Which option is faster...

[jicman]

On Monday, 5 August 2013 at 15:18:42 UTC, John Colvin wrote:
> On Monday, 5 August 2013 at 13:59:24 UTC, jicman wrote:
>>
>> Greetings!
>>
>> I have this code,
>>
>> foreach (...)
>> {
>>
>>  if (std.string.tolower(fext[0]) == "doc" ||
>>    std.string.tolower(fext[0]) == "docx" ||
>>    std.string.tolower(fext[0]) == "xls" ||
>>    std.string.tolower(fext[0]) == "xlsx" ||
>>    std.string.tolower(fext[0]) == "ppt" ||
>>    std.string.tolower(fext[0]) == "pptx")
>>   continue;
>> }
>>
>> foreach (...)
>> {
>>  if (std.string.tolower(fext[0]) == "doc")
>>    continue;
>>  if (std.string.tolower(fext[0]) == "docx")
>>    continue;
>>  if (std.string.tolower(fext[0]) == "xls")
>>    continue;
>>  if (std.string.tolower(fext[0]) == "xlsx")
>>    continue;
>>  if (std.string.tolower(fext[0]) == "ppt")
>>    continue;
>>  if (std.string.tolower(fext[0]) == "pptx")
>>   continue;
>>  ...
>>  ...
>> }
>>
>> thanks.
>>
>> josé
>
> better:
>
> foreach (...)
> {
>     auto tmp = std.string.tolower(fext[0]);
>     if(tmp == "doc" || tmp == "docx"
>        || tmp == "xls" || tmp == "xlsx"
>        || tmp == "ppt" || tmp == "pptx")
>     {
>         continue;
>     }
> }
>
> but still not super-fast as (unless the compiler is very 
> clever) it still means multiple passes over tmp. Also, it 
> converts the whole string to lower case even when it's not 
> necessary.
>
> If you have large numbers of possible matches you will probably 
> want to be clever with your data structures / algorithms. E.g.
>
> You could create a tree-like structure to quickly eliminate 
> possibilities as you read successive letters. You read one 
> character, follow the appropriate branch, check if there are 
> any further branches, if not then no match and break. Else, 
> read the next character and follow the appropriate branch and 
> so on.... Infeasible for large (or even medium-sized) 
> character-sets without hashing, but might be pretty fast for 
> a-z and a large number of short strings.

As the Hispanic speaking community say, muchas gracias.