CPP Tips and Tricks

1. Traversing Map or Priority Queue

• Specially when you have the class templates with multiple parameters i.e.<int, char> or <pair<int,int>> or <char,int>.
• You can traverse priority_queueusing the following code -

unordered_map<int,char> umap; 
for(auto &[firstValue, secondValue] : umap) {
	...
	cout << firstValue <<endl; 
	cout << secondValue << endl;
}

priority_queue<pair<int,char>> maxHeap; 
maxHeap.push({95,'a'}); 
auto [frequency, character] = maxHeap.top(); maxHeap.pop(); 

2. Returning vector from functions, when you have less values

• You can just use {a,b} to return the values as vector

vector<int> someFunction(int a,int b){
    return {a,b};
}

3. Iterating through an unordered_map

• Don’t expect unordered_map to follow $insertion$ order.

for(auto x : umap){
	...
    // access the keys using 
    cout << x.first;
    // access the values using
    cout << x.second; 
}

4. Using abs

#include<cstdlib>
..
..
int main(){
	cout << abs(-5) << endl; 
}
// Output : 5

5. Using Comparator for sort

• Sorting a 2D vector using first element of the vector
• We need to return , the order we wish to see in our $resultantarray$.
• So return v1[0] < v2[0]. ****

/* Comparator */
bool comparator(vector<int> &v1, vector<int> &v2) {
	return v1[0] < v2[1];
}
 
/* Call */
int main(){
	/* Intializing a 2D vector */
	vector<vector<int>> intervals{
		{1, 2},
		{3, 5},
		{4, 7},
		{6, 8},
		{9, 10}
	};
	
	/* Sorting */
	sort(intervals.begin(), intervals.end(), comparator);
 
	
	/* Return 0 */ 
	return 0; 
}

6. Using lambda function for comparator

• Sorting can be provided by the use of lambda function as well.

sort(arr.begin(),arr.end(),[](vector<int> a, vector<int> b){return a[0] < b[0];}); 

7. Swapping using two variables

void swap_one(int &a , int &b){
    a = a - b; 
    b = b + a; 
    a = b - a; 
}
 
void swap_two(int &a , int &b){
    a = a + b; 
    b = a - b; 
    a = a - b; 
}
 
void swap_three(int &a , int &b){
    a = a * b; 
    b = a / b; 
    a = a / b; 
}
 
void swap_four(int &a , int &b){
    b = (a+b) - (a = b);  
}

8. Basic CPP logger

#include <iostream>
#include <chrono>
 
#define LOG(msg) std::cout << __FILE__ << ":" << __LINE__ << " [" << \
    std::chrono::system_clock::now().time_since_epoch().count() << "] " << \
    msg << std::endl;