Re: Avoiding looping when applying a 'find' function to an array

[David Bateman]

Jeremy

Have you seen the "lookup" function in octave-forge? I believe it does 
exactly what you want. Follow your syntax below, it would be used 
something like

output_array = look_up_table (lookup (look_up_table, input_array));

Regards
David

Harbinson, Jeremy wrote:

> Hi,
> I have a problem which can be easily solved by looping, but which I
> would like to try to solve by avoiding loops.
>
> I need to transform one array to another equally sized array using a
> look-up table. 
>
> If the input array is A, then I take the value of each element of array
> A one by one, and using the 'find' function, I find the appropriate
> value for the matching element for the output array via the look-up
> table. Looping over every element in array A works just fine - for each
> element in A I can obtain an output value for the output array. However
> I can not think of way of doing this same task without looping - that
> is, applying the 'find' function to each element and producing an output
> array without looping. 
>
> If I input:
>
> Location_of_value_for_output_array = find(input_array(1,1) ==
> look_up_table)
>
> then I get the correct location in the look-up table for the value
> required in the output array, and then I increment the index of
> input_array etc - this is slow because the array is 512 times 512 array
> (a graphics file) and the look-up table is 574 rows long. 
>
> However if I try the following:
>
> Location_of_value_for_output_array = find(input_array(:,:) ==
> look_up_table)
>
> Or 
>
> Location_of_value_for_output_array = find(input_array == look_up_table)
>
> then I have no success.
> Any help would be much appreciated,
> Thanks,
> Jeremy Harbinson      
>
>
>
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