How do I tokenize a string in C++?

Bill_the_Lizard_1288 0
Viewed 627919

Java has a convenient split method:

String str = "The quick brown fox";
String[] results = str.split(" ");

Is there an easy way to do this in C++?

tokenize c++ split string
36 Answers

The Boost tokenizer class can make this sort of thing quite simple:

#include <iostream>
#include <string>
#include <boost/foreach.hpp>
#include <boost/tokenizer.hpp>

using namespace std;
using namespace boost;

int main(int, char**)
{
    string text = "token, test   string";

    char_separator<char> sep(", ");
    tokenizer< char_separator<char> > tokens(text, sep);
    BOOST_FOREACH (const string& t, tokens) {
        cout << t << "." << endl;
    }
}

Updated for C++11:

#include <iostream>
#include <string>
#include <boost/tokenizer.hpp>

using namespace std;
using namespace boost;

int main(int, char**)
{
    string text = "token, test   string";

    char_separator<char> sep(", ");
    tokenizer<char_separator<char>> tokens(text, sep);
    for (const auto& t : tokens) {
        cout << t << "." << endl;
    }
}
Ferruccio_40860

Here's a real simple one:

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

vector<string> split(const char *str, char c = ' ')
{
    vector<string> result;

    do
    {
        const char *begin = str;

        while(*str != c && *str)
            str++;

        result.push_back(string(begin, str));
    } while (0 != *str++);

    return result;
}
Adam_Pierce_53240

C++ standard library algorithms are pretty universally based around iterators rather than concrete containers. Unfortunately this makes it hard to provide a Java-like split function in the C++ standard library, even though nobody argues that this would be convenient. But what would its return type be? std::vector<std::basic_string<…>>? Maybe, but then we’re forced to perform (potentially redundant and costly) allocations.

Instead, C++ offers a plethora of ways to split strings based on arbitrarily complex delimiters, but none of them is encapsulated as nicely as in other languages. The numerous ways fill whole blog posts.

At its simplest, you could iterate using std::string::find until you hit std::string::npos, and extract the contents using std::string::substr.

A more fluid (and idiomatic, but basic) version for splitting on whitespace would use a std::istringstream:

auto iss = std::istringstream{"The quick brown fox"};
auto str = std::string{};

while (iss >> str) {
    process(str);
}

Using std::istream_iterators, the contents of the string stream could also be copied into a vector using its iterator range constructor.

Multiple libraries (such as Boost.Tokenizer) offer specific tokenisers.

More advanced splitting require regular expressions. C++ provides the std::regex_token_iterator for this purpose in particular:

auto const str = "The quick brown fox"s;
auto const re = std::regex{R"(\s+)"};
auto const vec = std::vector<std::string>(
    std::sregex_token_iterator{begin(str), end(str), re, -1},
    std::sregex_token_iterator{}
);
user_19680

Another quick way is to use getline. Something like:

stringstream ss("bla bla");
string s;

while (getline(ss, s, ' ')) {
 cout << s << endl;
}

If you want, you can make a simple split() method returning a vector<string>, which is really useful.

user_359780

Use strtok. In my opinion, there isn't a need to build a class around tokenizing unless strtok doesn't provide you with what you need. It might not, but in 15+ years of writing various parsing code in C and C++, I've always used strtok. Here is an example

char myString[] = "The quick brown fox";
char *p = strtok(myString, " ");
while (p) {
    printf ("Token: %s\n", p);
    p = strtok(NULL, " ");
}

A few caveats (which might not suit your needs). The string is "destroyed" in the process, meaning that EOS characters are placed inline in the delimter spots. Correct usage might require you to make a non-const version of the string. You can also change the list of delimiters mid parse.

In my own opinion, the above code is far simpler and easier to use than writing a separate class for it. To me, this is one of those functions that the language provides and it does it well and cleanly. It's simply a "C based" solution. It's appropriate, it's easy, and you don't have to write a lot of extra code :-)

Mark_44050

You can use streams, iterators, and the copy algorithm to do this fairly directly.

