How can I print a list of elements separated by commas?

Question

I know how to do this in other languages, but not in C++, which I am forced to use here.

I have a set of strings (keywords) that I'm printing to out as a list, and the strings need a comma between them, but not a trailing comma. In Java, for instance, I would use a StringBuilder and just delete the comma off the end after I've built my string. How can I do it in C++?

auto iter = keywords.begin();
for (iter; iter != keywords.end( ); iter++ )
{
    out << *iter << ", ";
}
out << endl;

I initially tried inserting the following block to do it (moving the comma printing here):

if (iter++ != keywords.end())
    out << ", ";
iter--;

Answer 1

Use an infix_iterator:

// infix_iterator.h 
// 
// Lifted from Jerry Coffin's 's prefix_ostream_iterator 
#if !defined(INFIX_ITERATOR_H_) 
#define  INFIX_ITERATOR_H_ 
#include <ostream> 
#include <iterator> 
template <class T, 
          class charT=char, 
          class traits=std::char_traits<charT> > 
class infix_ostream_iterator : 
    public std::iterator<std::output_iterator_tag,void,void,void,void> 
{ 
    std::basic_ostream<charT,traits> *os; 
    charT const* delimiter; 
    bool first_elem; 
public: 
    typedef charT char_type; 
    typedef traits traits_type; 
    typedef std::basic_ostream<charT,traits> ostream_type; 
    infix_ostream_iterator(ostream_type& s) 
        : os(&s),delimiter(0), first_elem(true) 
    {} 
    infix_ostream_iterator(ostream_type& s, charT const *d) 
        : os(&s),delimiter(d), first_elem(true) 
    {} 
    infix_ostream_iterator<T,charT,traits>& operator=(T const &item) 
    { 
        // Here's the only real change from ostream_iterator: 
        // Normally, the '*os << item;' would come before the 'if'. 
        if (!first_elem && delimiter != 0) 
            *os << delimiter; 
        *os << item; 
        first_elem = false; 
        return *this; 
    } 
    infix_ostream_iterator<T,charT,traits> &operator*() { 
        return *this; 
    } 
    infix_ostream_iterator<T,charT,traits> &operator++() { 
        return *this; 
    } 
    infix_ostream_iterator<T,charT,traits> &operator++(int) { 
        return *this; 
    } 
};     
#endif 

Usage would be something like:

#include "infix_iterator.h"

// ...
std::copy(keywords.begin(), keywords.end(), infix_iterator(out, ","));

Answer 2

In an experimental C++17 ready compiler coming soon to you, you can use std::experimental::ostream_joiner:

#include <algorithm>
#include <experimental/iterator>
#include <iostream>
#include <iterator>

int main()
{
    int i[] = {1, 2, 3, 4, 5};
    std::copy(std::begin(i),
              std::end(i),
              std::experimental::make_ostream_joiner(std::cout, ", "));
}

Live examples using GCC 6.0 SVN and Clang 3.9 SVN

Answer 3

Because everyone has decided to do this with while loops, I'll give an example with for loops.

for (iter = keywords.begin(); iter != keywords.end(); iter++) {
  if (iter != keywords.begin()) cout << ", ";
  cout << *iter;
}

Answer 4

Assuming a vaguely normal output stream, so that writing an empty string to it does indeed do nothing:

const char *padding = "";
for (auto iter = keywords.begin(); iter != keywords.end(); ++iter) {
    out << padding << *iter;
    padding = ", "
}

Answer 5

One common approach is to print the first item prior to the loop, and loop only over the remaining items, PRE-printing a comma before each remaining item.

Alternately you should be able to create your own stream that maintains a current state of the line (before endl) and puts commas in the appropriate place.

EDIT: You can also use a middle-tested loop as suggested by T.E.D. It would be something like:

if(!keywords.empty())
{
    auto iter = keywords.begin();
    while(true)
    {
        out << *iter;
        ++iter;
        if(iter == keywords.end())
        {
            break;
        }
        else
        {
            out << ", ";
        }
    }
}

I mentioned the "print first item before loop" method first because it keeps the loop body really simple, but any of the approaches work fine.

Answer 6

There are lots of clever solutions, and too many that mangle the code beyond hope of salvation without letting the compiler do its job.

