Avoid if statements for checking neighboring indexes in a 2D array

[Nicholas Wilson via Digitalmars-d-learn]

On Sunday, 16 July 2017 at 10:37:39 UTC, kerdemdemir wrote:
> My goal is to find connected components in a 2D array for 
> example finding connected '*'
> chars below.
>
>       x x x x x x
>       x x x x x x
>       x x * * x x
>       x x * * x x
>       x x x * * x
>       * x x x x x
>
>
> There are two connected '*' group in this example. First group 
> is composes of six '*' located closer to middle and the second 
> group composes only one '*' char located in the left bottom 
> corner.
>
> Do to this I generally implement a recursive algorithm which 
> repeat calling the same function by checking all neighbors 
> around the current index. I generally end up with something 
> like :
>
> void foo( int row, int col)
> {
>     //Do something here like caching the index
>
>     if ( twoDimensionData[row - 1][col] == '*')
>        foo(row- 1, col);
>     else if ( twoDimensionData[row + 1][col] == '*')
>        foo(row+ 1, col);
>     else if ( twoDimensionData[row - 1 ][col - 1] == '*')
>        foo(row - 1, col - 1);
>
> //..... I need 5 more of this bad boys I mean if checks
> }
>
> Is there any better way to achieve this with cool std functions 
> like enumerate or iota without needing to write eight if checks?

What you probably want is a convolution, used a lot in image 
processing.

insted of using recursion you walk left to right in blocks of 3x3 
and compute a "sum"
and then do the same vertically, then each cell contains the 
number of neighbours that are *.
In this case you want the kernel
111
101
111

        o o o x x x
        o o o x x x
        o o # * x x
        x x * * x x
        x x x * * x
        * x x x x x

        x o o o x x
        x o o o x x
        x o # # x x
        x x * * x x
        x x x * * x
        * x x x x x

        x x o o o x
        x x o o o x
        x x # # o x
        x x * * x x
        x x x * * x
        * x x x x x

        x x x o o o
        x x x o o o
        x x * # o o
        x x * * x x
        x x x * * x
        * x x x x x

Have a look at the video on http://halide-lang.org describing the 
different methods used.