#include <string>
#include <vector>
#include <iostream>
#include <istream>
#include <ostream>
#include <iterator>
#include <sstream>
#include <algorithm>

int main()
{
  std::string str = "The quick brown fox";

  // construct a stream from the string
  std::stringstream strstr(str);

  // use stream iterators to copy the stream to the vector as whitespace separated strings
  std::istream_iterator<std::string> it(strstr);
  std::istream_iterator<std::string> end;
  std::vector<std::string> results(it, end);

  // send the vector to stdout.
  std::ostream_iterator<std::string> oit(std::cout);
  std::copy(results.begin(), results.end(), oit);
}
KeithB_22980

No offense folks, but for such a simple problem, you are making things way too complicated. There are a lot of reasons to use Boost. But for something this simple, it's like hitting a fly with a 20# sledge.

void
split( vector<string> & theStringVector,  /* Altered/returned value */
       const  string  & theString,
       const  string  & theDelimiter)
{
    UASSERT( theDelimiter.size(), >, 0); // My own ASSERT macro.

    size_t  start = 0, end = 0;

    while ( end != string::npos)
    {
        end = theString.find( theDelimiter, start);

        // If at end, use length=maxLength.  Else use length=end-start.
        theStringVector.push_back( theString.substr( start,
                       (end == string::npos) ? string::npos : end - start));

        // If at end, use start=maxSize.  Else use start=end+delimiter.
        start = (   ( end > (string::npos - theDelimiter.size()) )
                  ?  string::npos  :  end + theDelimiter.size());
    }
}

For example (for Doug's case),

#define SHOW(I,X)   cout << "[" << (I) << "]\t " # X " = \"" << (X) << "\"" << endl

int
main()
{
    vector<string> v;

    split( v, "A:PEP:909:Inventory Item", ":" );

    for (unsigned int i = 0;  i < v.size();   i++)
        SHOW( i, v[i] );
}

And yes, we could have split() return a new vector rather than passing one in. It's trivial to wrap and overload. But depending on what I'm doing, I often find it better to re-use pre-existing objects rather than always creating new ones. (Just as long as I don't forget to empty the vector in between!)

Reference: http://www.cplusplus.com/reference/string/string/.

(I was originally writing a response to Doug's question: C++ Strings Modifying and Extracting based on Separators (closed). But since Martin York closed that question with a pointer over here... I'll just generalize my code.)

user_379460

Boost has a strong split function: boost::algorithm::split.

Sample program:

#include <vector>
#include <boost/algorithm/string.hpp>

int main() {
    auto s = "a,b, c ,,e,f,";
    std::vector<std::string> fields;
    boost::split(fields, s, boost::is_any_of(","));
    for (const auto& field : fields)
        std::cout << "\"" << field << "\"\n";
    return 0;
}

Output:

"a"
"b"
" c "
""
"e"
"f"
""
user_56610

Here is a sample tokenizer class that might do what you want 

//Header file
class Tokenizer 
{
    public:
        static const std::string DELIMITERS;
        Tokenizer(const std::string& str);
        Tokenizer(const std::string& str, const std::string& delimiters);
        bool NextToken();
        bool NextToken(const std::string& delimiters);
        const std::string GetToken() const;
        void Reset();
    protected:
        size_t m_offset;
        const std::string m_string;
        std::string m_token;
        std::string m_delimiters;
};

//CPP file
const std::string Tokenizer::DELIMITERS(" \t\n\r");

Tokenizer::Tokenizer(const std::string& s) :
    m_string(s), 
    m_offset(0), 
    m_delimiters(DELIMITERS) {}

Tokenizer::Tokenizer(const std::string& s, const std::string& delimiters) :
    m_string(s), 
    m_offset(0), 
    m_delimiters(delimiters) {}

bool Tokenizer::NextToken() 
{
    return NextToken(m_delimiters);
}

bool Tokenizer::NextToken(const std::string& delimiters) 
{
    size_t i = m_string.find_first_not_of(delimiters, m_offset);
    if (std::string::npos == i) 
    {
        m_offset = m_string.length();
        return false;
    }

    size_t j = m_string.find_first_of(delimiters, i);
    if (std::string::npos == j) 
    {
        m_token = m_string.substr(i);
        m_offset = m_string.length();
        return true;
    }

    m_token = m_string.substr(i, j - i);
    m_offset = j;
    return true;
}

Example:

std::vector <std::string> v;
Tokenizer s("split this string", " ");
while (s.NextToken())
{
    v.push_back(s.GetToken());
}
vzczc_2240

Check this example. It might help you..

#include <iostream>
#include <sstream>

using namespace std;

int main ()
{
    string tmps;
    istringstream is ("the dellimiter is the space");
    while (is.good ()) {
        is >> tmps;
        cout << tmps << "\n";
    }
    return 0;
}
user_5486000

MFC/ATL has a very nice tokenizer. From MSDN:

CAtlString str( "%First Second#Third" );
CAtlString resToken;
int curPos= 0;

resToken= str.Tokenize("% #",curPos);
while (resToken != "")
{
   printf("Resulting token: %s\n", resToken);
   resToken= str.Tokenize("% #",curPos);
};

Output

Resulting Token: First
Resulting Token: Second
Resulting Token: Third
Jim_In_Texas_150790

If you're using C++ ranges - the full ranges-v3 library, not the limited functionality accepted into C++20 - you could do it this way:

auto results = str | ranges::views::tokenize(" ",1);

... and this is lazily-evaluated. You can alternatively set a vector to this range:

auto results = str | ranges::views::tokenize(" ",1) | ranges::to<std::vector>();

this will take O(m) space and O(n) time if str has n characters making up m words.