The obvious solution, is to special-case the first iteration:

bool first = true;
for (auto const& e: sequence) {
   if (first) { first = false; } else { out << ", "; }
   out << e;
}

It's a dead simple pattern which:

1. Does not mangle the loop: it's still obvious at a glance that each element will be iterated on.
2. Allows more than just putting a separator, or actually printing a list, as the else block and the loop body can contain arbitrary statements.

It may not be the absolutely most efficient code, but the potential performance loss of a single well-predicted branch is very likely to be overshadowed by the massive behemoth that is std::ostream::operator<<.

Answer 7

Something like this?

while (iter != keywords.end())
{
 out << *iter;
 iter++;
 if (iter != keywords.end()) cout << ", ";
}

Answer 8

In python we just write:

print ", ".join(keywords)

so why not:

template<class S, class V>
std::string
join(const S& sep, const V& v)
{
  std::ostringstream oss;
  if (!v.empty()) {
    typename V::const_iterator it = v.begin();
    oss << *it++;
    for (typename V::const_iterator e = v.end(); it != e; ++it)
      oss << sep << *it;
  }
  return oss.str();
}

and then just use it like:

cout << join(", ", keywords) << endl;

Unlike in the python example above where the " " is a string and the keywords has to be an iterable of strings, here in this C++ example the separator and keywords can be anything streamable, e.g.

cout << join('\n', keywords) << endl;

Answer 9

My typical method for doing separators (in any language) is to use a mid-tested loop. The C++ code would be:

for (;;) {
   std::cout << *iter;
   if (++iter == keywords.end()) break;
   std::cout << ",";
}

(note: An extra if check is needed prior to the loop if keywords may be empty)

Most of the other solutions shown end up doing an entire extra test every loop iteration. You are doing I/O, so the time taken by that isn't a huge problem, but it offends my sensibilities.

Answer 10

Try this:

typedef  std::vector<std::string>   Container;
typedef Container::const_iterator   CIter;
Container   data;

// Now fill the container.


// Now print the container.
// The advantage of this technique is that ther is no extra test during the loop.
// There is only one additional test !test.empty() done at the beginning.
if (!data.empty())
{
    std::cout << data[0];
    for(CIter loop = data.begin() + 1; loop != data.end(); ++loop)
    {
        std::cout << "," << *loop;
    }
}

Answer 11

to avoid placing an if inside the loop, I use this:

vector<int> keywords = {1, 2, 3, 4, 5};

if (!keywords.empty())
{
    copy(keywords.begin(), std::prev(keywords.end()), 
         std::ostream_iterator<int> (std::cout,", "));
    std::cout << keywords.back();
}

It depends on the vector type, int, but you can remove it with some helper.

Answer 12

I suggest you simply switch the first character with the help of a lambda.

std::function<std::string()> f = [&]() {f = [](){ return ","; }; return ""; };                  

for (auto &k : keywords)
    std::cout << f() << k;

Answer 13

If the values are std::strings you can write this nicely in a declarative style with range-v3

#include <range/v3/all.hpp>
#include <vector>
#include <iostream>
#include <string>

int main()
{
    using namespace ranges;
    std::vector<std::string> const vv = { "a","b","c" };

    auto joined = vv | view::join(',');

    std::cout << to_<std::string>(joined) << std::endl;
}

For other types which have to be converted to string you can just add a transformation calling to_string.

#include <range/v3/all.hpp>
#include <vector>
#include <iostream>
#include <string>

int main()
{
    using namespace ranges;
    std::vector<int> const vv = { 1,2,3 };

    auto joined = vv | view::transform([](int x) {return std::to_string(x);})
                     | view::join(',');
    std::cout << to_<std::string>(joined) << std::endl;
}

Answer 14

There is a little problem with the ++ operator you are using.

You can try:

if (++iter != keywords.end())
    out << ", ";
iter--;

This way, ++ will be evaluated before compare the iterator with keywords.end().

