Thread: /= doesn't do what you think it does?

What I did is use the at method where the index for F comes from an "untrusted" source (in very indirect ways from a map). Try this:

Code:

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

int main(){
    try {
    	vector<vector<float> > kF;
    	vector<float> F;
    	map <int, string> namesmap;

    	namesmap[1] = "one";
    	namesmap[2] = "two";
    	namesmap[3] = "three";
    	namesmap[4] = "four";

    	vector<map<string,int> > kFmap;
    	kFmap.resize(2);
    	kFmap[0][namesmap[1]] = 5;
    	kFmap[0][namesmap[2]] = 10;
    	kFmap[0][namesmap[3]] = 5;
    	kFmap[0][namesmap[4]] = 20;

    	kFmap[1][namesmap[1]] = 5;
    	kFmap[1][namesmap[2]] = 5;
    	kFmap[1][namesmap[3]] = 5;
    	kFmap[1][namesmap[4]] = 20;

    	for (unsigned int k = 0; k < 2; k++){
    		cout << "k = " << k << endl;
    		F.resize(6);
    		for (int i = 1; i <= 4; i++){
    			F.at(kFmap[k][namesmap[i]])++;
    		}
    		for (int i = 1; i <= 4; i++){
    			cout << kFmap[k][namesmap[i]] << " kFmap | ";
    			cout << F.at(kFmap[k][namesmap[i]]) << " | ";
    			F.at(kFmap[k][namesmap[i]]) /= (10); //bug
    			cout << F.at(kFmap[k][namesmap[i]]) << " | ";
    			cout << endl;
    		} cout << endl;
    		kF.push_back(F);
    		F.clear();
    	}
    }
    catch (exception& e) {
        cout << e.what() << '\n';
    }
	return 0;
}

What is the reasoning behind this? Can you tell me about untrusted sources? I'd like to know how this stuff works...

Edit: that didn't do it for me, I get vector::_M_range_check error. And when I try it in my main program I don't even get that - just the same faulty results!

Originally Posted by Salem

If you extract this code, put a minimal support environment around it, and it works, then you can be pretty sure the real problem is somewhere else.

Indeed, hence the idea is not "to extend it to wider sets", but to use it to determine if the problem is in that portion of code, or likely to be elsewhere.

Originally Posted by elninio

What is the reasoning behind this? Can you tell me about untrusted sources? I'd like to know how this stuff works...

I think anon made the same mistake as me: talking too much

Here "untrusted" merely means that you cannot be sure that the index provided by the "source" is a valid index for use with the vector. You can either perform a check yourself (compare the index with the vector's size()), or use at() with the index, upon which a std::out_of_range exception will be thrown if the index is invalid (out of range).

Originally Posted by elninio

Edit: that didn't do it for me, I get vector::_M_range_check error.

That means that an exception was thrown, and you are using an index that is out of range (i.e., trying to access the vector out of bounds).

It looks that you are trying to use the values in the map, such as 10 and 20, to index into a vector that has only size 6. Your logic doesn't hold.

By "untrusted" I meant a value that doesn't come from an obvious place (such as a loop counter) and that isn't validated.

There's no such thing as C++99. There's C99, which is completely irrevelant to your problem.

Stop seeking the problem with the compiler. It does not contain bugs that any application that is not extremely complex or irregular will uncover.

One example of a possible out of bounds access is this:

Code:

for (int i = 1; i <= 4; i++){
    F.at(kFmap[k][namesmap[i]])++;
}

kFmap only has 2 elements, but you access kFmap[2], kFmap[3] and kFmap[4].

Originally Posted by laserlight

One example of a possible out of bounds access is this:

Code:

for (int i = 1; i <= 4; i++){
    F.at(kFmap[k][namesmap[i]])++;
}

kFmap only has 2 elements, but you access kFmap[2], kFmap[3] and kFmap[4].

Yep, it looks to me like that explains the problem too.