cpp_notes.txt

Here are all the cpp notes that I have written and understood!
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in CPP, sorting something a vector of pair by its second element, you can use the inbuilt function in c++

it goes like this:

---
sort(v.begin(), v.end(), sort_by_sec);
---

Driver fucntion fot it
---
bool sort_by_sec(cont std::pair &a, const std::pair &b){
	return a.second < b.second;
}
---

This method exploits the fact that, in any sorting method, comparisions are made. Ultimately, this code allows the inbuilt algorithm to compare directly the second element of the pair

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declaring a class object in C++
classname obj(parameters);
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Disjoint Set Note!:

Once you find parents make sure to union the sets. I.e. rreset their parents!

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in c++ 
bits/stdc++.h has strings in it.

string.length() : length of the string
string.at(index) : char at an index
string.substr(start, end) : gets substring between start and end index

About maps:
maps initialize all values to 0. This is quite frustrating in dynamic programming when generating the 
table. Coz all keys of the map are initialized to 0. Hence, if a fucntions return 0 as a value the function still things that the value isn't computed and hence can cause an infinite loop. instead initialize actual 0 values to -1. Then the next best thign you can do is, use "return (table[val] == -1?0:table[val]) "

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Ouuu... While doing merge sort I found a lot of usefull things

vectors; merging vectors in c ++

assume you have 
std::vector<int> AB, A = {1,2,3,4},B = {5,6,7,8};

then do:

AB.reserve(A.size()+B.size());
AB.insert(AB.end(), A.begin().A.end());
AB.insert(AB.end(), B.begin(), B.end());

this will give us AB = {1,2,3,4,5,6,7,8}

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Now, to check if a vector is empty: vector.empty() (boolean value return)

to erase an element (or range of elements) in the vector: vector.erase(vector.begin() + index, (optional) vector.begin()+end)

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passing arrays and initilazing values in array:

pass 2d array just by name. 
declare function as:type func(type array_name[][size]);

initialize memory:
memset(arrayname, value_to_be_initialized, sizeof(arrayname[0][0]...[0])*dim1*dim2*...dimr);
^ initializes a n dimentional array by value_to_be_initialized

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