STL - Vectors

CPP Reference: vector

Present inside the <vector> header, vectors are dynamic arrays with the ability to resize itself automatically when an element is inserted or deleted

Declaration: vector<Type> v1;

Iterators

• Forward: .begin() , .end()
• Reverse: .rbegin() , .rend()

Element access

• [idx] - Returns reference to element at index idx inside the vector.
• .front() – Returns reference to first element in the vector
• .back() – Returns reference to last element in the vector

Size functions

• .empty() - Returns whether the vector is empty or not
• .size() - Returns number of elements present inside the vector
• .capacity() - Returns number of elements worth of memory currently allocated to the vector
• .max_size() - Returns the theoretical maximum number of items that could be put in your vector
• .resize(n, val) - Resizes vector to only include first n elements. Can also pass extra argument val to which the new elements will be initialized

Modifiers

• .pop_back() - Removes last element from the vector. Takes O(1) time
• .push_back(val) - Appends one element having value val at the end of element. Takes O(1) time in most cases ,but O(N) when resizing
• .emplace_back(val) - Constructs the element in-place at the end of the vector. Same time complexity as push_back() but one less copy operation
• .clear() - Remove all elements. Takes O(N) time
• .erase() - Removes element(s) at specific position via erase(pos) or within a range via erase(start, end). Takes O(N) time on average

Difference between .emplace_back() and .push_back()

• Generally, emplace_back() is slightly faster than push_back() as it constructs the element in-place while push_back() creates the element and then copies it. So, prefer emplace_back() for complex objects or fast insertion.
• You can also pass the appropriate list of arguments to construct the vector element during emplace_back(), but for push_back(), you need to create the object first.

vector<pair<char, int>> v1;

v1.emplace_back('x', 12);
v1.push_back({'y', 15});
v1.push_back(make_pair('z', 17));

for (auto &p : v1) {
    cout << p.first << ", " << p.second << endl;
}

// x, 12
// y, 15
// z, 17

Ways to initialize a vector

1. Declare and .push_back() each element:

vector<int> v1;
v1.push_back(10);
v1.push_back(20);

2. Specify size and value (all elements will have same value):

vector<int> v1(4, 7);
// {7, 7, 7, 7}

3. List-initialization:

vector<int> v1{ 10, 20, 30 };

4. From another array or vector via (start_pos, end_pos):

// From another array
int arr[] = { 10, 20, 30 };
int n = sizeof(arr) / sizeof(arr[0]);
vector<int> v1(arr, arr + n);

// From another vector
vector<int> v2{10, 20, 30, 40, 50, 60};
vector<int> v3(v2.begin() + 2, v2.end());
// {30, 40, 50, 60}

5. Using fill() or fill_n() to set values:

vector<int> v1(5);
// fill(start_pos, end_pos, val)
fill(v1.begin(), v1.begin()+3, 12);
// {12, 12, 12, 0, 0}

vector<int> v2(7);
// fill_n(start_pos, count, val)
fill_n(v2.begin()+3, 2, 4);
//  {0, 0, 0, 4, 4, 0, 0}

6. Using iota() to set values as increments:

vector<int> v1(10);
// iota(start_pos, end_pos, start_val)
iota(v1.begin()+2, v1.end()-3, 20);
// { 0, 0, 20, 21, 22, 23, 24, 0, 0, 0 }

Initialising 2D vector

vector<vector<int>> mat(3, vector<int>(4, -1));

Above line initializes a 2D matrix mat of 3 rows and 4 columns with the default value -1 at each cell

Traversal

• Range-based loop: for(auto &element : v1)
• Using iterators: for(auto itr = v1.begin(); itr != v1.end(); itr++)
• Reverse traversal: for(auto itr = v1.rbegin(); itr != v1.rend(); itr++)

vector<int> v1{105, 48, 23, 6, 94, 52};

// Range-based for loop
cout << "{ ";
for (auto &x : v1) {
    cout << x << ", ";
}
cout << "}" << endl;
// { 105, 48, 23, 6, 94, 52, }

// for loop with iterators
cout << "{ ";
for (auto itr = v1.begin(); itr != v1.end(); itr++) {
    cout << *(itr) << ", ";
}
cout << "}" << endl;
// { 105, 48, 23, 6, 94, 52, }

// Reverse traversal: for loop with reverse iterators
cout << "{ ";
for (auto itr = v1.rbegin(); itr != v1.rend(); itr++) {
    cout << *(itr) << ", ";
}
cout << "}" << endl;
// { 52, 94, 6, 23, 48, 105, }

More functions:

• .insert(): Inserts element(s) at specific postition. Takes O(N) time on average

  • .insert(pos,val): Inserts element val at position pos
  • .insert(pos,reps,val): Inserts reps number of copies of val starting from position pos
  • .insert(pos,start,end): Inserts all elements of [start, end) at position pos

  vector<int> v1{10, 20, 30, 40, 50};
  vector<int> v2{-1, -2, -3, -4, -5, -6};
  
  v1.insert(v1.begin() + 3, 101);
  // 10 20 30 101 40 50
  
  v1.insert(v1.begin() + 2, 3, 420);
  // 10 20 420 420 420 30 101 40 50
  
  v1.insert(v1.begin() + 2, v2.begin() + 1, v2.begin() + 4);
  // 10 20 -2 -3 -4 420 420 420 30 101 40 50

• .emplace(pos,...args) - Inserts a new element into the container directly before pos by constructing it in-place.

Beware of vector<bool>

vector<bool> in C++ is a specialization of the standard vector container class that allows you to store a sequence of boolean values. Unlike a regular vector<T> where T can be any data type, vector<bool> doesn’t store each bool value as a full byte in memory. Instead, it utilizes bit manipulation techniques to pack multiple boolean values (typically 8) into a single byte. Due to the bit-packing optimization, operations like element access and modification are not as straightforward as with other types:

• No Direct Element Access: You cannot directly access individual elements via []. Instead, you need to use member functions like at() or iterators.
• No References: The reference type returned by std::vector<bool>::operator[] is a proxy class (often called std::vector<bool>::reference), which behaves like a bool& but is not actually a reference to a single bool value. This can lead to confusion and unexpected behavior. You cannot take the address of a bool element in a std::vector<bool>, as each element is not stored as a separate byte in memory.

Better alternatives:

• If the size is known at compile-time: use bistset<N> as suggested by CPP Reference
• For dynamic sizes: you can use vector<char> or boost::dynamic_bitset

Vector of custom data-type

struct Student {
    int id;
    string name;
    Student(int _id, string _name) : id(_id), name(_name) {}
};

bool compareIds(Student &s1, Student &s2) {
    // Assuming unique ids
    return (s1.id < s2.id);
}

void showStudents(vector<Student> &students) {
    for (auto &s : students) {
        cout << s.id << "\t" << s.name << endl;
    }
}

int main() {

    vector<Student> v1{
        // List-initialization
        Student{101, "Ramesh"},
        Student{103, "Shyam"},
        // Constructing Object
        Student(102, "Pankaj"),
        Student(104, "Sai"),
    };

    showStudents(v1);
    // 101     Ramesh
    // 103     Shyam
    // 102     Pankaj
    // 104     Sai

    sort(v1.begin(), v1.end(), compareIds);

    showStudents(v1);
    // 101     Ramesh
    // 102     Pankaj
    // 103     Shyam
    // 104     Sai

    return 0;
}