Intelligent method for particular traversing of a matrix

I have to traverse an n x n matrix in java (so indices are 0,...,n-1), to assign values to the single elements. I must start from the bottom right and arrive to the top left. The particularity is that I do not have to consider the matrix[n-1][n-1] element, that has been initialised before. The adjacent values depend on each other for initialazing and it must be initialized first.

n x n

matrix[n-1][n-1]

One way could be inserting an if in the for cycle

if

for

for (i = n-1; i >= 0; i--)
for (j = n-1; j >= 0; j--)
if (i == n - 1 && j == n - 1)
//initialize particular value
else
//initialize others


but it seems to me a bit inefficient.

Another way could be to initialize the value matrix[n-1][n-1] outside the cycle, then doing 3 for cycles (one for the bottom line, one for the rightest column, one for the other elements). But it seems a bit inelegant.

matrix[n-1][n-1]

for

So I'm searching, if exists, for a solution that involves only two annidate for, and without a control in every cycle (like first example).

for

store the value befor the nested for, and reatribute after the for loops?
– Kepotx
Jun 29 at 10:31

No. The init of adjacent elements depends on it
– Lore
Jun 29 at 10:33

2 Answers
2

Here is an approach that uses one loop through the matrix which makes it easy to avoid matrix[n-1][n-1]. Not sure how the calculations compares to an if though from a performance perspective

int matrix = new int[n][n];

int current = n * n - 2;
int row = 0;
int col = 0;
while (current >= 0) {
col = current % n;
row = current / n;

matrix[row][col] = //init stuff
current--;
}


I think the solution of Joakim is good except the % and / operations... inspired to this, I've found an interesting variant that avoid them. I've called column index j1 to avoid problems with other "normal" cycles.

j1

matrix[n-1][n-1] = //init code;
int j1 = n-2;

for (int i = n-1; i >= 0; i--) {
for (; j1 >= 0; j1--) {
matrix[i][j1] = //init code;
}
j1 = n-1;
}


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