'镜像'二维数组[Java]

Question

I am attempting to make "2-dimensional" array in Java that populates itself with random numbers between 1 and 6 (EDIT: I meant 0 and 5, as the code does currently - my apologies). However, I would like the numbers to be 'mirrored' over an imaginary line-of-best-fit, as if dealing with a graph. For example, if the number at indicies [1][4] is 3.0, I would like the number at [4][1] to also be 3.0. The snippet of code I currently have that deals with this is below, for what its worth (I have array.length established as 6 at another point in the code):

Random random = new Random();

        int n = array.length;

        double [][] populationArray = new double [n][n];

        for (int i=0; i<populationArray.length; i++){
            for (int j=0; j<populationArray[i].length; j++) {
                populationArray[i][j] = random.nextInt(6);
                if (populationArray[i][j] != 0) {
                    populationArray[j][i] = populationArray[i][j];
                }
            }
        }

        for (double[] p : populationArray) {
            System.out.println(Arrays.toString(p));
        }

This currently prints as:

[0.0, 1.0, 2.0, 1.0, 5.0, 2.0]
[1.0, 3.0, 5.0, 1.0, 3.0, 1.0]
[2.0, 5.0, 4.0, 1.0, 4.0, 1.0]
[1.0, 1.0, 1.0, 4.0, 2.0, 2.0]
[5.0, 3.0, 0.0, 2.0, 4.0, 4.0]
[2.0, 1.0, 0.0, 0.0, 4.0, 1.0]

As you can see some of the numbers reflect and some don't (I suspect the ones that do are purely out of luck), and I'm struggling with the logic of this. I would appreciate any suggestions - if this has been addressed somewhere else I'd also take a link, as I could not find it myself.

Thank you.

Answer 1

Because of the if(populationArray[i][j] != 0) only the values that aren't 0 are mirrored. if you remove the if statment the code will work.

Also random.nextInt(6) will generate an integer between 0 (inclusive) and 6 (exclusive), so that will generate either 0,1,2,3,4 or 5.

So to generate a number 1-5 (inclusive) you'll have to do random.nextInt(5)+1

So the for loop will become:

for (int i=0; i<populationArray.length; i++){
        for (int j=0; j<populationArray[i].length; j++) {
            populationArray[i][j] = random.nextInt(5)+1;
            populationArray[j][i] = populationArray[i][j];
    }
}

However I'd like to point out that all of the positions are given a random value twice. For creating a single 6x6 array the difference won't be very noticeable. But if you are planning to create bigger/many arrays I'd advise to optimise your code to avoid giving each spot in the array a value twice.

You can optimize this by changing j<populationArray[i].length to j<=i :

for (int i=0; i<populationArray.length; i++){
    for (int j=0; j<=i; j++) {
        populationArray[i][j] = random.nextInt(5)+1;
        populationArray[j][i] = populationArray[i][j];
    }
}

Answer 2

Your issue is with these lines

populationArray[i][j] = random.nextInt(6);
if(populationArray[i][j] != 0) {
    populationArray[j][i] = populationArray[i][j];
}

if populationArray[i][j] = random.nextInt(6); returns 0 than it will not be reflected. I suspect you want populationArray[i][j] = random.nextInt(5)+1; this will return a random number between 1 and 6.

Bonus: your code will now work but it is actually writing each cell twice. It will write [1, 4] and mirror to [4, 1] but then it will continue looping and write over [4, 1] and mirror to [1, 4] to fix this you will want to add a check to see if it's 0 before writing to it. Which coincidentally just ends up moving your if statement one line up.

    for (int i=0; i<populationArray.length; i++){
        for (int j=i; j<populationArray[i].length; j++) {
            populationArray[i][j] = random.nextInt(5)+1;
            populationArray[j][i] = populationArray[i][j];
        }
    }

EDIT: changed the solution based on Luke's comment. Note the change to int j=i on the second for statement.

Answer 3

In case your arrays get larger you might want to save time by not setting each element twice. Therefore your inner loop should not run until populationArray[i].length

for (int i=0; i<populationArray.length; i++){
        for (int j=0; j<populationArray[i].length-i; j++) {
            populationArray[i][j] = random.nextInt(6);
            int oppositI = populationArray.length -1 - i;
            int oppositJ = populationArray[i].length -1 - j;
            populationArray[oppositI][oppositJ] = populationArray[i][j];
    }
}

This assumes that your populationArray is a sqare. It will be kinda more complex otherwise...