Help in implementation (STL)

1 
#include <iostream>
#include<algorithm>
#include<bits/stdc++.h>
#include<string.h>
#define mod 1e18
using namespace std;
typedef long long int ll;
ll dx[4]={-1,0,0,1};
ll dy[4]={0,-1,1,0};
ll dijkstra(){
    ll n,i,j;
    cin>>n;
    ll mat[n][n];
    for(i=0;i<n;i++){
        for(j=0;j<n;j++){
            cin>>mat[i][j];
        }
    }
    ll dp[n][n];
    for(i=0;i<n;i++){
        for(j=0;j<n;j++){
            dp[i][j]=mod;
        }
    }
    dp[0][0]=mat[0][0];
    set<pair<ll,pair<ll,ll>>>q;
    q.insert(make_pair(mat[0][0],make_pair(0,0)));
    while(!q.empty()){
        ll x,y,len,p,q,u;
        set<pair<ll,pair<ll,ll>>> :: iterator it=q.begin();
        len=it.first;
        x=it.second.first;
        y=it.second.second;
        q.erase(q.begin());
        for(ll i=0;i<4;i++){
            p=x+dx[i];
            q=y+dy[i];
            if(p>=0 && q>=0 && p<n && q<n){
                u=dp[x][y]+mat[p][q];
                if(u<dp[p][q]){
                    q.erase({dp[p][q],{p,q}});
                    q.insert(make_pair(u,make_pair(p,q)));
                    dp[p][q]=u;
                }
            }
        }
    }
    return dp[n-1][n-1];
}
int main() {
	ll t;
	cin>>t;
	while(t--){
	    cout<<dijkstra()<<endl;
	}
	return 0;
}
2

It is giving me error
i.e.

 error: request for member ‘begin’ in ‘q’, which is of non-class type ‘ll {aka long long int}’
         set<pair<ll,pair<ll,ll>>> :: iterator it=q.begin();

Help @everule1

3

You’ve got two entirely different variables both named q:

line 26:

set<pair<ll,pair<ll,ll>>>q;

and

line 29:

ll x,y,len,p,q,u;

The first error occurs on line 29, where the latter q hides the former q - give the two variables different names.

Edit:

After you’ve done that, replace all:

it.

with

it->
3 Likes
4

Thank you so much…
Can you please tell when we use . and when we use ->

1 Like
5

a->b is, by convention, equivalent to (*a).b, so use -> whenever you would have to do (*a).b instead of a.b. Common situations include:

  1. When a is a pointer to an object of class type, and b is a member of the class; and
  2. When a is an iterator referring to an object of class type, and b is a member of the class (this is the situation occurring in your code, above).
3 Likes