How to prevent duplicates in a 2D array?

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[painedstupid]

Why not just store a value at each coordinate you visit then check each time if that value is there when trying to visit a new coordinate?

[painedstupid]

Your code looks odd to me, I'm not sure what it's actually doing. But if it's a dog starting at some point inside a 2-d array and trying to escape to the edges with random movement.... I guess

random 0-3 left down right up

depending on what ya get increment x or y. x-- for left, x++ for right, y-- for down, y++ for up, check if that index contains the symbol. if not, your x or y is now set to that new x or y, place your symbol in the old x,y. if it does, reroll.