Wrong multidiemnsional-array initialization trying to build the pascal triangle (JAVA)

Question

I am trying to write a class that creates an object of a pascal triangle using multidimensional arrays. Now i do have everything in place (at least i think so) except for the correct Initialization of the array. My program reads as follow:

class Pascal{

//Object variables
int size;
int[][] pascal;

Pascal(int size){ //Constructor

    this.size = size; //Defines how many rows/columns the triangle has.
    pascal = new int[size][];

    //Allocation of the arrays 
    for(int i=0;i<pascal.length;i++)
        pascal[i] = new int[i+1];

    pascal[0][0] = 1; //the tip of the triangle is always one. Also you need a value to start with.


    //Initialisation of the elements
    for(int x=0;x<pascal.length;x++){
        for(int y=0;y<pascal[x].length;y++){

            if(x>0){

                if(y==0 || y == (pascal[x].length)-1)
                    pascal[x][y] = 1; //Initialisation of the first and last element of the actual array x

                else
                    pascal[x-1][y-1] = pascal[x-1][y] + pascal[x-1][y-1];//Initialisation of all elements in between

            }
        }
    }

}


//The print method needed to display the pascal triangle
void print(){
    for(int i=0;i<pascal.length;i++){
        for(int k=pascal.length;k>i;k--)
            System.out.print(" ");
        for(int j=0;j<pascal[i].length;j++)
            System.out.print(pascal[i][j]+" ");
        System.out.println();
    }
}


//main function
public static void main(String... args){
    Pascal p = new Pascal(5); //example triangle consisting of 5 rows total
    p.print();
}
}

The output in this particiular example (new Pascal(5);) should be:

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

And yet it is:

    1
   2 1
  1 1 1
 1 0 1 1
1 0 0 0 1

I know the problem must be somewhere in the array initialization part of the code and it's probably a simple mistake, but starring at the monitor doesn't get me anywhere anymore :/

Just in case you don't just want to give me the answer: From my understanding the array element pascal[1][0] should be 1 instead of 2, because when the for-loops value x is 1 and value y is 0 the if-condition if( y==0 || y==pascal[x].length-1) should apply thus setting pascal[1][0] = 1.

Thanks for your help!

Answer 1

In the constructor, when initializing the 2D array, in the else, your assignment is incorrect. You want to initialize the current element, but the left hand side is incorrect (and inconsistent with the if):

pascal[x-1][y-1] = pascal[x-1][y] + pascal[x-1][y-1];

Try the [x][y] element itself.

pascal[x][y] = pascal[x-1][y] + pascal[x-1][y-1];

Making only this change, I get the correct output:

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