How to concatenate strings with space into one string with STL in c++

Question

Given a vector of string ["one", "two", "three"].

Question> How do I convert it into "one two three"?

I know the manual way to do it with a loop and would like to know whether there is a simpler way with STL function.

// Updated based on suggestion that I should use accumulate

#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
using namespace std;

struct BindTwoStrings
{
    string operator()(const string& s1, const string& s2) const {
        return s1.empty() ? s2 : s1 + " " + s2;
    }    
};

int main()
{    
   vector<string> vecString {"one", "two", "three"};   
   string ret2;   

   ret2 = accumulate(vecString.begin(), vecString.end(), ret2, 
           [] (const string& s1, const string& s2) -> string { 
              return s1.empty() ? s2 : s1 + " " + s2; });
   cout << "ret2:\"" << ret2 << "\"" << endl;

   string ret;
   ret = accumulate(vecString.begin(), vecString.end(), ret, BindTwoStrings());

   cout << "ret:\"" << ret << "\"" << endl;
   return 0;
}

Or `std::stringstream` with `std::ostream_iterator` and `std::copy`. — n0rd, Jan 08 '14 at 02:57
[Concatenating strings](http://stackoverflow.com/questions/720344/concatenating-strings) — Roger Stewart, Jan 08 '14 at 03:05

Dietmar Kühl · Answer 1 · 2014-01-08T03:02:50.227

4

You could use std::accumulate():

std::string concat = std::accumulate(std::begin(array) + 1, std::end(array), array[0],
    [](std::string s0, std::string const& s1) { return s0 += " " + s1; });

edited Jan 08 '14 at 03:02

answered Jan 08 '14 at 02:58

Dietmar Kühl

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@P0W: that will have a leading space followed by a sequence of not separated strings... – Dietmar Kühl Jan 08 '14 at 03:05
@DietmarKühl, Ah, sorry, it didn't seem to give a space at the beginning for me, which I find odd now that I think about it. – chris Jan 08 '14 at 03:07
@DietmarKühl Ahh yeah, accumulate always confuses me – P0W Jan 08 '14 at 03:10
@DietmarKühl, can you check my update post and see whether my solution work? – q0987 Jan 08 '14 at 03:14
This works but slow as hell. std::ostringstream is much better in performance. – frozenca May 12 '19 at 08:52

Gabriel L. · Answer 2 · 2014-01-08T03:06:53.123

3

With a std::stringstream, you could do that :

std::stringstream ss;
const int v_size = v.size();
for(size_t i = 0; i < v_size; ++i)  // v is your vector of string
{
  if(i != 0)
    ss << " ";
  ss << v[i];
}
std::string s = ss.str();

edited Jan 08 '14 at 03:06

answered Jan 08 '14 at 02:59

Gabriel L.

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score 2 · Answer 3 · edited Oct 01 '17 at 03:08

2

You can use std::ostream_iterator.

std::vector< std::string > vs{ "one", "two", "three" };

std::ostringstream result_stream;
std::ostream_iterator< std::string > oit( result_stream, " " );
std::copy( vs.begin(), vs.end(), oit );

std::string result = result_stream.str();

http://ideone.com/VfxWbd

If you want the result in a string, use std::ostringstream for the output iterator.

edited Oct 01 '17 at 03:08

PolGraphic

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answered Jan 08 '14 at 03:09

Potatoswatter

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Trailing space needs to be trimmed when `vs` is empty – P0W Jan 08 '14 at 03:14
@P0W How can you specify a requirement on someone else's question? Anyway just refer to the answer of the duplicate Q&A, which removes trailing space if the result is *not* empty. – Potatoswatter Jan 08 '14 at 06:59

How to concatenate strings with space into one string with STL in c++

3 Answers3