map,vector in c++ [duplicate] - c++

Question

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How do I print out the contents of a vector?
(31 answers)
Closed 2 years ago.
error: no match for ‘operator<<’ (operand types are ‘std::basic_ostream’ and ‘std::pair<const std::__cxx11::basic_string, std::vector >’)
i want to same key and mutiple values, for example key is 10 values are 2,3,4
but "*iter" is wrong..
how to cout map,vector in c++?

Answer 1

In your code snippet the value of the expression *iter is an object of the type std::pair<std::string, std::vector<int>> for which the operator << is not defined.
And the error message
error: no match for ‘operator<<’ (operand types are ‘std::basic_ostream’ and
‘std::pair<const std::__cxx11::basic_string, std::vector >’)
says about this.
The simplest way is to use the range-based for loop.
Here is a demonstrative program.
#include <iostream>
#include <string>
#include <vector>
#include <map>
int main()
{
std::map<std::string, std::vector<int>> m;
m["10"].assign( { 2, 3, 4 } );
for ( const auto &p : m )
{
std::cout << p.first << ": ";
for ( const auto &item : p.second )
{
std::cout << item << ' ';
}
std::cout << '\n';
}
return 0;
}
The program output is
10: 2 3 4
If you want to write ordinary for-loops using iterators then the loops can look the following way.
#include <iostream>
#include <string>
#include <vector>
#include <map>
#include <iterator>
int main()
{
std::map<std::string, std::vector<int>> m;
m["10"].assign( { 2, 3, 4 } );
for ( auto outer_it = std::begin( m ); outer_it != std::end( m ); ++outer_it )
{
std::cout << outer_it->first << ": ";
for ( auto inner_it = std::begin( outer_it->second );
inner_it != std::end( outer_it->second );
++inner_it )
{
std::cout << *inner_it << ' ';
}
std::cout << '\n';
}
return 0;
}
Again the program output is
10: 2 3 4

Answer 2

I suggest using structured bindings and range-based for loops:
std::map<std::string,std::vector<int>> m;
for (auto&[str, vec] : m) { // bind str to "first" in the pair and vec to "second"
std::cout << str << ':';
for(auto lineno : vec) std::cout << ' ' << lineno;
std::cout << '\n';
}

Answer 3

You can define how to print things via std::ostream like this:
#include <iostream>
#include <map>
#include <vector>
#include <string>
// define how to print std::pair<std::string, std::vector<int>>
std::ostream& operator<<(std::ostream& stream, const std::pair<std::string, std::vector<int>>& pair) {
stream << "(" << pair.first << ", {";
bool first = true;
for (int e : pair.second) {
if (!first) stream << ", ";
stream << e;
first = false;
}
stream << "})";
return stream;
}
int main(void) {
std::string yytext = "hoge";
int lineno = 42;
// below is copied from the question
std::map<std::string,std::vector<int>> m;
m[yytext].push_back(lineno);
std::map<std::string,std::vector<int>>::iterator iter;
for (iter=m.begin(); iter!=m.end(); iter++){
std::cout<<iter->first<<":"<<*iter<<std::endl;}
}