C++

Vector

1. useful functions

<algorithm>

• std::sort(v.begin(), v.end())
• std::unique(v.begin(), v.end())
• it moves duplicated elements on the back, and returns the number of the first duplicated elements.
• ! The vector has to be sorted.
• For example, { 0, 1, 1, 2, 2, 2, 3} => {0, 1, 2, 3, 1, 2, 2, 3} and returns 5. Therefore, you could use it as such:

    std::sort(v.begin(), v.end());
    v.resize(std::unique(v.begin(), v.end()) - v.begin());

• v1 == v2 is true when v1 and v2 has the same elements in the same orders.

  • &v1 == &v2 or *v1 == *v2 won’t be true tho.

Thoughts about data type Char

• So, as everyone knows, size of char is 8 bits = 1 byte. However, if you look at an ascii table, it only has 0 to 127, so total 128, which is only 2^7. I honestly don’t know why it uses 1 byte, maybe it’s just easy to remember that way. (Please make a comment if you know the history.) Anyways, this means -128 to -1 doesn’t mean anything while you are coding (maybe it has differnt meaning in it but idk atm). When you wanna encode your string, you could use it like this:

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

using namespace std;

int main()
{
    vector<char> v;
    
    string test = "this is a test";
    string ans;
    
    for(const char& el : test){
        ans += el*-1;
    }
    
    cout << ans <<'\n';
    
    for(char& el : ans){
        el *= -1;
    }

    cout << ans <<'\n';

    return 0;
}

output is,

��������������
this is a test

Passing parameters

• & - pass by references
  • It doesn’t make a copy of the value, but give you the real value. Thereby, if you change this, the original data(which is the same) will be changed.

void foo(int& value) {
    value = 6; 
} 
int main() {
    int value = 5;
    // 6
    cout << "value = " << value << '\n';
    foo(value);
    // 5
    cout << "value = " << value << '\n';
    return 0;
}

• const & - pass by references
  • by doing so, you can implicitly show that it’s not changeable, and also you can reduce time of making a copy.
• * - pass by address
  • literally.

void foo(int* ptr) {
    *ptr = 6; 
} 
int main() {
    int value = 5; 
    // 5
    std::cout << "value = " << value << '\n'; 
    foo(&value);
    // 6
    std::cout << "value = " << value << '\n'; 
    return 0; 
}

Declaration of functions in parameter

[](int a, int b){
    content;
}

For example:

sort(arr.begin(), arr.end(), [](const int& a, const int& b){
    return b < a;
});

Using outside values in Lambda expression

[outside_value_name](int a, int b){
    content;
}

For example:

string order = 'qwertyuiopalskdjfhgzmxncbv';

// & to get it by reference
sort(arr.begin(), arr.end(), [&order](const int& a, const int& b){
    return order.find(b) < order.find(a);
});

Use custom comparator with lambda expression on Priority Queue

//  first, define lambda expression 
auto comp = [](const pair<int,int> a, const pair<int,int> b){
    if(a.first != b.first) return (a.first < b.first);
    else return (a.second < b.second);
};
//  use decltype and on constructor you have to put the lambda expression 
priority_queue< pair<int,int>, vector<pair<int,int>>, decltype(comp)> pq(comp);        

Recursion vs Tail recursion

• Tail Recursion is to prevent SOF(stack overflow), it uses the same stack memory by not having calculations in the function.