如何使用 java 中的 do while 循环从第一个数组中填充第二个数组？

Question

So basically my problem is this:

I have an array (the first array, array A) which has 20 elements with integer values from 0 to 20 (they can be random) like:

1 2 3 4 5 6 1 2 3 4 

1 2 3 4 5 6 1 2 3 4 

I need to make another array with 40 elements (the second array, array B) which has the same values as the first array, but the catch is that the values of first array are used in second array, but in this pattern:

1 1 2 2 3 3 4 4 5 5
6 6 1 1 2 2 3 3 4 4
5 5 6 6 1 1 2 2 3 3
4 4 5 5 6 6 1 1 2 2

Basically, the second array's values from the start are:

B[0] = A[0]
B[1] = A[0]
B[2] = A[1]
B[3] = A[1]
etc.

and I need to achieve it by using do while loop.

     do {
        i++;
        B[i] = A[i];
        if(i>1) {
            B[i] = A[i-1];
        }
        System.out.print("\t" + B[i]+ " ");
        }
        while(i < 20);

Right now I am stuck with this, I can't figure out the part which states how the second array(B) is filled out. Any help is appreciated!

Answer 1

Similar to a for:

 int[] A = {1, 2, 3, 4, 5, 6, 1, 2, 3, 4};
 int[] B = new int[A.length * 2];
 int index_a = 0; 
 int index_b = 0;
 do {
      B[index_b++] = B[index_b++] = A[index_a++];
 }
 while(index_a < A.length);
 System.out.println(Arrays.toString(B));

You need two initial condition in this case index_a ; and index_b the operation that you want to do

     B[index_b++] = B[index_b++] = A[index_a++];

and the loop condition index_a < A.length .

This B[index_b++] = B[index_b++] = A[index_a++]; is the same as

B[index_b] = A[index_a];
index_b++;
B[index_b] = A[index_a];
index_b++;
index_a++;

Answer 2

You may do something like this:

int[] A = {1,2,3,4,5,6,1,2,3,4};
int[] B = new int[A.length * 2];
int i = 0;

do {
    int index = 2 * i;
    B[index] = A[i];
    B[index + 1] = A[i];
    i++;
} while(i < A.length);

What you do here is map each element from A to 2x and 2x + 1 of its index. Then you may fill in the "gaps" like you intended.

Answer 3

Here's an example of a do-while loop that does exactly what you asked.

int[] A = {1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 1, 2, 3, 4, 5, 6, 1, 2, 3, 4};
int[] B = new int[A.length*2];

int i = 0;
do {
    // Iterates through 0 & 1, then 2 & 3, etc.
    B[i*2] = A[i];
    B[i*2+1] = A[i];
    i++;
} while (i < A.length);

for (int num : B) {
    System.out.printf("%d ", num);
}

This prints 1 1 2 2 3 3 4 4 5 5 6 6 1 1 2 2 3 3 4 4 1 1 2 2 3 3 4 4 5 5 6 6 1 1 2 2 3 3 4 4 .