I have below two pieces of code:
// multiset::begin/end
#include <iostream>
#include <set>
#include <vector>
int main ()
{
int myints[] = {42,71,12};
std::set<int> mymultiset (myints,myints+2);
std::set<int> mymultiset1 (myints,myints+2);
std::set<int>::iterator it=std::prev(mymultiset.end());
std::set<int>::iterator it1=std::prev(mymultiset1.end());
std::cout << "mymultiset contains:";
for (; it!=std::prev(mymultiset.begin()) && it1!=std::prev(mymultiset1.begin()); --it,--it1)
std::cout << "PPP" << *it;
std::cout << "\nmymultiset contains:";
for (it=std::prev(mymultiset.end()); it!=std::prev(mymultiset.begin()) ; --it)
std::cout << "UUU" << *it;
std::cout << "\nmymultiset contains:";
for ( it=mymultiset.begin(); it!=(mymultiset.end()); it++)
std::cout << "KKK" << *it;
std::cout << '\n';
return 0;
}
but why the output is
mymultiset contains:PPP71
mymultiset contains:UUU71
mymultiset contains:KKK42KKK71
1) I'm not sure, but there's probably UB and it can cause segmentation fault (for example when you use clang) :
std::prev(container.begin())
For GCC, std::prev(container.begin() and container.begin() return the same value.
2) Why didn't you use reverse_iterator?
for (auto it = mymultiset.rbegin(); it != mymultiset.rend(); ++it)
std::cout << "PPP" << *it;
or
for (auto it: boost::adaptors::reverse(mymultiset))
std::cout << "PPP" << it;
Related
I create and modify a simple list. I replace the element at index 1 of the list. How would I semantically accomplish the same thing with a while loop. The tutorial instructor remarked that the current code is quite ugly and a while loop would accomplish the same thing in a much more simple and pretty fashion. I can't figure it out.
#include <iostream>
#include <list>
int main() {
std::list<int> numbers;
numbers.push_back(1);
numbers.push_back(2);
numbers.push_back(3);
numbers.push_front(0);
std::list<int>::iterator it = numbers.begin();
it++;
numbers.insert(it, 100);
std::cout << "Current element is: " << *it << '\n';
std::list<int>::iterator eraseIt = numbers.begin();
eraseIt++;
eraseIt = numbers.erase(eraseIt);
std::cout << "erasing at element: " << *eraseIt << '\n';
for (std::list<int>::iterator it = numbers.begin(); it != numbers.end();) {
if (*it == 2) {
numbers.insert(it, 1234);
}
if (*it == 1) {
it = numbers.erase(it);
} else {
it++;
}
}
for (std::list<int>::iterator it = numbers.begin(); it != numbers.end();
it++) {
std::cout << *it << '\n';
}
return 0;
}
You can probably do this if iterators are required:
// ...
std::list<int>::iterator it = ++numbers.begin();
numbers.insert(it, 100);
std::cout << "Current element is: " << *it << '\n';
std::list<int>::iterator eraseIt = ++numbers.begin();
eraseIt = numbers.erase(eraseIt);
std::cout << "erasing at element: " << *eraseIt << '\n';
it = numbers.begin();
while (it != numbers.end())
{
if (*it == 2) {
numbers.insert(it, 1234);
}
if (*it == 1) {
it = numbers.erase(it);
}
else {
++it;
}
}
for (auto& i : numbers)
{
std::cout << i << std::endl;
}
// ...
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";
}
}
As the title says I want to print out all the elements of "group1" in this 2D map. I tried this but I just get a huge error at the for loop.
#include<iostream>
#include<string>
#include<map>
using namespace std;
int main(){
map<string,map<int,double> > myMap;
myMap["group1"][13] = 10.41;
myMap["group1"][15] = 31.2;
//print all elements in "group1"
for (map< string, map<int,double> >::const_iterator iter =
myMap["group1"].begin(); iter != myMap["group1"].end(); ++iter)
{
cout << iter->first << '\t' << iter->second << '\n';
}
return 0;
}
Any help would be appreciated, thanks!
The type of myMap["group1"] is map<int,double> and not map<string,map<int,double>>
Then your code would look somehow like this:
const map<int,double> &grp1map = myMap["group1"];
for (map<int,double>::const_iterator iter = grp1map.begin(); iter != grp1map.end(); ++iter)
{
cout << iter->first << '\t' << iter->second << '\n';
}
You can simplify this with auto:
auto &grp1map = myMap["group1"];
for (auto iter = grp1map.begin(); iter != grp1map.end(); ++iter)
{
cout << iter->first << '\t' << iter->second << '\n';
}
But anyway you should do it like this:
for (auto &elem : myMap["group1"] )
{
cout << elem.first << '\t' << elem.second << '\n';
}
You want an iterator to your internal map<int,double>, whereas you are trying to get and iterator to your outer map<string,map<int,double> >:
int main() {
map<string, map<int, double> > myMap;
myMap["group1"][13] = 10.41;
myMap["group1"][15] = 31.2;
//print all elements in "group1"
for (map<int, double>::const_iterator iter =
myMap["group1"].begin(); iter != myMap["group1"].end(); ++iter)
{
cout << iter->first << '\t' << iter->second << '\n';
}
return 0;
}
In my C++ script, I want to insert some elements of a list into a vector (from the beginning of list to a specific position "it"), and then try to add the vector at the top of the list and keeping the same order of the vector but I get unwanted additional elements in the vector.
