MapGrocery, C++ (gcc)

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//g++  7.4.0
//Grocery List: use of map functions
//map is a container like array that stores elements come in a pair of key & data(aka mapped value); key is used to sort and identify the data element; there can’t be two similar keys; elements are stored here in a ordered way based on the key; so map container does the sorting work for you and helps to create an ordered list; here uses of important map functions are demonstrated;
//this code is created by Rezaul Hoque on July 26,2021;contact: jewelmrh@yahoo.com
//note: codes shared by Rezaul Hoque on rextester are not for sale; they are created and shared to facilitate the algorithm learning process; many like Hoque use this platform to practice programming ;Rezaul hopes his contribution helps others to fine tune their learning;

#include <iostream>
#include <map>
#include <string>
using namespace std;
int main(){
//creating a map object
map<char,int> grocery;
//inserting data with emplace(); note here key 'C' is entered before key 'A' and key 'B'; notice the difference when the list is displayed
grocery.emplace('C',10);
grocery.emplace( 'B',20);
grocery.emplace( 'A',30);
// inserting data with emplace_hint()
auto it= grocery.end();
grocery.emplace_hint(it,'P',10);
grocery.emplace_hint(it,'Q',14);
grocery.emplace_hint(it,'R',12);
cout<<"A grocery list:\n";
for(auto& x: grocery){
cout<<x.first<<":"<<x.second<<"\n";
}
//use of map at() to assign values; note here "LLL" is entered before "BBB" and "DDD";
map<string,int> list={{"LLL", 0}, {"BBB",0},{"DDD",0}};
list.at("LLL")= 55;
list.at("BBB")= 25;
list.at("DDD")= 35;
cout<<"Another list created with map at():\n";
for(auto& x: list){
cout<<x.first<<":"<<x.second<<"\n";
}
//use of map [] and = operators to assign values
map<char,int> grocery2;
grocery2['W']= 21;
grocery2['X']=22;
grocery2['Y']= 23;
grocery2['Z']= grocery2['Y'];
cout<<"Another list ,grocery2,created with map '[ ]'and '=':\n";
for(auto& x: grocery2){
cout<<x.first<<":"<<x.second<<"\n";
}

//swapping grocery 2 and grocery
grocery.swap(grocery2 );
cout<<"grocery list after swap:\n";
for(map<char,int>::iterator it=grocery.begin(); it != grocery.end();++it)
cout<<it->first<<" =>"<<it->second<<"\n";
//use of map lower_bound () and upper_bound() to select a range and clear it with erase()
map<char,int>::iterator low,up;
low=grocery2.lower_bound('B');
up = grocery2.upper_bound('Q');
grocery2.erase (low,up);

//use of map size()
cout<<"After erase() grocery2 has "<<grocery2.size()<<" elements\n";
cout<<"And they are: \n";
for(auto& x: grocery2){
cout<<x.first<<":"<<x.second<<"\n";
}
return 0;
}

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Compilation time: 1.13 sec, absolute running time: 0.18 sec, cpu time: 0.02 sec, memory peak: 5 Mb, absolute service time: 1,43 sec