How to convert a number to string and vice versa in C

Question

Since this question gets asked about every week  this FAQ might help a lot of users    How to convert an integer to a string in C   how to convert a string into an integer in C   how to convert a floating-point number to a string in C   how to convert a string to a floating-point number in C

User · Answer

How to convert a number to a string in C++03

Do not use the itoa or itof functions because they are non-standard and therefore not portable.

Use string streams

 #include <sstream>  //include this to use string streams
 #include <string> 

int main()
{    
    int number = 1234;

    std::ostringstream ostr; //output string stream
    ostr << number; //use the string stream just like cout,
    //except the stream prints not to stdout but to a string.

    std::string theNumberString = ostr.str(); //the str() function of the stream 
    //returns the string.

    //now  theNumberString is "1234"  
}

Note that you can use string streams also to convert floating-point numbers to string, and also to format the string as you wish, just like with cout

std::ostringstream ostr;
float f = 1.2;
int i = 3;
ostr << f << " + " i << " = " << f + i;   
std::string s = ostr.str();
//now s is "1.2 + 3 = 4.2"

You can use stream manipulators, such as std::endl, std::hex and functions std::setw(), std::setprecision() etc. with string streams in exactly the same manner as with cout

Do not confuse std::ostringstream with std::ostrstream. The latter is deprecated

Use boost lexical cast. If you are not familiar with boost, it is a good idea to start with a small library like this lexical_cast. To download and install boost and its documentation go here. Although boost isn't in C++ standard many libraries of boost get standardized eventually and boost is widely considered of the best C++ libraries.

Lexical cast uses streams underneath, so basically this option is the same as the previous one, just less verbose.
```
#include <boost/lexical_cast.hpp>
#include <string>

int main()
{
   float f = 1.2;
   int i = 42;
   std::string sf = boost::lexical_cast<std::string>(f); //sf is "1.2"
   std::string si = boost::lexical_cast<std::string>(i); //sf is "42"
}
```

How to convert a string to a number in C++03

The most lightweight option, inherited from C, is the functions atoi (for integers (alphabetical to integer)) and atof (for floating-point values (alphabetical to float)). These functions take a C-style string as an argument (const char *) and therefore their usage may be considered a not exactly good C++ practice. cplusplus.com has easy-to-understand documentation on both atoi and atof including how they behave in case of bad input. However the link contains an error in that according to the standard if the input number is too large to fit in the target type, the behavior is undefined.
```
#include <cstdlib> //the standard C library header
#include <string>
int main()
{
    std::string si = "12";
    std::string sf = "1.2";
    int i = atoi(si.c_str()); //the c_str() function "converts" 
    double f = atof(sf.c_str()); //std::string to const char*
}
```

Use string streams (this time input string stream, istringstream). Again, istringstream is used just like cin. Again, do not confuse istringstream with istrstream. The latter is deprecated.

#include <sstream>
#include <string>
int main()
{
   std::string inputString = "1234 12.3 44";
   std::istringstream istr(inputString);
   int i1, i2;
   float f;
   istr >> i1 >> f >> i2;
   //i1 is 1234, f is 12.3, i2 is 44  
}

Use boost lexical cast.

#include <boost/lexical_cast.hpp>
#include <string>

int main()
{
   std::string sf = "42.2"; 
   std::string si = "42";
   float f = boost::lexical_cast<float>(sf); //f is 42.2
   int i = boost::lexical_cast<int>(si);  //i is 42
}

In case of a bad input, lexical_cast throws an exception of type boost::bad_lexical_cast

