While putting sorted vector elements into map, it inserts them in ascending order after it has been sorted in descending order with the compare function. I know that map is in ascending order by default based on its key, but I am not sure why it sorts them again in ascending order based on its value. Is this common in std::map<std::string, int>?
I simply want to return a map in descending order by its value, I need to display the directory strings with a number of items found by decreasing order.
#include <iostream>
#include <vector>
#include <map>
#include <string>
#include <algorithm>
bool compare_dir(std::pair<std::string, int>&a,
std::pair<std::string, int>&b)
{
return a.second > b.second;
}
void sort_func(std::map<std::string, int>& M)
{
std::map<std::string, int>::iterator itr;
std::vector<std::pair<std::string, int>> temp;
std::map<std::string, int> result;
for(auto &it : M)
temp.push_back(it);
sort(temp.begin(), temp.end(), compare_dir);
for(auto &el : temp)
std::cout << el.first << " " << el.second << std::endl;
std::cout << std::endl;
for(auto & el : temp)
result.insert(el);
for(itr = result.begin(); itr != result.end(); itr++)
std::cout << itr->first << " " << itr->second << std::endl;
}
int main()
{
std::map<std::string, int> my_map{{"dir1", 1}, {"dir2", 2}, {"dir3", 3}};
for (const auto& [key, value] : my_map) {
std::cout << '[' << key << "] = " << value << "; " << std::endl;
}
std::cout << std::endl;
sort_func(my_map);
}
Related
How do I transverse through a nested map?
I want to access the value in the second nested map
int main ()
{
std::map<int, int> cool;
cool.insert(make_pair(8,9));
std::map<char, std::map<int, int> > mmap;
mmap.insert(std::make_pair('a', cool));
cout << mmap['a'][8];
std::map<char, map<int, int> >::iterator itr;
for(itr = mmap.begin(); itr != mmap.end(); itr++){
cout << itr->second << endl;
}
return 0;
}
Your itr points to a pair<char, map<int, int> >,
so its second is a map<int, int>,
so an iterator into that will do the trick:
for(map<int, int>::iterator itr2 = itr->second.begin(); itr2 != itr->second.end(); ++itr2){
cout << itr2->second << endl;
}
With C++17 you can use structured binding to accomplish this:
for (auto const& [key, val] : mmap) {
for (auto const& [k, v] : val) {
std::cout << key << " " << k << " " << v << '\n';
}
}
I am having a map like this
typedef vector< pair<int, string> > Categories;
map<string,Categories> cats;
but when I am trying to read elements of the map like
for(map<string,Categories>::const_iterator it = cats.begin(); it != cats.end(); ++it)
{
std::cout << it->first << " " << it->second.first<<"\n";
}
I do get errors
error: 'const class std::vector<std::pair<int, std::basic_string<char>
' has no member named 'first' std::cout << it->first << " " << it-> second.first<<"\n";
error: 'const class std::vector ' has no member named 'first'
std::cout << it->first << " " << it->second.first<<"\n";
Its clear as Crystal, that you might have many elements in your values of your map, which is a std::vector< std::pair<int, std::string>>
Then how would you print elements of a vector? The options are:
random access (i.e, vec[index])
iterator (i.e, std::vector< std::pair<int, std::string>>::const_iterator itr;)
or by a range based for loop (i.e, for(const auto& it: vec) )
In your case, if you wanna have something simple and easy code is using a range based loop:
for(const auto& it: cats)
{
std::cout << it.first << " = "; // keys
for(const auto& ve: it.second) // values vec
std::cout << ve.first << "-" << ve.second << "\t";
std::cout << std::endl;
}
If you still wanna have long iterator loops, here is it.
see live action here: https://www.ideone.com/3bS1kR
#include <string>
#include <iostream>
#include <vector>
#include <map>
#include <iterator>
typedef std::vector< std::pair<int, std::string>> Categories;
int main()
{
std::map<std::string, Categories> cats;
cats["key1"] = {std::make_pair(1, "pair1"), std::make_pair(1, "pair2"), std::make_pair(1, "par3")};
cats["key2"] = {std::make_pair(2, "pair1"), std::make_pair(2, "pair2")};
cats["key3"] = {std::make_pair(3, "pair1")};
std::cout << "Range based loop \n";
for(const auto& it: cats)
{
std::cout << it.first << " = "; // keys
for(const auto& ve: it.second) // values vec
std::cout << ve.first << "-" << ve.second << "\t";
std::cout << std::endl;
}
std::cout << "\nIterator loop \n";
std::map<std::string, Categories>::const_iterator it;
std::vector< std::pair<int, std::string>>::const_iterator curr_val_it;
for(it = cats.cbegin(); it != cats.cend(); ++it)
{
std::cout << it->first << " = "; // keys
for(curr_val_it = it->second.cbegin(); curr_val_it != it->second.cend(); ++curr_val_it )
std::cout << curr_val_it->first << "-" << curr_val_it->second << "\t"; // values vec
std::cout << std::endl;
}
return 0;
}
you need to access a element of the vector first, then the pair within.
