Searching in vectors of pairs - c++

Question

I have a 2d vector of pairs and I would like get in which row is a given pair.
I tried to iterate through all the elements but I didn't succeed.
Here is the adjacency matrix of an undirected, weighted graph:
vector<pair<int,int >> adj[20];
for (int i = 1; i <= nodes; i++){
f >> x >> y >> weight;
adj[x].push_back(make_pair(weight, y));
adj[y].push_back(make_pair(weight, x));
}
And it looks like something like this:
(4,2),(5,3)
(4,1),(6,3)
(5,1),(6,2)
I am trying to find in which row is the pair (6,2)
typedef pair<int, int>p;
p pair= make_pair(6, 2);
cout << which_row(adj, pair);
Here is what I tried first:
int which_row(vector<p>adj[20], p pair) {
vector <pair<int, int>> ::iterator it;
for (int i = 0; i < adj->size(); i++) {
for (int j = 0; j < adj[i].size(); i++)
if (pair.first == adj[i][j].first && pair.second == adj[i][j].second)
return i;
}
return -1;
}
but it does not step in the for loop.
Second:
int which_row(vector<p>adj[20], p pair) {
vector <pair<int, int>> ::iterator it;
for (int i = 0; i <= adj->size(); i++) {
for (it = adj[i].begin(); it != adj[i].end(); it++)
if (pair.first == it->first && pair.second == it->second)
return i;
}
}
But it does not step in the second for loop.
What can I do?

Answer 1

Since we can infer your x and y inputs from the pairs you gave us, you never push anything into adj[0]. adj->size() is the same as adj[0].size(). You never pushed anything into this so it will be 0. It seems you want to go over every row? Each row contains a vector? Each vector contains some pairs? Make adj a vector<vector<pair<int, int>>> so you can access adj.size(). That's likely what you want. Or iterate from i = 0; i < 20; i++ to go over each row.
Change your iteration to:
int which_row(vector<p>adj[20], p pair) {
for (int i = 0; i < 20; i++) { // <-- changed to < 20 (not <= 20)
for (auto it = adj[i].begin(); it != adj[i].end(); it++)
if (pair == *it) // <-- pairs compare as you'd expect, no need to compare each element
return i;
}
return -1; // need to return something if not found
}
Or we can take advantage of std::find in the <algorithm> library:
#include <algorithm>
int which_row(vector<p>adj[20], p pair) {
for (int i = 0; i < 20; i++) { // <-- changed to < 20 (not <= 20)
if (std::find(adj[i].begin(), adj[i].end(), pair) != adj[i].end())
return i;
}
return -1; // need to return something if not found
}
If you change your adj initialization to std::vector<std::vector<p>> adj(20); you can change your which_row to:
// in main or whatever:
// makes a vector of 20 default-initialized (empty) vector<p>
std::vector<std::vector<p>> adj(20);
// elsewhere:
int which_row(vector<vector<p>> adj, p pair) {
for (auto i = 0; i < adj.size(); i++) { // <-- adj is a vector now
if (std::find(adj[i].begin(), adj[i].end(), pair) != adj[i].end())
return i;
}
}
return -1; // need to return something if not found
}