I ahve a vector of doubles taht I need to extract values from. I was
just about to use the STL find() algo, but I have a couple of questions:

first: can you specify the tolerance threshold to match on doubles?
second: if yes, how may this be done (i.e. how many one specify the
tolerance threshold for comparing doubles?)

Anonymous wrote:

> I ahve a vector of doubles taht I need to extract values from. I was
> just about to use the STL find() algo, but I have a couple of questions:
>
> first: can you specify the tolerance threshold to match on doubles?
> second: if yes, how may this be done (i.e. how many one specify the
> tolerance threshold for comparing doubles?)

Note that find() has two signatures: one that takes a value to be found and
one that takes a unary predicate instead. Use the later and provide a
functor:

class is_approximately {

double x;

public:

is_approximately ( double what )
: x ( what )
{}

bool operator() ( double y ) const {
// return true if y is near x.
}

};

Now you can do:

find( whatever.begin(), whatever.end(), is_approximately( 0.5 ) );



Best

Kai-Uwe Bux

Kai-Uwe Bux wrote:
> Anonymous wrote:
>
>> I ahve a vector of doubles taht I need to extract values from. I was
>> just about to use the STL find() algo, but I have a couple of questions:
>>
>> first: can you specify the tolerance threshold to match on doubles?
>> second: if yes, how may this be done (i.e. how many one specify the
>> tolerance threshold for comparing doubles?)
>
> Note that find() has two signatures: one that takes a value to be found and
> one that takes a unary predicate instead. Use the later and provide a
> functor:

Isn't the second form find_if(), not find()?
>
> class is_approximately {
>
> double x;
>
> public:
>
> is_approximately ( double what )
> : x ( what )
> {}
>
> bool operator() ( double y ) const {
> // return true if y is near x.
> }
>
> };
>
> Now you can do:
>
> find( whatever.begin(), whatever.end(), is_approximately( 0.5 ) );
>
>
>
> Best
>
> Kai-Uwe Bux

red floyd wrote:

> Kai-Uwe Bux wrote:
>> Anonymous wrote:
>>
>>> I ahve a vector of doubles taht I need to extract values from. I was
>>> just about to use the STL find() algo, but I have a couple of questions:
>>>
>>> first: can you specify the tolerance threshold to match on doubles?
>>> second: if yes, how may this be done (i.e. how many one specify the
>>> tolerance threshold for comparing doubles?)
>>
>> Note that find() has two signatures: one that takes a value to be found
>> and one that takes a unary predicate instead. Use the later and provide a
>> functor:
>
> Isn't the second form find_if(), not find()?

My bad. I had the page from the standard on my screen right in front of me.
Clearly, I just can't read :-(


>>
>> class is_approximately {
>>
>> double x;
>>
>> public:
>>
>> is_approximately ( double what )
>> : x ( what )
>> {}
>>
>> bool operator() ( double y ) const {
>> // return true if y is near x.
>> }
>>
>> };
>>
>> Now you can do:
>>
>> find( whatever.begin(), whatever.end(), is_approximately( 0.5 ) );

So that would be:

find_if( whatever.begin(), whatever.end(), is_approximately( 0.5 ) );


Thanks

Kai-Uwe Bux

Another option would also be std::find_first_of.

Regards
Klaas