... it->second[0].first<< ...
better impl of loop:
for(const auto& cat : cats)
{
string mapidx = cat.first;
vector<pair<int, std::string>> catvect = cat.second;
}
then you can have a seperate loop to read the contents of the vector:
for(const auto& cat : cats)
{
string mapidx = cat.first;
vector<pair<int, std::string>> catvect = cat.second;
for (const auto& entry : catvect)
{
int number = entry.first;
string whatever = entry.second;
}
}
the temp variables are just for readability, no need for all the copies ;)
Error is exacly what compiler told you:
const class std::vector ' has no member named 'first'
so, you have do decide how to print your map by overloading ostream opeartor, below example how it can be achived:
#include <iostream>
#include <map>
#include <string>
#include <utility>
#include <vector>
typedef std::vector<std::pair<int, std::string>> Categories;
std::map<std::string,Categories> cats;
std::ostream& operator << (std::ostream& os, const std::vector<std::pair<int, std::string>>& v)
{
os << "[";
for (auto& el : v) {
os << " " << el.first << " : " << el.second;
}
os << "]";
return os;
}
int main() {
cats.emplace("cat1", std::vector<std::pair<int, std::string>>(1, std::make_pair(1, "category1")));
for(auto& cat : cats) {
std::cout << cat.first << " " << cat.second << "\n";
}
}
Since we are storing Vector Categories inside a Map we will have to iterate Vector too:
for(map<string,Categories>::const_iterator it = cats.begin(); it != cats.end(); ++it)
{
//iterator for vector (it->second)
for(Categories::const_iterator it2 = it->second.begin(); it2 != it->second.end(); it2++ )
{
std::cout << it->first << " " << it2->first<<" "<<it2->second <<"\n";
}
}
I have a map which has two string as key and one vector as value
how can i print the value of map.
Below is my approach which is bad Can Someone help me thanks in advance
NOTE : i want to print by key not iterating on vector
int main()
{
vector<string>value;
std::map<std::pair<string,string> ,vector<string>> myMap;
string input1,input2,MyvectorValue;
for(int i=0;i<5;++i)
{
cin>>input1;
cin>>input2;
cin>>MyvectorValue;
myMap[std::make_pair(input1,input2)].push_back(MyvectorValue);
}
int j=0;
for( auto it = myMap.begin(); it != myMap.end(); ++it )
{
std::vector<std::string>& value = it->second.at(j++);
cout<<value // This is bad
//how can i print all map value ??
}
}
The value of the map is a vector, assuming you can use C++11, the following code would do what you need.
#include <string>
#include <iostream>
#include <map>
#include <utility>
#include <vector>
int main()
{
std::vector< std::string >value;
std::map< std::pair<std::string , std::string> , std::vector<std::string> > myMap;
std::string input1,input2,MyvectorValue;
for(int i=0;i<5;++i)
{
std::cin>>input1;
std::cin>>input2;
std::cin>>MyvectorValue;
myMap[std::make_pair(input1,input2)].push_back(MyvectorValue);
}
//If you have a particular key (string1, string2), and want to print the values for that specific key...
auto particularKey = std::make_pair("stringA", "stringB");
for(auto val : myMap[particularKey])
std::cout << val << " ";
std::cout << std::endl;
// If you want to iterate through all keys of your map
for(auto &elem : myMap)
{
std::cout << "for the pair with key (" << elem.first.first << "," << elem.first.second << "), the value is the following vector" << std::endl;
for(auto s : elem.second)
{
std::cout << s << " ";
}
std::cout << std::endl << std::endl;
}
return 0;
}
You can print the keys by accessing the pair and then using first and second to get the first and second member of the pair respectively.
You can also print the values, by accessing the vectors and iterating over them, printing every string separately.
for(auto& element : myMap)
{
std::cout << "Key: {" << element.first.first << ", " << element.first.second << "}\n";
std::cout << "Value is a vector with the following strings: \n";
for(auto& str: element.second)
std::cout << str << std::endl;
}
If you want to print by key not iterating on vector , then you may declare map as "std::map,string> myMap". Then, you can do following modification to your code as given below.
int main() {
vector<string>value;
std::map<std::pair<string,string>,string> myMap;
string input1,input2,MyvectorValue;
for(int i=0; i<5; ++i) {
cin>>input1;
cin>>input2;
cin>>MyvectorValue;
myMap[std::make_pair(input1,input2)]+=MyvectorValue;
myMap[std::make_pair(input1,input2)]+= " ";
}
for( auto it = myMap.begin(); it != myMap.end(); ++it ) {
std::string& value = it->second;
cout<<value<<endl;
}
}
I ran into the following problem: suppose I have
pair<int, int> p(1,2)
vector<pair<int, int>> vec;
I want to use find to get the iterator pointing to the element p in the vector
find(vec.begin(), vec.end(), p)
But it gave me the error
type 'std::__1::pair<int, int>' does not provide a call operator
How should I proceed?
