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I have a 2D array as follows

int[][] seatsPrices = {{10,10,10,10,10,10,10,10,10,10},
                        {10,10,10,10,10,10,10,10,10,10},
                        {10,10,10,10,10,10,10,10,10,10},
                        {10,10,20,20,20,20,20,20,10,10},
                        {10,10,20,20,20,20,20,20,10,10},
                        {10,10,20,20,20,20,20,20,10,10},
                        {20,20,30,30,40,40,30,30,20,20},
                        {20,30,30,40,50,50,40,30,30,20},
                        {30,40,50,50,50,50,50,50,40,30}};

I'm asking the user to give a number. Let's say he gives 20. So, I want to code to compare that value with the seatsPrices array iteratively and find any i,j indexes of the seatsPrices array which its values is 20, and print it. I should probably use findAny() but I cannot figure out how to use it. Note: I need to find only one "20" and stop. Thus, using two nested loop, it causes some problems for me.

ufuk Yavuz
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  • `findAny` returns the element, not the index of it – Alberto Sinigaglia Jun 04 '21 at 19:35
  • @AlbertoSinigaglia If I can somehow find any match with the input, I can manage to return its index, no problem. I cannot manage to find a match – ufuk Yavuz Jun 04 '21 at 19:37
  • @ufukYavuz Why go through that extra step? I don't think there is any standard java function that would go through a 2d array, but two nested `for` loops should do the trick – Ayush Garg Jun 04 '21 at 19:39
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    Since comparing int values is obvious (use `==`) my guess is that you are asking how to [Iterate through 2 dimensional array](https://stackoverflow.com/q/25798958)? While iterating simply compare held values at specific position. If it holds searched value like 20 handle those indexes as you want (add them to some list, print them, etc..). – Pshemo Jun 04 '21 at 19:40
  • @megargayu I have tried to do with two nested for loops. but the thing is, as soon as I find a match with the user input, I need to exit the loop. And I cannot find an algorithm to do that. It always creates some problems. – ufuk Yavuz Jun 04 '21 at 19:43
  • What's wrong with just using `break`? – azurefrog Jun 04 '21 at 19:48
  • ... or `return` when you find the match? – Pshemo Jun 04 '21 at 19:49
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    "*as soon as I find a match with the user input, I need to exit the loop. And I cannot find an algorithm to do that. It always creates some problems*" possibly related: [How do I break out of nested loops in Java?](https://stackoverflow.com/q/886955) – Pshemo Jun 04 '21 at 19:54
  • @azurefrog I think OP means to break out of the whole loop, meaning they would need a named loop to break out of the top loop instead of only breaking out of the nested loop. – Ayush Garg Jun 04 '21 at 19:56

3 Answers3

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I don't know why you would use findAny(), it seems much simpler to just iterate over the arrays and search for 20 and print out the i,j whenever you come across one.

for (int i = 0; i < seatPrices.length; i++) {
    for (int j = 0; j < seatPrices[0].length; j++) {
        if (seatPrices[i][j] == 20) 
            System.out.println(i + " " + j);
    }
}

If you know anything about the arrays (like they're sorted) you can come up with a quicker algorithm to find the indices.

If you want to find only one:

boolean found = false;
for (int i = 0; i < seatPrices.length && !found; i++) {
    for (int j = 0; j < seatPrices[0].length && !found; j++) {
        if (seatPrices[i][j] == 20) {
            System.out.println(i + " " + j);
            found = true;
        }
    }
}
Alex
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  • So the code you write iterates through all indeces of the array which its value is 20 and print the least "20". I mean the "20" at 7th row 9th column. But I need for the code to find the first "20" only and stop – ufuk Yavuz Jun 04 '21 at 19:49
  • Edited. It's still going to be worst case n search time. – Alex Jun 04 '21 at 19:51
  • His array seems to be ordered, he can do a binary search there instead of sequential search. This would reduce to log(n). – Lucas Montenegro Carvalhaes Jun 04 '21 at 19:55
  • @Alex thank you that last code is what I'm looking for! – ufuk Yavuz Jun 04 '21 at 19:56
  • Another (in my opinion, better) way of doing this would be to name the first loop something like `findLoop:` and then do `break findLoop;` instead of using a new variable `found`. – Ayush Garg Jun 04 '21 at 19:58
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The .findAny() and .findFirst() stream methods are not the best for you. They both work by finding values and not indexes. The best here is to indeed iterate through your array until you find the value you want and stopping the iteration when the value is found. You could do as Alex said.

If you know your array is ordered (like it seems to be) you could do a binary search on it instead. Start by looking at the middle of the array, then check if the value there is your value, or if it is greater than it, or less than it.

  • If the value (middle of the array) is your value (20), the search ended.
  • If the value is greater than your value, the value (if present) is on the left side of your split in the middle.
  • If the value is smaller than your value, then the value you want is on the right side of your split.

You just have to repeat this algorithm on the side of the array your value is, until you find it.

Example 1D algorithm:

int binarySearch(int arr[], int lIndex, int rIndex, int searchValue)
{
    if (rIndex >= lIndex) {
        int pivot = lIndex + (rIndex - lIndex) / 2;
        if (arr[pivot] == searchValue)
            return pivot;
        if (arr[pivot] > searchValue)
            return binarySearch(arr, lIndex, pivot - 1, searchValue);
        return binarySearch(arr, pivot + 1, rIndex, searchValue);
    }
    return -1;
}

You can unwrap your 2D array into a 1D array and get the index, than just remap the index to a 2D index:

x = index / size;
y = index % size;
Lucas Montenegro Carvalhaes
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1

If you want to provide the user with a really any value, I'd suggest you getting all the appropriate items and get a random one thereafter. You can collect all the items matching with such a one-liner:

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class SpecificValueIn2DArray {
    static int[][] seatsPrices = {{10, 10, 10, 10, 10, 10, 10, 10, 10, 10},
    {10, 10, 10, 10, 10, 10, 10, 10, 10, 10},
    {10, 10, 10, 10, 10, 10, 10, 10, 10, 10},
    {10, 10, 20, 20, 20, 20, 20, 20, 10, 10},
    {10, 10, 20, 20, 20, 20, 20, 20, 10, 10},
    {10, 10, 20, 20, 20, 20, 20, 20, 10, 10},
    {20, 20, 30, 30, 40, 40, 30, 30, 20, 20},
    {20, 30, 30, 40, 50, 50, 40, 30, 30, 20},
    {30, 40, 50, 50, 50, 50, 50, 50, 40, 30}};

public List<int[]> getItemIndexCollection(int itemValue) {
    return IntStream.range(0, seatsPrices.length)
        .boxed()
        .flatMap(i ->
            IntStream.range(0, seatsPrices[i].length)
                .boxed()
                .map(j -> seatsPrices[i][j] == itemValue ? new int[]{i, j} : new int[]{-1, -1})
        )
        .filter(item -> item[0] != -1 && item[1] != -1)
        .collect(Collectors.toList());
}

}

and get a random element from the result list:

    final var actualResult = new SpecificValueIn2DArray().getItemIndexCollection(itemValue);
    final var random = new Random();
    final var pair = actualResult.get(random.nextInt(actualResult.size()));
    System.out.println(pair[0] + " " + pair[1]);
    System.out.println(SpecificValueIn2DArray.seatsPrices[pair[0]][pair[1]]);
Alexander Petrovskiy
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