User · Answer

In C  17  new functions std  to chars and std  from chars are introduced in header charconv       std  to chars  is locale-independent  non-allocating     and non-throwing        Only a small subset of formatting policies used by    other libraries  such as  std  sprintf  is provided     From std  to chars  same for std  from chars      The guarantee that  std  from chars  can recover    every floating-point value formatted    by  to chars  exactly is only provided if both    functions are from the same implementation       See en cppreference com for more information  including format control   include  lt cstdio gt   include  lt cstddef gt   include  lt cstdlib gt   include  lt cassert gt   include  lt charconv gt   using Type       Any fundamental type      std  size t buffer size                  noreturn   void report and exit int ret  const char  output  noexcept        std  printf   s n   output       std  exit ret     void check const std  errc  amp ec  noexcept       if  ec      std  errc  value too large          report and exit 1   Failed      int main         char buffer buffer size               Type val to be converted  result of converted back       auto result1   std  to chars buffer  buffer   buffer size   val to be converted       check result1 ec        result1 ptr     0        auto result2   std  from chars buffer  result1 ptr  result of converted back       check result2 ec        assert val to be converted    result of converted back       report and exit 0  buffer       Although it s not fully implemented by compilers  it definitely will be implemented

User · Answer

Update for C  11  As of the C  11 standard  string-to-number conversion and vice-versa are built in into the standard library  All the following functions are present in  lt string gt   as per paragraph 21 5    string to numeric  float              stof const string amp  str  size t  idx   0   double             stod const string amp  str  size t  idx   0   long double        stold const string amp  str  size t  idx   0   int                stoi const string amp  str  size t  idx   0  int base   10   long               stol const string amp  str  size t  idx   0  int base   10   unsigned long      stoul const string amp  str  size t  idx   0  int base   10   long long          stoll const string amp  str  size t  idx   0  int base   10   unsigned long long stoull const string amp  str  size t  idx   0  int base   10     Each of these take a string as input and will try to convert it to a number  If no valid number could be constructed  for example because there is no numeric data or the number is out-of-range for the type  an exception is thrown  std  invalid argument or std  out of range    If conversion succeeded and idx is not 0  idx will contain the index of the first character that was not used for decoding  This could be an index behind the last character   Finally  the integral types allow to specify a base  for digits larger than 9  the alphabet is assumed  a 10 until z 35   You can find more information about the exact formatting that can parsed here for floating-point numbers  signed integers and unsigned integers   Finally  for each function there is also an overload that accepts a std  wstring as it s first parameter   numeric to string  string to string int val   string to string unsigned val   string to string long val   string to string unsigned long val   string to string long long val   string to string unsigned long long val   string to string float val   string to string double val   string to string long double val     These are more straightforward  you pass the appropriate numeric type and you get a string back  For formatting options you should go back to the C  03 stringsream option and use stream manipulators  as explained in an other answer here    As noted in the comments these functions fall back to a default mantissa precision that is likely not the maximum precision  If more precision is required for your application it s also best to go back to other string formatting procedures   There are also similar functions defined that are named to wstring  these will return a std  wstring

User · Answer

I stole this convienent class from somewhere here at StackOverflow to convert anything streamable to a string      make string class make string   public    template  lt typename T gt    make string amp  operator lt  lt   T const  amp  val         buffer   lt  lt  val      return  this        operator std  string   const       return buffer  str        private    std  ostringstream buffer        And then you use it as   string str   make string    lt  lt  6  lt  lt  8  lt  lt   hello     Quite nifty   Also I use this function to convert strings to anything streamable  althrough its not very safe if you try to parse a string not containing a number    and its not as clever as the last one either      parse string template  lt typename RETURN TYPE  typename STRING TYPE gt  RETURN TYPE parse string const STRING TYPE amp  str      std  stringstream buf    buf  lt  lt  str    RETURN TYPE val    buf  gt  gt  val    return val      Use as   int x   parse string lt int gt   78      You might also want versions for wstrings

User · Answer

include  lt iostream gt   include  lt string h gt  using namespace std  int main        string s  quot 000101 quot      cout lt  lt s lt  lt  quot  n quot      int a   stoi s      cout lt  lt a lt  lt  quot  n quot      s to string a      s   1      cout lt  lt s     return 0      Output    000101 101 1011

[c++] How to convert a number to string and vice versa in C++

How to convert a number to a string in C++03

How to convert a string to a number in C++03

Examples related to c++

Examples related to string

Examples related to type-conversion

Examples related to numeric

Examples related to c++-faq