See also the library's own tokenization example, here.

user_15930770

If you're willing to use C, you can use the strtok function. You should pay attention to multi-threading issues when using it.

On_Freund_21500

For simple stuff I just use the following:

unsigned TokenizeString(const std::string& i_source,
                        const std::string& i_seperators,
                        bool i_discard_empty_tokens,
                        std::vector<std::string>& o_tokens)
{
    unsigned prev_pos = 0;
    unsigned pos = 0;
    unsigned number_of_tokens = 0;
    o_tokens.clear();
    pos = i_source.find_first_of(i_seperators, pos);
    while (pos != std::string::npos)
    {
        std::string token = i_source.substr(prev_pos, pos - prev_pos);
        if (!i_discard_empty_tokens || token != "")
        {
            o_tokens.push_back(i_source.substr(prev_pos, pos - prev_pos));
            number_of_tokens++;
        }

        pos++;
        prev_pos = pos;
        pos = i_source.find_first_of(i_seperators, pos);
    }

    if (prev_pos < i_source.length())
    {
        o_tokens.push_back(i_source.substr(prev_pos));
        number_of_tokens++;
    }

    return number_of_tokens;
}

Cowardly disclaimer: I write real-time data processing software where the data comes in through binary files, sockets, or some API call (I/O cards, camera's). I never use this function for something more complicated or time-critical than reading external configuration files on startup.

user_75310

I thought that was what the >> operator on string streams was for:

string word; sin >> word;
Daren_Thomas_22600

I know this question is already answered but I want to contribute. Maybe my solution is a bit simple but this is what I came up with:

vector<string> get_words(string const& text, string const& separator)
{
    vector<string> result;
    string tmp = text;

    size_t first_pos = 0;
    size_t second_pos = tmp.find(separator);

    while (second_pos != string::npos)
    {
        if (first_pos != second_pos)
        {
            string word = tmp.substr(first_pos, second_pos - first_pos);
            result.push_back(word);
        }
        tmp = tmp.substr(second_pos + separator.length());
        second_pos = tmp.find(separator);
    }

    result.push_back(tmp);

    return result;
}

Please comment if there is a better approach to something in my code or if something is wrong.

UPDATE: added generic separator

user_55173780

Here's my Swiss® Army Knife of string-tokenizers for splitting up strings by whitespace, accounting for single and double-quote wrapped strings as well as stripping those characters from the results. I used RegexBuddy 4.x to generate most of the code-snippet, but I added custom handling for stripping quotes and a few other things.

#include <string>
#include <locale>
#include <regex>

std::vector<std::wstring> tokenize_string(std::wstring string_to_tokenize) {
    std::vector<std::wstring> tokens;

    std::wregex re(LR"(("[^"]*"|'[^']*'|[^"' ]+))", std::regex_constants::collate);

    std::wsregex_iterator next( string_to_tokenize.begin(),
                                string_to_tokenize.end(),
                                re,
                                std::regex_constants::match_not_null );

    std::wsregex_iterator end;
    const wchar_t single_quote = L'\'';
    const wchar_t double_quote = L'\"';
    while ( next != end ) {
        std::wsmatch match = *next;
        const std::wstring token = match.str( 0 );
        next++;

        if (token.length() > 2 && (token.front() == double_quote || token.front() == single_quote))
            tokens.emplace_back( std::wstring(token.begin()+1, token.begin()+token.length()-1) );
        else
            tokens.emplace_back(token);
    }
    return tokens;
}
user_35434370

I wrote a simplified version (and maybe a little bit efficient) of https://stackoverflow.com/a/50247503/3976739 for my own use. I hope it would help.

void StrTokenizer(string& source, const char* delimiter, vector<string>& Tokens)
{   
   size_t new_index = 0;
   size_t old_index = 0;

   while (new_index != std::string::npos)   
   {
      new_index = source.find(delimiter, old_index);
      Tokens.emplace_back(source.substr(old_index, new_index-old_index));

      if (new_index != std::string::npos)
          old_index = ++new_index;
   }
}
user_39767390
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