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vector

Initialising vector​

Different ways to initialise a Vector​

vector<int> v1;

vector<int> v2(n);

vector<int> v3(n, 2);  //{2,2,2,2,2,2,2}

vector from vector​

vector<int> vec(vecName.begin(), vecName.end());
caution

Only problem in above code is it can only be used during Initialisation not to update existing vector

vector from deque​

vector<int> vec(dequeName.begin(), dequeName.end());
caution

Only problem in above code is it can only be used during Initialisation not to update existing vector

vector from set​

vector<int> vec(setName.begin(), setName.end());
caution

Only problem in above code is it can only be used during Initialisation not to update existing vector

Copy the vector or deque elements to another vector or deque using insert()​

info
// vector "vec" is given

deque<int> dq; // empty "deque" initialised
dq.insert(dq.end(), vec.begin(), vec.end());

deque to vector​

// deque "dq" is given

vector<int> vec; // empty "deque" initialised
vec.insert(vec.end(), dq.begin(), dq.end());

array to vector​

:::

Taking unknown number of User Inputs for one vector​

info
vector<int> v;
int temp;
vector<int> v;

while (cin >> temp)
{
    v.emplace_back(temp);
}

Taking input for multiple vectors from multiple single lines​

info

Using istringstream​

/* Inputs are
10 22 2  1 5 54       16651 6516 6165 2
65  6565   65  65 56 6565
*/
// #include <sstream>   ~ "<sstream>" contains "istringstream"
// #include <string>
vector<int> v1;
vector<int> v2;

int tmp;    // "tmp" will be used to store the "integers" from the "istingstream"
string str1, str2;  // declaring the "strings" where to "store the lines"

// Initialising the string👇
getline(cin, str1); // "str1" becomes 👉  [ 10 22 2  1 5 54       16651 6516 6165 2 ]
getline(cin, str2); // "str2" becomes 👉  [   65 6565 65 65 56 6565 ]

istringstream ss1(str1);    // 👉 ASSIGNING THE STRING "str1" to 'istringstream' "ss1"

// Giving the inputs to the vector "al1" // Initialising "al1"👇
while (ss1 >> tmp) {
    v1.emplace_back(tmp);
}   // "v1" becomes 👉  [  10 22 2 1 5 54 16651 6516 6165 2  ]

istringstream ss2(str2);    // 👉 ASSIGNING THE STRING "str2" to 'istringstream' "ss2"

// Giving the inputs to the vector "al2" // Initialising "al2"👇
while (ss2 >> tmp) {
    v2.emplace_back(tmp);
}   // "v2" becomes 👉  [  65 6565 65 65 56 6565  ]
  • Output is 👇
v1
10 22 2 1 5 54 16651 6516 6165 2
v2
65 6565 65 65 56 6565
caution

😱Single Input First, then vector input😱​

If the single inputs are given in the lines above the vector then must add :~ String line; -- getline(cin, line); where line will take the ending "" from the line of the sigle Input will before taking the NEXT LINE as input for vector.​

  • Otherwise the first vector v1 will only get "" i.e. NOTHING 🤯😡
// #include <bits/stdc++.h>
#include <algorithm>
#include <vector>
#include <climits>
#include <string>
#include <iostream>
#include <sstream>

using namespace std;

#define ll long long
#define endl "\n"

int main()
{
    vector<int> v1;
    vector<int> v2;

    int tmp;                 // "tmp" will be used to store the "integers" from the "istingstream"
    string str1, str2; // declaring the "strings" where to "store the lines"

    int num1;
    cin >> num1;

    cout << num1;
    cout << endl;

    string line;
    getline(cin, line);

    // Initialising the string👇
    getline(cin, str1); // "str1" becomes 👉  [ 10 22 2  1 5 54       16651 6516 6165 2 ]
    cout << "str1" << str1 << endl;

    getline(cin, str2); // "str2" becomes 👉  [   65 6565 65 65 56 6565 ]
    cout << "str2" << str2 << endl;

    istringstream ss1(str1); // 👉 ASSIGNING THE STRING "str1" to 'istringstream' "ss1"

    // Giving the inputs to the vector "al1" // Initialising "al1"👇
    while (ss1 >> tmp)
    {
        v1.emplace_back(tmp);
    } // "v1" becomes 👉    [  10 22 2 1 5 54 16651 6516 6165 2  ]

    istringstream ss2(str2); // 👉 ASSIGNING THE STRING "str2" to 'istringstream' "ss2"