Answer 15

Following should do:-

 const std::vector<__int64>& a_setRequestId
 std::stringstream strStream;
 std::copy(a_setRequestId.begin(), a_setRequestId.end() -1, std::ostream_iterator<__int64>(strStream, ", "));
 strStream << a_setRequestId.back();

Answer 16

It's very easy to fix that (taken from my answer here):

bool print_delim = false;
for (auto iter = keywords.begin(); iter != keywords.end( ); iter++ ) {
    if(print_delim) {
        out << ", ";
    }
    out << *iter;
    print_delim = true;
}
out << endl;

---

I am using this idiom (pattern?) in many programming languages, and all kind of tasks where you need to construct delimited output from list like inputs. Let me give the abstract in pseudo code:

empty output
firstIteration = true
foreach item in list
    if firstIteration
        add delimiter to output
    add item to output
    firstIteration = false

In some cases one could even omit the firstIteration indicator variable completely:

empty output
foreach item in list
    if not is_empty(output)
        add delimiter to output
    add item to output

Answer 17

I use a little helper class for that:

class text_separator {
public:
    text_separator(const char* sep) : sep(sep), needsep(false) {}

    // returns an empty string the first time it is called
    // returns the provided separator string every other time
    const char* operator()() {
        if (needsep)
            return sep;
        needsep = true;
        return "";
    }

    void reset() { needsep = false; }

private:
    const char* sep;
    bool needsep;
};

To use it:

text_separator sep(", ");
for (int i = 0; i < 10; ++i)
    cout << sep() << i;

Answer 18

Using boost:

std::string add_str("");
const std::string sep(",");

for_each(v.begin(), v.end(), add_str += boost::lambda::ret<std::string>(boost::lambda::_1 + sep));

and you obtain a string containing the vector, comma delimited.

EDIT: to remove the last comma, just issue:

add_str = add_str.substr(0, add_str.size()-1);

Answer 19

Another possible solution, which avoids an if

Char comma = '[';
for (const auto& element : elements) {
    std::cout.put(comma) << element;
    comma = ',';
}
std::cout.put(']');

Depends what you're doing in your loop.

Answer 20

I think this should work

while (iter != keywords.end( ))
{

    out << *iter;
    iter++ ;
    if (iter != keywords.end( )) out << ", ";
}

Answer 21

Could be like so..

bool bFirst = true;
for (auto curr = keywords.begin();  curr != keywords.end(); ++curr) {
   std::cout << (bFirst ? "" : ", ") << *curr;
   bFirst = false;
}

Answer 22

I think this variant of @MarkB's answer strikes optimal balance of readability, simplicity and terseness:

auto iter= keywords.begin();
if (iter!=keywords.end()) {
    out << *iter;
    while(++iter != keywords.end())
        out << "," << *iter;
}
out << endl;

Answer 23

This one overloads the stream operator. Yes global variables are evil.

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <iterator>

int index = 0;
template<typename T, template <typename, typename> class Cont>
std::ostream& operator<<(std::ostream& os, const Cont<T, std::allocator<T>>& vec)
{
    if (index < vec.size()) {
        if (index + 1 < vec.size())
            return os << vec[index++] << "-" << vec;
        else
            return os << vec[index++] << vec;
    } else return os;
}

int main()
{
    std::vector<int> nums(10);
    int n{0};
    std::generate(nums.begin(), nums.end(), [&]{ return n++; });
    std::cout << nums << std::endl;
}

Answer 24

You can use a do loop, rewrite the loop condition for the first iteration, and use the short-circuit && operator and the fact that a valid stream is true.

auto iter = keywords.begin();
if ( ! keywords.empty() ) do {
    out << * iter;
} while ( ++ iter != keywords.end() && out << ", " );
out << endl;

Answer 25

I would go with something like this, an easy solution and should work for all iterators.

int maxele = maxele = v.size() - 1;
for ( cur = v.begin() , i = 0; i < maxele ; ++i)
{
    std::cout << *cur++ << " , ";
}
if ( maxele >= 0 )
{
  std::cout << *cur << std::endl;
}

Answer 26

Can use functors:

#include <functional>

string getSeparatedValues(function<bool()> condition, function<string()> output, string separator)
{
    string out;
    out += output();
    while (condition())
        out += separator + output();
    return out;
}

Example:

if (!keywords.empty())
{
    auto iter = keywords.begin();
    cout << getSeparatedValues([&]() { return ++iter != keywords.end(); }, [&]() { return *iter; }, ", ") << endl;
}

Answer 27

A combination of c++11 lambda and macro:

#define INFIX_PRINTER(os, sep)([&]()->decltype(os)&{static int f=1;os<<(f?(f=0,""):sep);return os;})()

Usage:

for(const auto& k: keywords)
    INFIX_PRINTER(out, ", ") << k;

Answer 28

Here are two methods you could use, which are both essentially the same idea. I like these methods because they do not contain any unnecessary conditional checks or assignment operations. I'll call the first one the print first method.

