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How to use something like s.end() in for loop with auto?

I have a below program, I want to see how to condition on last element in the container when using auto in C++11, that's why this question.

#include <iostream>
#include <set>

using namespace std;

void show(set<string>& s)
cout << "<";
for (const auto &e: s)
if (e != s.end()) // This is WRONG -- results in compilation error !!
cout << e << ",";
else
cout << e;
cout << ">" << endl;


int main(int argc, char *argv)
set <string> s = "a", "e", "i", "o" ;
show(s);
return 0;



above program outputs <a,e,i,o,>

<a,e,i,o,>

I tried to use s.end() in the for loop in the show(), but compilation fails.

s.end()

for

show()

desired output :

<a,e,i,o>


std::copy(s.cbegin(), s.cend(), nstd::join_iterator<std::string>(std::cout, ", "));

std::experimental::ostream_joiner

std::vector<int> sink; nstd::join_iterator<std::vector<int>> i(sink, 42);

4 Answers
4

Use this:

if (&e != &*s.rbegin())


Or even better, print comma before an element. I assume it could be slightly faster.

if (&e != &*s.begin())
cout << "," << e;
else
cout << e;


bool first = 1; for (...) if (first) first = 0; else std::cout << ','; std::cout << e;

&

The range based for loops sacrifice power for simplicity. Making use of auto looping over iterators is less fiddly than it used to be:

auto

for (auto it = s.begin(); it != s.end(); ++it)

if (std::next(it) != s.end())
cout << e << ",";
else
cout << e;



It'd be nice if there was a portable way to get your current position in a range based for loop but until that gets added to the standard you'll just have to make the choice between simplicity and flexibility.

I think, the shortest way is to test a 'bool' variable before adding a comma to the output stream. The following code should fits your needs.

#include<iostream>
#include<set>
using namespace std;

void show(set<string>& s);

int main(int argc, char *argv)
set <string> s = "a", "e", "i", "o" ;
show(s);
return 0;


void show(set<string>& s)
cout<<'<';
bool add_comma=false;
for (const auto& e: s)
if (add_comma)cout<<',';
cout<<e;
add_comma=true;

cout<<'>';



You usually get more readable code by printing first element outside of the main loop.

void show(const set<string>& s)
cout << "<";
auto it = s.begin();
if (it != s.end())
cout << *it++;

for (; it != s.end(); ++it)
cout << "," << *it;

cout << ">" << endl;



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