CODEWITHSUNDEEP
c++vector
John Smith
John Smith

Reputation: 429

How to implement 2D vector array?

I'm using the vector class in the STL library for the first time. How should I add to a specific row of the vector array?

struct x{
     vector <vector <int> > v;
     int row;
 };

vector< int* > my ints;
int add;

if i wanted to add to first row of v with the first pointer of integers, could I do 

myints[0]->v[myints[0]->row].push_back(add);

Is this method fine to create a 2 D vector of vector ints where each row could potentially be of different length (i.e. have a different number of columns)?

Upvotes: 38

Views: 275059

Answers (9)

Abhishek Kumar
Abhishek Kumar

Reputation: 53

vector<int> adj[n]; // where n is number of rows in 2d vector.

Upvotes: 1

Satakshi Pandey
Satakshi Pandey

Reputation: 432

vector<vector> matrix(row, vector(col, 0));

This will initialize a 2D vector of rows=row and columns = col with all initial values as 0. No need to initialize and use resize.

Since the vector is initialized with size, you can use "[]" operator as in array to modify the vector.

matrix[x][y] = 2;

Upvotes: 2

rashedcs
rashedcs

Reputation: 3725

We can easily use vector as 2d array. We use resize() method for this purpose. The code below can be helpful to understand this issue.

Code Snippet :

#include<bits/stdc++.h>
using namespace std;

int main()
{
    ios::sync_with_stdio(false);
    int row, col;
    cin>>row>>col;

    vector <vector<int>> v;
    v.resize(col,vector<int>(row));

    //v = {{1,2,3}, {4,5,6}, {7,8,9}}; 

    /** input from use **/
    for(int i=0; i<row; i++)
    {
       for(int j=0; j<col; j++)
       {
          cin>>v[i][j];
       }
    }

    for(int i=0;i<row; i++)
    {
       for(int j=0;j<col;j++)
       {
          cout<<v[i][j]<<" ";
       }
    }
    return 0;
}

Upvotes: 8

Aditya Goel
Aditya Goel

Reputation: 381

Just use the following methods to create a 2-D vector.

int rows, columns;        

// . . .

vector < vector < int > > Matrix(rows, vector< int >(columns,0));

OR 

vector < vector < int > > Matrix;

Matrix.assign(rows, vector < int >(columns, 0));

//Do your stuff here...

This will create a Matrix of size rows * columns and initializes it with zeros because we are passing a zero(0) as a second argument in the constructor i.e vector < int > (columns, 0).

Upvotes: 18

Madhurya Gandi
Madhurya Gandi

Reputation: 892

//initialize the 2D vector first

vector<vector<int>> matrix;

//initialize the 1D vector you would like to insert into matrix

vector<int> row;

//initializing row with values

row.push_back(val1);

row.push_back(val2);

//now inserting values into matrix

matrix.push_back(row);

//output- [[val1,val2]]

Upvotes: 12

Ridowan Ahmed
Ridowan Ahmed

Reputation: 95

I use this piece of code . works fine for me .copy it and run on your computer. you'll understand by yourself .

#include <iostream>
#include <vector>
using namespace std;
int main()
{
    vector <vector <int> > matrix;
    size_t row=3 , col=3 ;
    for(int i=0,cnt=1 ; i<row ; i++)
    {
        for(int j=0 ; j<col ; j++)
        {
            vector <int> colVector ;
            matrix.push_back(colVector) ;
            matrix.at(i).push_back(cnt++) ;
        }
    }
    matrix.at(1).at(1) = 0;     //matrix.at(columns).at(rows) = intValue 
    //printing all elements
    for(int i=0,cnt=1 ; i<row ; i++)
    {
        for(int j=0 ; j<col ; j++)
        {
            cout<<matrix[i][j] <<" " ;
        }
        cout<<endl ;
    }
}

Upvotes: 2

Varun Garg
Varun Garg

Reputation: 595

Another way to define a 2-d vector is to declare a vector of pair's. 

 vector < pair<int,int> > v;

**To insert values**
 cin >> x >>y;
 v.push_back(make_pair(x,y));

**Retrieve Values**
 i=0 to size(v)
 x=v[i].first;
 y=v[i].second;

For 3-d vectors take a look at tuple and make_tuple.

Upvotes: 2

Richard
Richard

Reputation: 8758

If you know the (maximum) number of rows and columns beforehand, you can use resize() to initialize a vector of vectors and then modify (and access) elements with operator[]. Example:

int no_of_cols = 5;
int no_of_rows = 10;
int initial_value = 0;

std::vector<std::vector<int>> matrix;
matrix.resize(no_of_rows, std::vector<int>(no_of_cols, initial_value));

// Read from matrix.
int value = matrix[1][2];

// Save to matrix.
matrix[3][1] = 5;

Another possibility is to use just one vector and split the id in several variables, access like vector[(row * columns) + column].

Upvotes: 60

Moritz
Moritz

Reputation: 4751

I'm not exactly sure what the problem is, as your example code has several errors and doesn't really make it clear what you're trying to do. But here's how you add to a specific row of a 2D vector:

// declare 2D vector
vector< vector<int> > myVector;
// make new row (arbitrary example)
vector<int> myRow(1,5);
myVector.push_back(myRow);
// add element to row
myVector[0].push_back(1);

Does this answer your question? If not, could you try to be more specific as to what you are having trouble with?

Upvotes: 77

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