UndyingJellyfish - 1 year ago 82
Java Question

# Creating N by N diagonal matrix using basic logic

I want to create a matrix of size N by N where N is a constant value defined globally, for now I just want to create a matrix where N=6. Where I fall short is I want to make it diagonally, like so:

``````0 1 2 3 4 5
1 0 1 2 3 4
2 1 0 1 2 3
3 2 1 0 1 2
4 3 2 1 0 1
5 4 3 2 1 0
``````

Currently I have this method:

``````public static void drawMatrix(){
for (int line = 0; line < N; line++){
for (int j = 0; j < N; j++){
System.out.print(j + " ");
}
System.out.println();
}
}
``````

Unfortunately it's only able to print 0 1 2 3 4 5 in every line, so I suppose I need another nested for-loop, however I'm not sure how to set it up.

`j` is the column number, so it will be the same for all rows. What you need to do is to add or subtract `j` from the line number, depending on the line number, in order to make a "shift". Since the result could become negative, you will need to add `N` and mod by `N`:

``````if (j > line) {
System.out.print((N-line+j)%N + " ");
} else {
System.out.print((line-j+N)%N + " ");
}
``````

Demo.

You can also rewrite it without an `if` using a conditional expression:

``````int sign = j > line ? -1 : 1;
System.out.print((N+sign*(line-j))%N + " ");
``````

Demo.

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