[computer-go] C++ pointer question

[Peter Drake]

This is an even better solution -- thanks!

Peter Drake
Assistant Professor of Computer Science
Lewis & Clark College
http://www.lclark.edu/~drake/




On Aug 8, 2006, at 12:52 PM, Dave Denholm wrote:

> Peter Drake <drake at lclark.edu> writes:
>
>
> (not cc-ing the list, but feel free to quote anything in a followup to
> the list if you want)
>
>
>> Okay, I've got this method in my BitBoard class:
>>
>> void dilate() // Expand to include all adjacent points
>> {
>> 	BitBoard temp;
>> 	temp.bits[0] = bits[0] | (bits[0] << 1) | (bits[0] >> 1) | bits[1];
>> 	temp.bits[board_width - 1] = bits[board_width - 1] | (bits
>> [board_width - 1] << 1)
>> 		| (bits[board_width - 1] >> 1) | bits[board_width - 2];
>> 	for (unsigned row = 1; row < board_width - 1; row++) {
>> 		temp.bits[row] = bits[row] | (bits[row] << 1) | (bits[row] >>
>> 1) |  bits[row - 1] | bits[row + 1];
>> 	}
>> 	for (unsigned row = 0; row < board_width; row++) {
>> 		bits[row] = temp.bits[row];
>> 	}
>> }
>>
>> Is there some way to avoid that second for loop? I tried "this =
>> temp" and "*this = *temp", but the compiler isn't happy with either.
>>
>
> If I understand your code correctly, I think you can eliminate the
> temporary array entirely... you just need to preserve one value in a
> temporary scalar across the loop.
>
>   unsigned prev = 0;  // within loop, holds old value of bits[row-1]
>   for (unsigned row = 0; row < board_width - 1; row++) {
>      unsigned new_value =
>          bits[row] |
>          (bits[row] << 1) |
>          (bits[row] >> 1) |
>          prev  |  // preserved value of bits[row - 1], or 0 for row  
> == 0
>          bits[row + 1];
>      prev = bits[row];
>      bits[row] = new_value;
>  	}
>
>    bits[board_width-1] = ...
>
> (loop handles [0] for free by initialising prev to 0)
>
> bear in mind I'm a c programmer, not a c++ programmer, so I may have
> missed some subtlety. But I assume bits[] is just a scalar array
>
>
> dd
> -- 
> Dave Denholm              <ddenholm at esmertec.com>       http:// 
> www.esmertec.com

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