Here is my code:
#include <iostream>
#include <iterator>
#include <vector>
#include <list>
int main() {
std::list<int> mylist;
for (int i = 0; i < 10; i++)
mylist.push_back(i * 10);
for (std::list<int>::iterator i = mylist.begin(); i <= mylist.end(); ++i) {
std::cout << *i << ", ";
}
std::cout << std::endl;
std::advance(it, 6);
std::cout << "The 6th element in mylist is: " << *it << std::endl;
// The vector that will contain mylist elements
std::vector<int> intAdeplacer;
intAdeplacer.insert(intAdeplacer.end(), mylist.begin(), it);
std::cout << std::endl;
std::cout << "Nombre d'éléments dans le vecteur : " << intAdeplacer.size() << std::endl;
std::cout << std::endl;
// see the content of the vector
std::cout << "Le vecteur de deplacement contient : " << std::endl;
for (std::vector<int>::const_iterator i = intAdeplacer.begin();
i <= intAdeplacer.end(); ++i) {
std::cout << *i << ", ";
}
I get this output:
Le vecteur de deplacement contient :
0, 10, 20, 30, 40, 134985,
134985 is not wanted..
// Insert in front of the list the values of the vector and keeping the same order of elements in the vector
for (std::vector<int>::const_iterator i = intAdeplacer.end();
i >= intAdeplacer.begin(); --i) {
mylist.push_front(*i);
}
std::cout << std::endl;
std::cout << std::endl;
std::cout << "nouvelle composition de mylist : " << std::endl;
for (std::list<int>::const_iterator j = mylist.begin(); j != mylist.end();
++j) {
std::cout << *j << ", ";
}
std::cout << std::endl;
std::cout << std::endl;
// erasing the added elements from mylist
std::list<int>::iterator debut = mylist.begin();
std::list<int>::iterator fin = mylist.end();
std::advance(fin, 6);
mylist.erase(debut, fin);
for (std::list<int>::iterator j = mylist.begin(); j <= mylist.end(); ++j) {
std::cout << *j << ", ";
}
return 0;
}
If it is an iterator pointing at the 6th element of your list, the following insert() will insert from begin() (included) to it (excluded) into the vector:
intAdeplacer.insert(intAdeplacer.end(), mylist.begin(), it);
So you'll have only 5 elements, from 0 to 40. Unfortunately your printing loop does include the end() of the vector, which is out of range. This is why you get this strange trailing number.
Just correct your loop into:
for (auto i = intAdeplacer.begin(); i != intAdeplacer.end(); ++i) {
std::cout << *i << ", ";
}
Or consider this alternate range-for syntax:
for (auto &element: intAdeplacer) {
std::cout << element << ", ";
}
I have to print out the size of the vector and all the contents inside it. But my for loop doesnt iterate, it doesn't go up by one but instead stays at the value 0 for the whole loop.
#include "stdafx.h"
#include <string.h>
#include <string>
#include <iostream>
#include <cctype>
#include <vector>
#include <list>
using std::string;
using std::vector;
vector<int> v2(10);
for( auto i : v2)
{
if( i == 0 )
{
std::cout << v2.size() << std::endl;
}
std::cout << "Element value " << (i) << " is " << v2[i] << std::endl;
}
So I only want to print the size once, at the start. Then print out each element value which I know will be 0 by default. But it just prints out "Element value 0 is 0" 9 times.
If you want at first to print out the size of the vector then place this output statement before the range based for statement
std::vector<int> v2(10);
std::cout << v2.size() << std::endl;
size_t i = 0;
for ( auto x : v2 )
{
std::cout << "Element value " << i++ << " is " << x << std::endl;
}
But if you are using a count then it would be better to use the ordinary for statement
std::vector<int> v2(10);
std::cout << v2.size() << std::endl;
for ( std::vector<int>::size_type i = 0; i < v2.size(); i++ )
{
std::cout << "Element value " << i << " is " << v2[i] << std::endl;
}
Your question indicates that you want to treat the first element differently. This does not work with range-based for loops out of the box. You have two options:
Use an extra variable to remember whether the first iteration was made.
Use a traditional loop, using an index variable or an iterator.
Example for 1:
bool first_iteration = true;
for( auto i : v2)
{
if (first_iteration)
{
std::cout << v2.size() << std::endl;
first_iteration = false;
}
// ...
}
Example for 2:
for (auto iter = v2.begin(); iter != v2.end(); ++iter)
{
if (iter == v2.begin())
{
std::cout << v2.size() << std::endl;
}
// ...
}