I am wanting to iterate through my entire column that contains the type double and divide each value by the size of the column. This will give me the frequency. I would have not issue doing this in an array or any other datatype. I am still learning about the vectors. Here is the 2D vector type that I am trying to manipulate
vector<pair<char, double>> output;
If you have C++11:
for (auto & p : output)
{
cout << p.first << " " << p.second << "\n";
}
std::cout << std::endl;
or with C++03
for (std::vector<std::pair<char, double> >::iterator it = output.begin();
it != output.end(); ++it)
{
cout << it .first << " " << it .second << "\n";
}
std::cout << std::endl;
You can treat a vector just like an array, and access the elements using []. For example:
for (size_t i = 0; i < output.size(); ++i)
{
pair<char, double>& element = output[i]; // access element i in the vector
cout << element.first; // prints the first member of the pair
cout << element.second; // prints the second member of the pair
}
std::pair is a struct template that provides a way to store two
heterogeneous objects as a single unit.
if you have a vector of pairs this means you will access data through pair interface . You can get first data of pair via first member and second via second.
for(std::vector<std::pair <char, double> >::const_iterator vpci = arg.begin();
vpci != arg.end(); ++vpci) {
cout << vpci->first << "->" << vpci->second;
}
or even better, maybe create a template:
template <typename T1, typename T2>
void prn_vecOfPair(const std::vector<std::pair <T1, T2> > &arg, string sep ="") {
for(std::vector<std::pair <T1, T2> >::const_iterator vpci = arg.begin();
vpci != arg.end(); ++vpci) {
cout << vpci->first << "->" << vpci->second << sep;
}
}
in C++11 this (as usual) can be done much easier:
for (auto & i : output)
{
cout << i->first << "->" << i->second << "\n";
}
Related
First one being:
map <int,int> m;
//... some elements inserted
auto i= m.begin();
cout<<(*i).first<<(*i).second;
Here we are required to use the dereference operator *
Second:
map <int,int> m;
//... some elements inserted
for(auto i: m)
cout<<i.first<<i.second;
Why am I not required to use the * operator this time?
One more doubt:
for(auto &i: m)
what difference does '&' make here?
auto i=m.begin() will give you iterator .. which is accessed more like a pointer (syntactically) when you want to access the value...
for(auto i:m) will copy current element of m (a pair) into i , i is a copy of element, not the element itself...
for (auto &i: m) will work on a reference, the original map is affected
As explained in the below code snippet, the first i is of type iterator where as the i in the for loops are of pair type.
#include <iostream>
#include <map>
int main()
{
std::map <int,int> m;
m[1] = 5;
m[10] = 60;
m[100] = 800;
// Below i is map iterator (std::map<int, int>::iterator)
auto i = m.begin();
std::cout << typeid(i).name() << '\n';
std::cout << (*i).first << " : " << (*i).second << '\n';
std::cout << i->first << " : " << i->second << '\n';
for(auto i: m) {
// Below i is pair<int, int>
std::cout << typeid(i).name() << '\n';
std::cout << i.first << " : " << i.second << '\n';
}
for(auto& i: m) {
// Below i is reference of pair<int, int>)
// modifying this would result in updated values in the map.
std::cout << typeid(i).name() << '\n';
std::cout << i.first << " : " << i.second << '\n';
}
return 0;
}
I want to printout my vector from vector list
std::list < std::vector< unsigned int >> lista;
for (std::list< std::vector< unsigned int > >::iterator i = lista.begin(); i != lista.end(); ++i){
std::cout << std::hex << whats_here << " " << std::endl;
}
I don't know what should I place on whats_here. Any help will be appreciated.
Really simple:
for (auto &list_item : lista)
{
for (auto &item : list_item)
{
std::cout << std::hex << item << " " << std::endl;
}
std::cout << std::endl;
}
Make any required adjustment to get the desired output. Since the expected output is not given in the question, the above code is my best guess given your non working code.
I am trying to display the contents of a multimap with a string and a vector as key and value, respectively. I run into problems when I try to dislpay the contents of the vector (value in multimap). What I have done so far is:
multimap<string, vector<string> > someMultimap;
vector<string> someVector;
someVector.push_back("test");
someMultimap.insert(pair<string, vector<string> >("KEY", someVector));
//So for it works fine...
multimap <string, vector<string> >::iterator it;
for (it = someMultimap.begin(); it != someMultimap.end(); ++it)
{
cout << it->first << endl << endl << it->second << endl << endl;
^^^^^^^^^^
this is the problem, can´t do this with a vector(?)
}
Does anyone have a suggestion for solving the problem?
