Thread: use of private members

Here's another clunky experiment that don't work right:

Code:

#include <iostream>
#include <vector>
using namespace std;

class fibvec  {
	vector<int> series;
	unsigned int len;
	public:
		fibvec() {
			series.reserve(2);
			series[0] = series[1] = 1;
			len = 2;
		}
		int digit (int pos) {
			int i, j = 0;
			if (pos >= len) {
				int nlen = pos+1;
				series.reserve(nlen);
				for (i=len;i<nlen;i++)
					series[i] = series[i-1] + series[i-2];
				len = nlen;
			}
			return series[pos];
		}
};


int main() {
	int i;
	fibvec eg;

	for (i=5;i<10;i++) cout << eg.digit(i) << '\n';

	return 0;
}

It doesn't work because within fibvec.digit values for "series" are not kept -- everything is always 0. How can I render this "static" or something? Eg, in perl, a constructor returns a special reference to a hash that contains any number and type of elements (it's special in the sense that the class methods can then be used on it). But I cannot "return" anything with a C++ constructor -- do I have to make fibvec derived from vector (I would assume not, since apparently deriving things from the standard containers is a bad practice).

Yeah, you should be using resize() instead of reserve(), and you do not need len. However, suppose you wanted to create a vector of two ints, both set to 1. There is an easy way using an appropriate constructor:

Code:

std::vector<int> series(2, 1); // 2 ints, both set to 1.

You can get this same effect for a member variable using the constructor initialisation list:

Code:

fibvec() : series(2, 1) {}

Originally Posted by EVOEx

I have no idea what the problem is you're describing. But one problem is that you'll need resize rather than reserve. Also, why even have a length, isn't that what the .size() function on a vector is for? Of course, that only works if you use resize rather than reserve.

Whew! Using resize instead of reserve made the difference (why?). "This informs the vector of a planned increase in size," -- does that mean you would call reserve then resize? So what is the purpose of reserve?


The len got left in there from an earlier version where I was trying to work around the problem caused by reserve, by using an init method...blah blah blah.

Thanks.

Originally Posted by laserlight

Yeah, you should be using resize() instead of reserve(), and you do not need len. However, suppose you wanted to create a vector of two ints, both set to 1. There is an easy way using an appropriate constructor:

Okay, but I cannot do that here:

Code:

class fibvec  {
	vector<int> series(2,1);   

for whatever reason.

[edit] alright, I get it: constructor initialisation list

Originally Posted by EVOEx

Reserve is useless except for optimization.

There is another use: to avoid invalidation of iterators, pointers and references due to a reallocation.

Hmmm. This seems to work then:

Code:

				int *ref;
				series.reserve(nlen);
				for (i=series.size();i<nlen;i++) {
					ref = &series.back();
					series.push_back(*ref + *(--ref));
				}

Altho I would assume it is less optimal than just resizing and using [].

Originally Posted by EVOEx

I don't think this line is valid, as the behaviour, I think, is undefined:

Yeah, hadn't thought about that. I do get this:
warning: operation on ‘ref’ may be undefined
But thot it was about the range...

Well, I ended up with something pretty close to that, here's the whole shebang:

Code:

class fibvec  {
	vector<int> series;
	public:
		fibvec() : series(2,1) {}
		int digit (unsigned int pos) {
			if (pos >= series.size()) {
				int start = series.size(), nlen = pos+1;
				series.resize(nlen);
				for (;start<nlen;start++) series[start] = series[start-1] + series[start-2];
			}
			return series[pos];
		}
};

Originally Posted by _Mike

For any (pos) it outputs the fibonacci number for (pos+1)

That's just my ignorance of math -- a set in math starts with an index of 1, not 0, right?