C++ adding an extra vector stops code from working

timhardy · 08-17-2005, 05:45 PM

I'm almost definitely being an idiot but can anyone please explain why adding another vector<int> to the following code would stop it from working.

Working code:

Code:

#include <iostream>
#include <vector>

using std::cin;		using std::cout;
using std::endl;	using std::vector;

int main ()
{
	//generate list of cubes separated into collections by digit
	vector<int> cubes5;
	vector<int> cubes4;
	vector<int> cubes3;
	int cube;
	int root = 1;
	while (cube < 100000){
		cube = root*root*root;
		if (cube > 100 && cube < 1000)
			cubes3.push_back(cube);
		if (cube > 1000 && cube < 10000)
			cubes4.push_back(cube);
		if (cube > 10000 && cube < 100000)
			cubes5.push_back(cube);
		++root;
		//output for debugging purposes
		cout << root << " " << cube << endl;
		}
	return 0;
}

This works perfectly (please note this is the start of something longer chopped down to make a shorter, working example for this post)

However, if I declare another vector<int> at the beginning, the code compiles without any warnings but when run outputs nothing.

Problematic code:

Code:

#include <iostream>
#include <vector>

using std::cin;		using std::cout;
using std::endl;	using std::vector;

int main ()
{
	//generate list of cubes separated into collections by digit
	vector<int> cubes5;
	vector<int> cubes4;
	vector<int> cubes3;
	vector<int> trouble;
	int cube;
	int root = 1;
	while (cube < 100000){
		cube = root*root*root;
		if (cube > 100 && cube < 1000)
			cubes3.push_back(cube);
		if (cube > 1000 && cube < 10000)
			cubes4.push_back(cube);
		if (cube > 10000 && cube < 100000)
			cubes5.push_back(cube);
		++root;
		//output for debugging purposes
		cout << root << " " << cube << endl;
		}
	return 0;
}

I am sure there is a very simple explanation as to why declaring another vector<int> would have this effect but so far it escapes me.

Any advice or pointers would be most appreciated. Thank you.

timhardy · 08-17-2005, 06:05 PM

Extra info:

I'm guessing that the machine is running out of memory when it automatically allocates a max size for each vector although I don't know why it fails silently rather than warning me of this.

If I declare each vector with an explicit size, eg:

Code:

vector<int> cubes5(100);
	vector<int> cubes4(100);
	vector<int> cubes3(100);
	vector<int> trouble(100);

then everything works as expected.

alpha_foobar · 08-17-2005, 06:56 PM

Cool problem.

The issue is that you haven't initialised the cube variable prior to entering the loop (and therefor the conditional is evaluated based on whatever information is in memory at the time of execution!)

It is therefor just a fluke that it run without the 4th vector. It doesn't run on my machine without the 4th vector due to the data in memory.

The reason it does and then doesn't on your machine is obviously that the value in memory without the 4th vector is less than 100000, but when you add the 4th vector, the position in memory of cube moves and it uses some random data that happens to be greater than 100000.

Try outputting the value of cube before the loop to see for youself.

and to fix:
int cube = 0;

alpha_foobar · 08-17-2005, 06:59 PM

Quote:

Originally posted by timhardy
Extra info:

I'm guessing that the machine is running out of memory when it automatically allocates a max size for each vector although I don't know why it fails silently rather than warning me of this.

If I declare each vector with an explicit size, eg:

Code:

vector<int> cubes5(100); vector<int> cubes4(100); vector<int> cubes3(100); vector<int> trouble(100);

then everything works as expected.

This could work, again if it flukes a memory value that is less than the initial condition. But not on my machine... again try printing cube before the loop to check the initial value of cube.

timhardy · 08-17-2005, 07:19 PM

alpha_foobar thank you so much for that.

What's the emoticon for slapping myself on the head for being so dumb?

As you say, it was pure luck that the code was working originally but I assumed it was correct and that I'd stumbled upon a bizarre C++ quirk instead of checking it properly when things stopped working.... There's a lesson there.

Thanks for your patient and detailed explanation. It all makes sense again now!