This is what I've used and it's worked great.
#include <iostream>
#include <vector>
#include <algorithm>
struct FindPair {
FindPair (int first, int second)
: m_first_value(first)
, m_second_value(second) { }
int m_first_value;
int m_second_value;
bool operator()
( const std::pair<int, int> &p ) {
return (p.first == m_first_value && p.second == m_second_value);
}
};
int main()
{
std::vector< std::pair<int, int> > myVec;
std::vector< std::pair<int, int> >::iterator it;
myVec.push_back(std::make_pair(1,1));
myVec.push_back(std::make_pair(1,2));
it = std::find_if(myVec.begin(), myVec.end(), FindPair(1, 2));
if (it != myVec.end())
{
// We Found it
std::cout << "Matched Found on Current Iterator!" << std::endl;
std::cout << "it.first: " << (*it).first << std::endl;
std::cout << "it.second: " << (*it).second << std::endl;
}
else
{
std::cout << "Nothing Matched!" << std::endl;
}
return 0;
}
Output:
Matched Found on Current Iterator!
it.first: 1
it.second: 2
I want to iterate through each element in the map<string, int> without knowing any of its string-int values or keys.
What I have so far:
void output(map<string, int> table)
{
map<string, int>::iterator it;
for (it = table.begin(); it != table.end(); it++)
{
//How do I access each element?
}
}
You can achieve this like following :
map<string, int>::iterator it;
for (it = symbolTable.begin(); it != symbolTable.end(); it++)
{
std::cout << it->first // string (key)
<< ':'
<< it->second // string's value
<< std::endl;
}
With C++11 ( and onwards ),
for (auto const& x : symbolTable)
{
std::cout << x.first // string (key)
<< ':'
<< x.second // string's value
<< std::endl;
}
With C++17 ( and onwards ),
for (auto const& [key, val] : symbolTable)
{
std::cout << key // string (key)
<< ':'
<< val // string's value
<< std::endl;
}
Try the following
for ( const auto &p : table )
{
std::cout << p.first << '\t' << p.second << std::endl;
}
The same can be written using an ordinary for loop
for ( auto it = table.begin(); it != table.end(); ++it )
{
std::cout << it->first << '\t' << it->second << std::endl;
}
Take into account that value_type for std::map is defined the following way
typedef pair<const Key, T> value_type
Thus in my example p is a const reference to the value_type where Key is std::string and T is int
Also it would be better if the function would be declared as
void output( const map<string, int> &table );
The value_type of a map is a pair containing the key and value as it's first and second member, respectively.
map<string, int>::iterator it;
for (it = symbolTable.begin(); it != symbolTable.end(); it++)
{
std::cout << it->first << ' ' << it->second << '\n';
}
Or with C++11, using range-based for:
for (auto const& p : symbolTable)
{
std::cout << p.first << ' ' << p.second << '\n';
}
As #Vlad from Moscow says,
Take into account that value_type for std::map is defined the following way:
typedef pair<const Key, T> value_type
This then means that if you wish to replace the keyword auto with a more explicit type specifier, then you could this;
for ( const pair<const string, int> &p : table ) {
std::cout << p.first << '\t' << p.second << std::endl;
}
Just for understanding what auto will translate to in this case.
As P0W has provided complete syntax for each C++ version, I would like to add couple of more points by looking at your code
Always take const & as argument as to avoid extra copies of the same object.
use unordered_map as its always faster to use. See this discussion
here is a sample code:
#include <iostream>
#include <unordered_map>
using namespace std;
void output(const auto& table)
{
for (auto const & [k, v] : table)
{
std::cout << "Key: " << k << " Value: " << v << std::endl;
}
}
int main() {
std::unordered_map<string, int> mydata = {
{"one", 1},
{"two", 2},
{"three", 3}
};
output(mydata);
return 0;
}
if you just want to iterate over the content without changing values
do:
for(const auto & variable_name : container_name(//here it is map name)){
cout << variable_name.first << " : " << variable_name.second << endl;
}
If you want to modify the contents of the map, remove the const and keep & (if you want to modify directly the contents inside container). If you want to work with a copy of the container values, remove the & sign too; after that, you can access them by using .first and .second on "variable_name".
it can even be done with a classic for loop.
advancing the iterator manually.
typedef std::map<int, int> Map;
Map mymap;
mymap['a']=50;
mymap['b']=100;
mymap['c']=150;
mymap['d']=200;
bool itexist = false;
int sizeMap = static_cast<int>(mymap.size());
auto it = mymap.begin();
for(int i = 0; i < sizeMap; i++){
std::cout << "Key: " << it->first << " Value: " << it->second << std::endl;
it++;
}
Other way :
map <int, string> myMap = {
{ 1,"Hello" },
{ 2,"stackOverflow" }
};
for (auto iter = cbegin(myMap); iter != cend(myMap); ++iter) {
cout << iter->second << endl;
}