std::vector<>::iterator and recognizable bounds

[Rune Allnor]

Hi folks.

I have a function that takes an element in a vector as
argument. The naive interface goes as

float computeSomething(const std::vector<float>& v, size_t i)
{
size_t j = i-1;
size_t k = i+1;

if (j<0){/* Handle start boundary condition */}
if (k>=v.size()){/* Handle terminal boundary condition */}
}

I would like to implement this in terms of iterators.
But it seems one needs several iterators to get this
done:

typedef std::vector<float>::const_iterator pos;

float computeSomething(pos i, pos b, pos e)
// Call function with b=v.begin(), e=v.end()
{
pos j = i; --j;
pos k =i; ++k;

if (i==b){/* Handle start boundary condition */}
if (k==e){/* Handle terminal boundary condition */}
}

So my question is: Do there exist generally recognizable
invalid values for the std::vector<>::iterators? Like std::string::npos
() ?

If yes I could make a greatly simplified (as well as
safer) interface to the function:

float computeSomething(pos i)
{
pos j=i;--j;
pos k = i;++k;

if(j==std::vector::npos){}
if(k==std::vector::npos) {}
}

But I can't find any analogy to std::string::npos defined for
vectors. Does such a value exist?

Rune

 

[KK]

Hi folks.

I have a function that takes an element in a vector as
argument. The naive interface goes as

float computeSomething(const std::vector<float>& v, size_t i)
{
   size_t j = i-1;
   size_t k = i+1;

   if (j<0){/* Handle start boundary condition */}
   if (k>=v.size()){/* Handle terminal boundary condition */}

}

I would like to implement this in terms of iterators.
But it seems one needs several iterators to get this
done:

typedef std::vector<float>::const_iterator pos;

float computeSomething(pos i, pos b, pos e)
// Call function with b=v.begin(), e=v.end()
{
    pos j = i; --j;
    pos k =i; ++k;

    if (i==b){/* Handle start boundary condition */}
    if (k==e){/* Handle terminal boundary condition */}

}

So my question is: Do there exist generally recognizable
invalid values for the std::vector<>::iterators? Like std::string::npos
() ?

If yes I could make a greatly simplified (as well as
safer) interface to the function:

float computeSomething(pos i)
{
    pos j=i;--j;
    pos k = i;++k;

    if(j==std::vector::npos){}
    if(k==std::vector::npos) {}

}

But I can't find any analogy to std::string::npos defined for
vectors. Does such a value exist?

Rune

/* apply const attribute where applicable */
float computeSomething(vector<float> vec, vector<float>::iterator
iter)
{
vector<float>::iterator fIter = iter, bIter =iter;
if (iter != vec.begin() || iter != vec.end())
{
advance(fIter,+1);
advance(bIter,-1);
}
else {/* fill this up */ }
if (bIter == vec.begin())
{/* Handle start boundary condition */
}
if (fIter == vec.end())
{/* Handle terminal boundary condition */
}
}

 

[James Kanze]

I have a function that takes an element in a vector as
argument. The naive interface goes as

float computeSomething(const std::vector<float>& v, size_t i)
{
size_t j = i-1;
size_t k = i+1;

if (j<0){/* Handle start boundary condition */}
if (k>=v.size()){/* Handle terminal boundary condition */}
}

I would like to implement this in terms of iterators. But it
seems one needs several iterators to get this done:

Yep. That's part of the design of iterators---never use one
parameter when two can do. It makes a lot of things very
painful.

typedef std::vector<float>::const_iterator pos;

float computeSomething(pos i, pos b, pos e)
// Call function with b=v.begin(), e=v.end()
{
pos j = i; --j;
pos k =i; ++k;

if (i==b){/* Handle start boundary condition */}
if (k==e){/* Handle terminal boundary condition */}
}

That's more or less an exact equivalent of your original
version, yes. Now if you make your function a template, it will
work with any sequence which is defined by bidirectional
iterators.

So my question is: Do there exist generally recognizable
invalid values for the std::vector<>::iterators? Like
std::string::npos () ?
No.

If yes I could make a greatly simplified (as well as
safer) interface to the function:

float computeSomething(pos i)
{
pos j=i;--j;
pos k = i;++k;

if(j==std::vector::npos){}
if(k==std::vector::npos) {}
}

In other words, you want the iterator to encapsulate both the
current position and both upper and lower bounds somehow. It's
certainly possible, but I'm not sure what you'd gain by it.

But I can't find any analogy to std::string::npos defined for
vectors. Does such a value exist?

No, although you could probably define one arbitrarily.
Something like static_cast< size_t >( -1 ). Like
std::string::npos, it would be an index, however, and not an
iterator.