0; i < inputs.length; i++)
            if (iter[i].hasNext())
                result.add(iter[i].next());
    }
    return result;
}

public static <T> List<T> alternate(Collection<? extends Iterable<? extends T>> inputs) {
    @SuppressWarnings("unchecked")
    Iterable<? extends T>[] iterables = inputs.toArray(new Iterable[0]);
    return alternate(iterables);
}

Tests

System.out.println(alternate(
        Arrays.asList(1, 2, 3),
        Arrays.asList(44, 55, 66, 77, 88),
        Arrays.asList(12, 34),
        Arrays.asList(5, 6, 7, 8)));
System.out.println(alternate(Arrays.asList(
        Arrays.asList(1, 2, 3),
        Arrays.asList(44, 55, 66, 77, 88),
        Arrays.asList(12, 34),
        Arrays.asList(5, 6, 7, 8))));

Output

[1, 44, 12, 5, 2, 55, 34, 6, 3, 66, 7, 77, 8, 88]
[1, 44, 12, 5, 2, 55, 34, 6, 3, 66, 7, 77, 8, 88]

How do I alternate chars from two arrays?

2 Answers2