You can add a second loop for the vector.
multimap <string, vector<string> >::const_iterator it;
for (it = someMultimap.begin(); it != someMultimap.end(); ++it)
{
cout << it->first << endl ;
vector<string>::const_iterator itVec;
for (itVec = it->second.begin(); itVec != it->second.end(); ++itVec)
{
cout << *itVec <<" ";
}
cout<<endl;
}
If you don't plan to modify any value you can make your iterators const
Use std::copy to output the vector's contents with a single function:
#include <iterator>
#include <algorithm>
#include <iostream>
//..
using namespace std;
for (it = someMultimap.begin(); it != someMultimap.end(); ++it)
{
cout << it->first << "\n\n";
copy(it->second.begin(), it->second.end(), ostream_iterator<string>(cout, " "));
cout << "\n\n";
}
Loop through the vector and print:
cout << it->first << endl << endl;
for (vector<string>::iterator j(it->second.begin()); j != it->second.end(); ++j)
cout << *j << " ";
cout << endl << endl;
Also, don't use std::endl, it flushes on each call which can hinder performance.
Well, you can define what it means to send a std::vector to a std::ostream.
Here's an example of one way to do it:
template<typename T>
std::ostream& operator <<(std::ostream& stream, std::vector<T> const& v) {
for (auto&& e : v) {
stream << e << ' ';
}
return stream;
}
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;
}
How can I print the contents of a nested map? I am counting the number of times a word appears in a file, reporting it by line number and number of times per line. The words, lines, and occurrences per line are being stored in the following container:
map<string, map<int, int>> tokens;
However, I'm not sure on the syntax. I am printing the outer map that lists all of the words using the following code, but can't figure out how to print the inner values (the line number and number of times the word appears on each line) as well. I assume I can just include it inline in the for loop, but I can't figure out how:
for (map <string, map<int, int>>::iterator it = tokens.begin(); it != tokens.end(); ++it){
cout << it->first << " : " << /* assume I can include another statement here to print the values? */ endl;
}
I am trying to get an output similar to this:
(word : line:occurrences, line:occurrences, ...)
about : 16:1, 29:1, 166:1, 190:1, 191:1
above : 137:1
accompanied : 6:1
across : 26:1
admit : 20:1
advancing : 170:1
.
.
.
It is actually pretty simple.
You just get the internal map with it->second, and you iterate through that the same way.
Thereby, you would write something like this:
for (map <string, map<int, int>>::iterator it = tokens.begin(); it != tokens.end(); ++it){
cout << it->first << " : ";
map<int, int> &internal_map = it->second;
for (map<int, int>::iterator it2 = internal_map.begin(); it2 != internal_map.end(); ++it2){
if (it2 != internal_map.begin())
cout << ",";
cout << it2->first << ":" << it2->second;
}
cout << endl;
}
You could write something like this if you have C++11 support:
for (auto it : tokens) {
cout << it->first << " : ";
map<int, int> &internal_map = it->second;
for (auto it2: internal_map) {
if (it2 != internal_map.begin())
cout << ",";
cout << it2->first << ":" << it2->second;
}
cout << endl;
}
C++17
Since C++17 you can use range-based for loops together with structured bindings for iterating over maps.
This way, the readability of lpapp's C++11 solution can be further improved as follows:
for (auto const &[k1, v1] : tokens) {
std::cout << k1 << " : ";
for (auto const &[k2, v2] : v1) {
if (&k2 != &v1.begin()->first)
std::cout << ", ";
std::cout << k2 << ":" << v2;
}
std::cout << std::endl;
}
Note: I admit, the check for printing a comma is a bit messy. If you are looking for a nicer solution, you
might want to take a look at this Q&A.
Code on Coliru
Since we are only printing the elements, not modifying it, I would make more use of const, (a) const reference and (b) const iterators.
Also, for pre-C++11, it helps to define typedef's for complex types.
#include <string>
#include <map>
#include <iostream>
using namespace std;
typedef map<int, int> InnerMap;
typedef map<string, InnerMap> OuterMap;
void printClassic( OuterMap const & tokens ) {
for( OuterMap::const_iterator cit = tokens.begin();
cit != tokens.end(); ++cit ) {
cout << cit->first << " : ";
InnerMap const & imap = cit->second;
for( InnerMap::const_iterator cit2 = imap.begin();
cit2 != imap.end(); ++cit2 ) {
cout << cit2->first << ":" << cit2->second << ",";
}
cout << endl;
}
}
void printCpp11( OuterMap const & tokens ) {
for( auto const & cit : tokens ) {
cout << cit.first << " : ";
auto const & imap = cit.second;
for( auto const & cit2 : imap ) {
cout << cit2.first << ":" << cit2.second << ",";
}
cout << endl;
}
}