    // Giving the inputs to the vector "al2" // Initialising "al2"👇
    while (ss2 >> tmp)
    {
        v2.emplace_back(tmp);
    } // "v2" becomes 👉    [  65 6565 65 65 56 6565  ]

    cout << "v1" << endl;
    for (auto el : v1)
    {
        cout << el << " ";
    }
    cout << endl;

    cout << "v2" << endl;
    for (auto el : v2)
    {
        cout << el << " ";
    }
    cout << endl;

    int num2;
    cin >> num2;
    cout << num2;
    return 0;
}
  • Output is👇
56
str1 10 22 2  1 5 54       16651 6516 6165 2
str2  65 6565 65 65 56 6565
v1
10 22 2 1 5 54 16651 6516 6165 2
v2
65 6565 65 65 56 6565
45
#include <iostream>
#include <vector>

using namespace std;

int main()
{
    //taking the vector size from the user
    int n;
    cin >> n;

//Different ways to initialise a Vector
    vector<int> v1;
    vector<int> v2(n);
    vector<int> v3(n, 2);  //{2,2,2,2,2,2,2}

//Ways to give values to the vector
    for (int i = 0; i < n; i++) {
        int val;
        cin >> val;
        v1.emplace_back(val);  //push_back can also be used
    }

    for (int i = 0; i < v2.size(); i++) {
        cin >> v2.at(i);
        //cin>>v2[i];   also can be used
    }

    //using iterator which I don't know
    //to be leart later using iterator and pointer


//WAYS TO PRINT THE VECTOR
    //my favourite method
    for (auto el : v1)
        cout << el << " ";

    cout << endl;

    for (int i = 0; i < n; i++) {
        cout << v1.at(i) << " ";
        //cout<<v1[i]<<" ";
    }


    //using iterator which I don't know
    //to be learnt later using iterator and pointer

    return 0;
}

Printing the vector​

//my favourite method
for (auto el : v1)
    cout << el << " ";

Printing 2D vector​

info
// vector< vector<int> > v
for (auto el : v) {
    for (auto el2 : el) {
        cout << el << " ";
    }
    cout << endl;
}

Using Iterator​

caution
to be added

2D vectors​

Initialising 2D vector​

info
  • Only Declaration
vector<vector<int>> v;
  • Initialisation with both ROW and COLUMN
vector<vector<int>> v(n, vector<int>(m));   // ROW = "n" ; COL = 'm'
vector<vector<int>> v(n, vector<int>(m, 0));   // ROW = "n" ; COL = 'm'
// Each Col of each Row will be initsialised with "0" from (m, 0) above

/*
0 0 0 0
0 0 0 0
0 0 0 0
*/

v.at(i).at(j) = temp;

  • Here all elements need to be replaced like this: ⏩ v.at(i).at(j) = temp;
  • Initialising with only ROW
vector<vector<int>> v(n);   // ROW = "n"

Taking user input with ROW & COL known​

info

Here ROW must be initialised as vector<vector<int>> v(row);​

Otherwise the code will show Runtime Error :~ SIGABRT ~ terminate called after throwing an instance of 'std::out_of_range' what(): vector::_M_range_check: __n (which is 0) >= this->size() (which is 0)

/* Input is:-
4
2 4 6 8 10 12
4 5 6 7 8 9
1 2 2 2 51 5
56 5 15 5 56 5
 */
int row, col = 6;   // "ROW" & "COLUMN" known
cin >> row;
cout << row << " " << col << endl;

vector<vector<int>> v(row); // Initialising 2D vector

for (int i = 0; i < row; i++)
{
    // v.at(i) == vector<int>(col); // NOT REQUIRED
    for (int j = 0; j < col; j++)
    {
        int tmp;
        cin >> tmp;
        (v.at(i)).emplace_back(tmp);
    }
}

// Printing "2D Vector"
for (auto el : v)    // "( vector<int> el : v )" can also be used, "auto" just makes it easier
{
    for (auto el2 : el)     // "( int el2 : el )"  can also be used, "auto" just makes it easier
    {
        cout << el2 << " ";
    }
    cout << endl;
}

