counting negative numbers in array

Question

I am trying to find the counts for negative numbers in a 2d-array ( square- matix). In matrix if you go from up to down and left to write number increases. Logic here is to start from last column and proceed to the left. If you find the neg num then increase the row index and proceed the same way till the last row. I am getting the error in java code but not in python.

public class O_n 

{

public static void main(String[] args)

{

    int firstarray[][] = {{-2,-1,0},{-1,0,1},{0,1,2}};
    int secondarray[][] = {{-4,-3,-2},{-3,-2,-1},{-2,-1,0}};
    System.out.print ("First array has"+count_neg(firstarray));
    System.out.println("negative numbers");
    System.out.print ("Second array has"+count_neg(secondarray));
    System.out.println("negative numbers");
}

public static int count_neg(int x[][]){
    int count = 0;
    int i = 0; //rows
    int j = x.length - 1; //columns

    System.out.println("max column index is: "+j);
    while ( i >=0 && j<x.length){
        if (x[i][j] < 0){ // negative number
            count += (j + 1);// since j is an index...so j + 1
            i += 1;
        }
        else { // positive number
            j -= 1;
        }
    }
    return (count);
    }
}

I am getting this output

max column index is: 2
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
    at O_n.count_neg(O_n.java:22)
    at O_n.main(O_n.java:9)
/home/eos/.cache/netbeans/8.1/executor-snippets/run.xml:53: Java 
returned: 1
BUILD FAILED (total time: 0 seconds)

What is wrong with this code? the same thing worked in python...

def count_neg(array):
    count = 0
    i = 0 # row
    j = len(array) -1 # col
    # since we are starting from right side

    while j >= 0 and i < len(array):
        if array[i][j] < 0: 
            count += (j + 1)
            i += 1
        else:
            j -= 1
    return count
print(count_neg([[-4,-3,-1,1],[-2,-2,1,2],[-1,1,2,3],[1,2,4,5]]))

Answer 1

I would write the method like this, just go through the 2d array and increase count each time a negative number is found

public static int count_neg(int x[][]){
    int count = 0;

    for(int i = 0; i < x.length; i++){
        for(int j = 0; j < x[i].length; j++){
            if(x[i][j] < 0){
                count++;
            }
        }
    }
    return (count);
}

Answer 2

Your indexes are reversed from the python version:

while j >= 0 and i < len(array)

To the Java version:

while (i >= 0 && j < x.length)
// Change to
while (j >= 0 && i < x.length)

Output:

max column index is: 2
3

If you are using Java8, you can use streams to implement count_neg:

public static int countNeg(int x[][]) {
    long count = Stream.of(x)
            .flatMapToInt(arr -> IntStream.of(arr))
            .filter(i -> i < 0)
            .count();
    return Math.toIntExact(count);
}