Method 1: the print first method

if (!keywords.empty()) {
    out << *(keywords.begin()); // First element.
    for (auto it = ++(keywords.begin()); it != keywords.end(); it++)
        out << ", " << *it; // Every subsequent element.
}

This is the method I used at first. It works by printing the first element in your container by itself, and then prints every subsequent element preceded by a comma and space. It's simple, concise, and works great if that's all you need it to do. Once you want to do more things, like add an "and" before the last element, this method falls short. You'd have to check each loop iteration for if it's on the last element. Adding a period, or newline after the list wouldn't be so bad, though. You could just add one more line after the for-loop to append whatever you desire to the list.

The second method I like a lot more. That one I'll call the print last method, as it does the same thing as the first but in reverse order.

Method 2: the print last method

if (!keywords.empty()) {
    auto it = keywords.begin(), last = std::prev(keywords.end());
    for (; it != last; it++) // Every preceding element.
        out << *it << ", ";
    out << "and " << *it << ".\n"; // Last element.
}

This one works by printing every element except for the last with a comma and space, allowing you to optionally add an "and" before it, a period after it, and/or a newline character. As you can see, this method gives you a lot more options on how you can handle that last element without affecting the performance of the loop or adding much code.

If it bothers you to leave the first part of the for-loop empty, you could write it like so:

if (!keywords.empty()) {
    auto it, last;
    for (it = keywords.begin(), last = std::prev(keywords.end()); it != last; it++)
        out << *it << ", ";
    out << "and " << *it << ".\n";
}

Answer 29

I like a range-based for with a is_last_elem test. That imho it's very readable:

for (auto& e : range)
{
    if (!is_last_elem(e, range)) [[likely]] 
        os << e << ", ";
    else
        os << e;
}
os << std::endl;

Full code:

C++20:

#include <iostream>
#include <list>
#include <ranges>
#include <utility>
#include <type_traits>
#include <memory>

template <std::ranges::bidirectional_range R>
bool is_last_elem(const std::ranges::range_value_t<R>& elem, const R& range)
{
    auto last_it = range.end();
    std::advance(last_it, -1);
    return std::addressof(elem) == std::addressof(*last_it);
}

template <std::ranges::bidirectional_range R, class Stream = std::ostream>
void print(const R& range, std::ostream& os = std::cout)
{
    for (auto& e : range)
    {
        if (!is_last_elem(e, range)) [[likely]] 
            os << e << ", ";
        else
            os << e;
    }
    os << std::endl;
}

int main()
{
    std::list<int> v{1, 2, 3, 4, 5};
    print(v);
}

C++17:

#include <iostream>
#include <list>
#include <utility>
#include <type_traits>
#include <memory>

template <class Range>
using value_type_t = std::remove_reference_t<decltype(*std::begin(std::declval<Range>()))>;

template <class Range>
bool is_last_elem(const value_type_t<Range>& elem, const Range& range)
{
    auto last_it = range.end();
    std::advance(last_it, -1);
    return std::addressof(elem) == std::addressof(*last_it);
}

template <class Range, class Stream = std::ostream>
void print(const Range& range, std::ostream& os = std::cout)
{
    for (auto& e : range)
    {
        if (!is_last_elem(e, range))
            os << e << ", ";
        else
            os << e;
    }
    os << std::endl;
}

int main()
{
    std::list<int> v{1, 2, 3, 4, 5};
    print(v);
}

Answer 30

C++20 brings the formatting library. However as of now (april 2021) neither gcc nor clang implement it yet. But we can use the fmt library on which it is based on:

std::list<int> v{1, 2, 3, 4, 5};
fmt::print("{}", fmt::join(v, ", "));

Answer 31

Since C++20, if you are looking for a compact solution and the solution by bolov is not yet supported by your compiler, you can use a range-based for loop with an init-statement for the first flag and a conditional operator as follows:

std::set<std::string> keywords {"these", "are", "my", "keywords"};

for (bool first{true}; auto const& kw : keywords)
    std::cout << (first ? first = false, "" : ", ") << kw;

Output:

are, keywords, my, these

Note: I found this solution in the Example section on this page at cppreference.com.

Code on Wandbox