Another method (But not a very good method)​

ROW need not be initialisedL:~ vector<vector<int>> v;

  • Creating a vector<int> tempVector; and taking the Input LINES into the tempVector
  • Then adding the v.emplace_back(tempVector); to the main vector v ; So that we will not need to call any index of vector v like v.at(i) because vector v is a 2D vector and to access the "COLUMN" part of 2D Vector "ROW" must be initialsised like vector<vector<int>> v(row);
/* Input is:-
4
2 4 6 8 10 12
4 5 6 7 8 9
1 2 2 2 51 5
56 5 15 5 56 5
 */
int row, col = 6;
cin >> row;
cout << row << " " << col << endl;

vector<vector<int>> v;

for (int i = 0; i < row; i++)
{
    vector<int> tempVector;
    for (int j = 0; j < col; j++)
    {
        int tmp;
        cin >> tmp;
        tempVector.emplace_back(tmp);
    }
    v.emplace_back(tempVector);
}

for (auto el : v)
{
    for (auto el2 : el)
    {
        cout << el2 << " ";
    }
    cout << endl;
}

Taking user input with COL unknown || Varying COLUMNs ~ sstream​

info
  • ROW is REQUIRED

Here ROW must be initialised as vector<vector<int>> v(row);​

Otherwise the code will show Runtime Error :~ SIGABRT ~ terminate called after throwing an instance of 'std::out_of_range' what(): vector::_M_range_check: __n (which is 0) >= this->size() (which is 0)

/* Input is:-
4
2 4 6 8 10 12
4 5 6 7 8 9
1 2 2 2 51 5
56 5 15 5 56 5
 */
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int row;    // "Column" not given
    cin >> row;
    // cout << row << endl;

    vector<vector<int>> v(row);

    string line;
    getline(cin, line);

    string str;
    int temp;

    for (int i = 0; i < row; i++)
    {
        getline(cin, str);
        istringstream ss(str);

        while (ss >> temp)
        {
            (v.at(i)).emplace_back(temp);
        }
        // istringstream.swap(ss);    /* 👉 It is used only when we want to "RESET" the "istringstream ss(str);" ; So that it can be used again and another variable of "istringstream ss(str);" wil not be needed */
        // 👆 But in this case we are using "FOR" or "WHILE" loops, so "istringstream ss(str);" gets "RESET" in each loop; Hence istringstream.swap(ss); is not required
    }

    for (auto el : v)
    {
        for (auto el2 : el)
        {
            cout << el2 << " ";
        }
        cout << endl;
    }

    return 0;
}

Taking user input with ROW unknown ~ istringstream​

info

ROW or COLUMN NOT NEEDED​

Using istringstream from <sstream> header​

ROW need not be initialisedL:~ vector<vector<int>> v; So it will without taking any ROW as INPUT

  • Creating a vector<int> tempVector; and taking the Input LINES into the tempVector
  • Then adding the v.emplace_back(tempVector); to the main vector v ; So that we will not need to call any index of vector v like v.at(i) because vector v is a 2D vector and to access the "COLUMN" part of 2D Vector "ROW" must be initialsised like vector<vector<int>> v(row);
  • ☠️ If the single inputs are given in the lines above the vector then must add :~ String line; -- getline(cin, line); where line will take the ending "" from the line of the sigle Input will before taking the NEXT LINE as input for vector. Otherwise the first vector v1 will only get "" i.e. NOTHING 🤯😡
/* Input is:-
4
2 4 6 8 10 12
4 5 6 7 8 9
1 2 2 2 51 5
56 5 15 5 56 5
 */
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int row;
    cin >> row; // "ROW" is Not Required". 🤪But need to take as Its given,,but will NOT USE😉. "COLUMN" is not given and also NOT REQUIRED

    vector<vector<int>> v;

    string line;        //👉 If the single inputs are given in the lines above the vector then must add :~ String line; -- getline(cin, line); where line will take the ending "" from the line of the sigle Input will before taking the NEXT LINE as input for vector.
    getline(cin, line); // ## Otherwise the first vector v1 will only get "" i.e. NOTHING 🤯😡

    string str; // it's the string which will store the Input "LINES"
    int temp;   // "tmp" will be used to store the "integers" from the "istingstream"

