Assigning a vector from an array (pointer)

Question

Is there a standard way to accomplish this that is better than a for loop?

If I had an array type supposedly I can do this:

double d_array[] = { 1.0, 2.0, 3.0 };
std::vector<double> d_vector(d_array, d_array+3);

But I can't do this when I only have a double * and an int indicating its length.

Edit: Actually, I think I actually can do this. The error messages are quite a handful, though, if you get your type parameters wrong (which is why it didn't work for me at first).

It seems to work now. When trying to do this with mis-matched types the error messages are incredibly verbose although not entirely cryptic. — Steven Lu
– Steven Lu, Commented Mar 1, 2012 at 4:44
You might wanna look at this question stackoverflow.com/questions/231491/… — bisarch
– bisarch, Commented Mar 1, 2012 at 5:10

Steven Lu · Accepted Answer · 2016-10-06 20:39:15Z

5

Of course you can do it the same way

int length;
double *d;
//allocate memory and data to pointer
std::vector<double> d_vector(d, d+length);

edited Oct 6, 2016 at 20:39

Steven Lu

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answered Mar 1, 2012 at 4:44

ken

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Matt · Accepted Answer · 2012-03-01 04:45:08Z

1

Yep. You should be able to do this. The relevant constructor is template <class InputIterator> vector ( InputIterator first, InputIterator last, const Allocator& = Allocator() ); - and pointers are iterators too (since you can do integer arithmetic on them).

That being said, all that the vector is doing is iterating over the elements of d_array, so don't expect the performance to be significantly better than the for loop version.

answered Mar 1, 2012 at 4:45

Matt

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4 Comments

Steven Lu Over a year ago

It's really quite cool that it's able to transparently use pointers like this. That this could in any way have better performance than the for loop version is incredible (though not really of course because a naive for loop would cause the vector to resize itself). Anyway, I like things like this because it helps cuts down on bugs.

Wyzard Over a year ago

It may be smart enough to subtract the two pointers to find out how many values it'll be looping over, and allocate enough space for them initially. Default-constructing a vector and inserting the values yourself may require it to reallocate as the vector grows.

Steven Lu Over a year ago

Right, in my naive implementation I needed to figure out whether i wanted to use resize or reserve... Using the ctor or assign with pointers is just so much neater.

Matt Over a year ago

It's possible it might be smart enough to do a pre-allocation, but it's not guaranteed that it can even make an educated guess. It just takes an InputIterator, and for those there's no guarantee that the iteration is contiguous. You can create an InputIterator that jumps ahead an arbitrary amount - say, 4 * sizeof(double), whatever you want - though it would certainly be possible to optimize, it's a lot of special casing to do so. (though, if I were implementing the standard, I would make that "guess" by pre-allocating the delta and trusting it to resize if my guess was wrong...)

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