STL vector

[gizzmo]

Hi,

I am trying to do the following with the STL vector. Say we have two vectors, both of the same dimension M. I want to copy part of a vector B into the other vector A, up to the element k-1. And the remaining part replace with something else. Here is Matlab code that does that

A(1:k−1) = B(1:k−1);
A(k) = B(k) − 1;
A(k+1:M) = 0;

How can I do this in c++? I'm trying to do this using iterators, but I don't know how to access single element instead of a range of the vector.

[helios]

http://www.cplusplus.com/reference/algorithm/copy/
http://www.cplusplus.com/reference/algorithm/fill/

[Mathhead200]

//like this?

vector<double> a, b;
... //fill a and b
for( int i = 0; i < k; i++ )
  a[i] = b[i]; //copy from b to a
for( int i = k; i < a.size(); i++ )
  a[i] = 0; //assign the remainder of a to 0 (or whatever) 

[gizzmo]

I keep reading about how it is not a good practice to iterate over STL containers like Mathhead200 wrote. Is there more elegant way to do this?

[Mathhead200]

Ug... This is why I just reinvent the standard library as I need it.

http://cplusplus.com/reference/stl/vector
http://cplusplus.com/reference/std/iterator
and what helios said: http://cplusplus.com/forum/general/43408/#msg234571

[firedraco]

I keep reading about how it is not a good practice to iterate over STL containers like Mathhead200 wrote. Is there more elegant way to do this?

Sort of...but he's iterating over a vector, so it's ok as long as you don't ever need to change the container type. If you want to be the most general...do what helios said.

[gizzmo]

Yes, I want to be able to change the container from vector to something else (don't know to what at the moment). That is why I don't want to do this in such a simple manner as Mathhead200 described. copy and fill work fine and it is easy to do lines 1 and 3 but I am still puzzled by line 2.

Next thing would be the question what kind of container does what I'm planning to do. For now, in the vector, I store some objects. I'd like to be able to search for a particular object. The search needs to be fast (binary search is preferable). To do that I need some hashing function (I saw that STL has some hashing algorithms. Will they hash objects?) than I need to sort those tags and than I'll be able to do the binary search. What container should I use?

[helios]

I am still puzzled by line 2

Simply a[k]=b[k]-1.
Alternatively,

std::copy(b.begin(),b.begin()+k,a.begin());
a[k]--;

[Athar]

The search needs to be fast (binary search is preferable). To do that I need some hashing function (I saw that STL has some hashing algorithms. Will they hash objects?) than I need to sort those tags and than I'll be able to do the binary search. What container should I use?

std::set should be what you're looking for. It requires that the contained objects can be ordered (i.e. they need an operator<). There's also std::[tr1::]unordered_set, which uses hashing instead.

[gizzmo]

OK. I rewrote my code so that I do not need what I wrote in my first post. But I need help with another problem. Say I want to store objects BV of class BaseVector in vector container. First object I generate with BV.first() and all next objects with BV.next() which returns true/false depending on how many objects I have created. The code below doesn't work. All elements of the vector are the same.

	BaseVector BV;
	std::vector<BaseVector>::iterator state;
	resize(1000);
	BV.first();
	state = begin();
	*state = BV;
	while (BV.next())
	{
		++state;
		*state = BV;
	}

[helios]

...What?

[gizzmo]

I take from this reaction that what I wrote should work.

[helios]

It was meant to convey "I found no interpretation of what you just said that made any sense whatsoever".

[gizzmo]

I did not define operator=. Now it works.
helios: Yes, I know it is hard to discuss something that was taken out of context.