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

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

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; } 

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 = ", " } 

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.

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<<.

Something like this?

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

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.

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; 

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.

I think simplicity is better for me, so after I look through all answers I prepared my solution(c++14 required):

#include <iostream> #include <vector> #include <utility> // for std::exchange c++14 int main() { std::vector nums{1, 2, 3, 4, 5}; // c++17 const char* delim = ""; for (const auto value : nums) { std::cout << std::exchange(delim, ", ") << value; } } 

Output example:

1, 2, 3, 4, 5 

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; 

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; } } 

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.

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; } 

Starting with C++23 you can use std::format or std::print (or std::println) to do this.

#include <print> #include <vector> int main() { std::print("{}", std::vector{2, 3, 5, 7}); } 

[2, 3, 5, 7] 

If you need the result as std::string use std::format. Note that formatting ranges requires C++23 support, specifically the implementation of P2286R8 (check C++23 library features).

#include <format> #include <vector> int main() { std::string text = std::format("{}", std::vector{2, 3, 5, 7}); } 

If you are stuck to an older standard, you can use the fmt library to print ranges.

#include <fmt/ranges.h> #include <vector> int main() { // direct print fmt::print("{}", std::vector{2, 3, 5, 7}); // create std::string object std::string str = fmt::format("{}", std::vector{2, 3, 5, 7}); } 

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().

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; 

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(); 

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; 

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 

I think this should work

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

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); 

Could be like so..

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

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"; } 

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, ", ")); 

I use this:

template<class T> void print(vector<T> v, ostream& f, const char* separator) { std::copy(v.begin(), v.end()-1, std::ostream_iterator<T>(f, separator)); f << *(v.end()-1) << endl;} 

you call it like this:

print(v, std::cout, ","); 

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; } 

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; 

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; }