pointer mayhem

Hi.

I'm trying to understand how pointers work. I have an algorithms implemented in two ways: one using pointers, the other not so much. The thing is, the second implementation works, the first doesn't and I'm not sure why.

Here's my main (IN_FILE, N_DATA and E_GOOD are globally defined variables: name of an input-file, 1000, and the exit code):

Code:

int main(const int argc, const char *argv){
	ifstream fin(IN_FILE);
	vector< Supplier >* supps = new vector< Supplier > (N_DATA);
	
	for(int i = 0; i < N_DATA; ++i)
		fin >> supps->at(i).companyID >> supps->at(i).price;
	
	printSuppliers( &selectionSort(*supps), 2);
	supps = sort(supps);
	cout << "<--------------------------->" << endl;
	printSuppliers(supps, 2);
	
	return E_GOOD;
}

Here's the pointer based sorting:

Code:

vector< Supplier > * sort(vector< Supplier > *v){
	int i, j, min;

	for (i = 0; i < v->size() - 1; i++){
		min = i;
		for (j = i+1; j < v->size(); j++){
			if (v->at(i).price < v->at(min).price){          
				min = j;
			}
		}
		swap(v->at(i), v->at(min));
	}
	
	return v;
}

and the non-pointer based:

Code:

vector<Supplier> selectionSort(vector<Supplier> a){
	int i, j, min;

	for (i = 0; i < a.size() - 1; i++){
		min = i;
		for (j = i+1; j < a.size(); j++){
			if (a[j].price < a[min].price){          
				min = j;
			}
		}
		swap(a[i], a[min]);
	}
	
	return a;
}

here's the printing function:

Code:

void printSuppliers(const vector< Supplier > *v, int n){
	for(int i = 0; i < n; i++){
		cout 
			<< "Supplier: " << i << endl
			<< "\tID: " << v->at(i).companyID << endl
			<< "\tPrice: " << v->at(i).price << endl << endl
			;
	}
}

My first output (from the dereferenced sorting) is in correct order, but the second output is unsorted:

Code:

Supplier: 0
        ID: C956
        Price: 828

Supplier: 1
        ID: C38
        Price: 893

<--------------------------->
Supplier: 0
        ID: C0
        Price: 1869

Supplier: 1
        ID: C1
        Price: 1728

Anyone care to help me understand why this is happening?

Code:

>			if (v->at(i).price < v->at(min).price){

Shouldn't that be j?

Code:

			if (v->at(j).price < v->at(min).price){

Originally Posted by swoopy

Code:

>			if (v->at(i).price < v->at(min).price){

Shouldn't that be j?

Code:

			if (v->at(j).price < v->at(min).price){

...yes...yes it should. Thanks for pointing that out, and boy do I feel stupid for missing that. At least I know that my understanding of pointers isn't drastically off.

There is no reason really to allocate the vector itself dynamically (using new), and the usual way to pass it to functions is via references.

For example

Code:

void selectionSort(vector<Supplier>& a)

This lets you modify the vector in the function.

I can see that you want selectionSort to return the vector to be used as an argument for another function. It is safe to return a reference to the parameter that you passed in by reference. Thus:

Code:

vector<Supplier>& selectionSort(vector<Supplier>& a)

Your non-pointer version is not very efficient: you pass a copy of the vector and you return a copy.