    while (getline(cin, str))   //🤪 using while loop to traverse without needing "ROW" as Input.🤯Here "getline(cin, str)" will give input to "str" and will also "END / Terminate" the "while loop" when there is no input is left
    {
        istringstream ss(str);
        vector<int> tempVector;
        while (ss >> temp)
        {
            tempVector.emplace_back(temp);
        }
        v.emplace_back(tempVector);
        // istringstream.swap(ss);    /* 👉 It is used only when we want to "RESET" the "istringstream ss(str);" ; So that it can be used again and another variable of "istringstream ss(str);" wil not be needed */
        // 👆 But in this case we are using "FOR" or "WHILE" loops, so "istringstream ss(str);" gets "RESET" in each loop; Hence istringstream.swap(ss); is not required
    }

    for (auto el : v)
    {
        for (auto el2 : el)
        {
            cout << el2 << " ";
        }
        cout << endl;
    }

    return 0;
}

Taking user input with both ROW and COLUMN unknown ~ istringstream​

info

ROW or COLUMN NOT NEEDED​

Using istringstream from <sstream> header​

ROW need not be initialisedL:~ vector<vector<int>> v; So it will without taking any ROW as INPUT

  • Creating a vector<int> tempVector; and taking the Input LINES into the tempVector
  • Then adding the v.emplace_back(tempVector); to the main vector v ; So that we will not need to call any index of vector v like v.at(i) because vector v is a 2D vector and to access the "COLUMN" part of 2D Vector "ROW" must be initialsised like vector<vector<int>> v(row);
  • ☠️ If the single inputs are given in the lines above the vector then must add :~ String line; -- getline(cin, line); where line will take the ending "" from the line of the sigle Input will before taking the NEXT LINE as input for vector. Otherwise the first vector v1 will only get "" i.e. NOTHING 🤯😡
/* Input is:-
4
2 4 6 8 10 12
4 5 6 7 8 9
1 2 2 2 51 5
56 5 15 5 56 5
 */
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int row;
    cin >> row; // "ROW" is Not Required". 🤪But need to take as Its given,,but will NOT USE😉. "COLUMN" is not given and also NOT REQUIRED

    vector<vector<int>> v;

    string line;        //👉 If the single inputs are given in the lines above the vector then must add :~ String line; -- getline(cin, line); where line will take the ending "" from the line of the sigle Input will before taking the `NEXT LINE` as input for vector.
    getline(cin, line); // ## Otherwise the first vector v1 will only get "" i.e. NOTHING 🤯😡

    string str; // it's the string which will store the Input "LINES"
    int temp;   // "tmp" will be used to store the "integers" from the "istingstream"

    while (getline(cin, str))   //🤪 using while loop to traverse without needing "ROW" as Input.🤯Here "getline(cin, str)" will give input to "str" and will also "END / Terminate" the "while loop" when there is no input is left
    {
        istringstream ss(str);
        vector<int> tempVector;
        while (ss >> temp)
        {
            tempVector.emplace_back(temp);
        }
        v.emplace_back(tempVector);
        // istringstream.swap(ss);    /* 👉 It is used only when we want to "RESET" the "istringstream ss(str);" ; So that it can be used again and another variable of "istringstream ss(str);" wil not be needed */
        // 👆 But in this case we are using "FOR" or "WHILE" loops, so "istringstream ss(str);" gets "RESET" in each loop; Hence istringstream.swap(ss); is not required
    }

    for (auto el : v)
    {
        for (auto el2 : el)
        {
            cout << el2 << " ";
        }
        cout << endl;
    }

    return 0;
}

Sub vector​

TO DO

vector in built Methods​

Replace an element of a vector at()​

// "v" is: [ 10 20 30 40 50 60  ]
v.at(3) = 2222;     // replace Index='3' with "2222"      // replaced "40" with  "2222"

// "v" becomes: [ 10 20 30 2222 50 60 ]

v.size()​

cout << v.size();

vector begin()​

1st iterator

vector end()​

after last iterator

v.end() is not included as it doesnt't exists in (v.begin(), v.end())

vector rbegin()​

caution

returns a reverse iterator pointing to the last element in the container. i.e. it points to the index size() - 1 or we can say v.end() - 1

printing vector in reverse order using v.rbegin() & v.rend()
for (auto it = v.rbegin(); it != v.rend(); it++)
{
    cout << *it << " ";
}

vector rend()​

::caution

returns a reverse iterator pointing to the theoretical element right before the first element in the vector container. i.e. it points to the index 0 or we can say v.begin()

v.rend() is excluded in (v.rbegin(), v.rend())​

printing vector in reverse order using v.rbegin() & v.rend()
for (auto it = v.rbegin(); it != v.rend(); it++)
{
    cout << *it << " ";
}

