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Iterating over the boundary of a grid
By 0-jij-0, history, 3 years ago, In English

Hello,

I was wondering if there's any trick that allows us to iterate over the boundary of an $$$N \times M$$$ grid using a single loop instead of 4 or 2.

To iterate over the whole grid for example we can iterate from 0 to $$$NM$$$ and have $$$i = x / M$$$ and $$$j = x \% M$$$.

So is there any smart thing like this to do for the boundary only (i.e pairs $$$(i, j)$$$ such that either $$$i = 0$$$ or $$$i = N - 1$$$ or $$$j = 0$$$ or $$$j = M - 1$$$)

Thanks for future help!

EDIT: Problem Solved!

Solution 1
Solution 2
My Solution inspired by the previous one
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3 years ago, # |
← Rev. 2   Vote: I like it +4 Vote: I do not like it

Here you go

code
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    3 years ago, # ^ |
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    This is clever! Thank you for the suggestion

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3 years ago, # |
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I would probably do it this way:

code

But note that it doesn't work when $$$n = 1$$$ or $$$m = 1$$$ so you will probably have to run a simple loop for that.

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    3 years ago, # ^ |
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    I got your point and your idea to skip the stuff in between actually inspired me to get the following and the complexity would be simply $$$O(N + M)$$$

    Code

    Thank you!

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3 years ago, # |
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