如何在C ++中创建向量的向量图？

Question

I'm trying to make a map containing vectors containing vectors but I am getting an error when trying to access an element.

This is my code:

#include <iostream>
#include <map>
#include <vector>
#include <string>

using namespace std;

int main()
{
    map<int, vector<vector<string>>> m;
    m[0][0].push_back("hi");
    return 0;
}

You can try it here: https://onlinegdb.com/HytLmjQJV

The error I'm getting is in Visual Studio:

Debug assertion failed.Program : Expression:vector subscript out of range.

Answer 1

m[0] is an empty vector in your code so you cannot access its first element (m[0][0]). You need to create a vector inside, eg

m[0].push_back({});
m[0][0].push_back("hi");

Answer 2

The "problem" here is that you have 3 nested containers, so you need to add values to all of them to store your "hi" value.
When you do this m[0][0].push_back("hi"); you are basically accessing the position 0 of each vector inside the map , but you didn't set any values in them, they are empty.

I will first break it down to better explain.

With nested containers, like map<int, vector<vector<string>>> , the easiest way to understand how they work is to break it down.

map<int, vector<vector<string>>> m;

vector<string> inner; // Inner vector
vector<vector<string>> outer; // Outer vector


inner.push_back("Hi"); // Inner vector value
outer.push_back(inner); // Outer vector value

    // Map holds pairs
m.insert(pair<int, vector<vector<string>>>(10, outer));

    // Accessing data
cout << m[10][0][0] << endl;

Output

> Hi

Simplifying

That said you can easily and more cleanly write something like this.

m.insert({20, {{"Hi2"}}});

    // Accessing data
cout << m[20][0][0] << endl << endl; 

Output

> Hi2 

Where the first set of curly braces, {20, {...}} , is the pair that is inserted into the map . Think of it as the map only caring about a pair of an int and a vector .
Then you have that vector , outer curly braces {{...}} , knowing it holds another vector .
This vector , inner curly braces {""} , knows it holds strings.

Essentially with the curly braces we are doing the same as described above, creating separate values for each of the containers, a pair and two vectors.

Extending

Then you can easily scale it up with more data.

m.insert({25, {{"Hi2", "Hi3", "Hi4"}, {"By1", "By2"}}});

    // Accessing data
cout << m[25][0][0] << endl;
cout << m[25][0][1] << endl;
cout << m[25][0][2] << endl << endl;

cout << m[25][1][0] << endl;
cout << m[25][1][1] << endl;

Output

> Hi2 
> Hi3
> Hi4 
>
> By1
> By2

Note

You can check the project here to see it working.
Also in c++ a map doesn't have a push_back() but if you want something similar to it you can use emplace() and do something like this m.emplace(20, outer) , although it doesn't work with the shorter versions with the curly braces.

Answer 3

While you are trying to add to this vector properly, you haven't added anything to your vector. You're attempting to grab something that isn't there.

You'll need to put something in that first vector.

m[0].pushback();
m[0][0].pushback("hi");

alternatively, you could create the vector that you're going keep inside that first vector, first.

vector<string> vec1;
vec1.pushback("hi");

m[0].pushback(vec1);