:::

back()​

v.back() is used to access the last element of the vector. v.back() is same as v.at(v.size() - 1)

/* vector<vector<int>> ans; */ /* vector<vector<int>>& v */

if (ans.back().at(1) >= vect[i][0])
{
    ans.back().at(1) = max( ans.back().at(1), vect[i][1] );
}

front()​

v.back() is used to access the first element of the vector. v.front() is same as v.at(0)

emplace_back()​

v2.emplace_back(tmp);

at()​

cout << vec.at(i) << endl;

insert()​

info

Inserts elements at the specified location in the container (vector).

  • v.insert(v.begin() + IndexNo) inserts an element at the Index No.
  • insert() function adds the element to vector and the elements at that Index No. gets pused back to the right side.
// Input is: [ 1 2 3 4 5 6 7 ]
v.insert(v.begin() + 2, 555);   // element at that `Index No` gets pused to the right side

// Output: [ 1 2 555 3 4 5 6 7 ]
// "v" = [ 1 2 3 4 5 6 7 ]
// "v1" = [ 100 200 300 400 500 600 700 ]
v.insert(v.begin() + 5, v1.begin(), v1.end());    // elements at that `Index No` gets pused to the right side
// Inserted " 100 200 300 400 500 600 700 " at Index="5"


// Output is:     [ 1  2  3  4  5  100  200  300  400  500  600  700  6   7  ]
// Index are :~    0  1  2  3  4   5    6    7    8    9    10   11  12  13
tip

Copy the vector or deque elements to another vector or deque using insert()​

add vector to deque​

// vector "vec" is given

deque<int> dq; // empty "deque" initialised
dq.insert(dq.end(), vec.begin(), vec.end());

add deque to vector​

// deque "dq" is given

vector<int> vec; // empty "deque" initialised
vec.insert(vec.end(), dq.begin(), dq.end());

emplace()​

caution

Only availablde for single insertions,NOT VALID for range insertions operations like adding deque elements to vector is not AVAILABEL in emplace();; but these are available in insert()​

When to use emplace():​

  • Single insertion of single elements at certain positions

When to use insert():​

  • Range of elements like vector to deque and so on.
info

TO ADD

  • emplace() vs insert()

assign()​


erase()​

info
  • erase(int position): Removes the element present at position. Ex: v.erase(v.begin()+4); (erases the fifth element of the vector v)
// Input is [ 10 5 2 3 10 ]
// To remove/erase Index = "1"
cin >> temp;    // temp = 1
v.erase(v.begin() + temp);  // v.erase(1)

// output is: [ 10  2 3 10 ]    // 5 at Index=1 has been erased/removed
  • erase(int start,int end): Removes the elements in the range from start to end; inclusive of the start and exclusive of the end. Ex:v.erase(v.begin()+2,v.begin()+5); (erases all the elements from the 3rd element to the 4th element not the 5th element.)
// Input is [ 100 90 80 70 40 10 56 ]
// To remove/erase Index="2" to Index="5"
cin >> num1 >> num2;    // num1 = 2 , num1 = 5
v.erase(v.begin() + num1, v.begin() + num2);  // v.erase(2, 5)

// output is: [ 100 90  10 56 ]    // [ 80 70 40 ] has been erased/removed

clear()​

  • clears the complete container
  • After using this method the container becomes empty.
// vec => {1 2 3 4 5 6}

vec.clear();
// 🔼 vec => { }     /* empty */

pop_back()​


empty()​

v.empty() return bool true or false

--

methods only available to vector​

caution
  • capacity()
  • reserve()
  • data()

vector specific solution tricks​

erase element by value​

first find(..) then v.erase()

auto it = find(v2.begin(), v2.end(), el); // iterator
if (it != v2.end())
{
    v2.erase(it);   // erase takes iterators like .. 'v.begin() + 2'
}

remove all occurances of an element​

remove(v.begin(), v.end(), "hello");