std string to float or double

Question

I m trying to convert std  string to float double  I tried   std  string num    0 6   double temp    double atof num c str       But it always returns zero  Any other ways

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You can use boost lexical cast    include  lt boost lexical cast hpp gt   string v  0 6    double dd   boost  lexical cast lt double gt  v   cout  lt  lt  dd  lt  lt  endl    Note  boost  lexical cast throws exception so you should be prepared to deal with it when you pass invalid value  try passing string  xxx

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std  string num    0 6   double temp     atof num c str       Does it for me  it is a valid C   syntax to convert a string to a double   You can do it with the stringstream or boost  lexical cast but those come with a performance penalty     Ahaha you have a Qt project      QString winOpacity  0 6    double temp   winOpacity toDouble      Extra note  If the input data is a const char   QByteArray  toDouble will be faster

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The C   11 way is to use std  stod and std  to string  Both work in Visual Studio 11

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The Standard Library  C  11  offers the desired functionality with std  stod    std  string  s     0 6  std  wstring ws    0 7  double d    std  stod s   double dw   std  stod ws     Generally for most other basic types  see  lt string gt   There are some new features for C strings  too  See  lt stdlib h gt

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If you don t want to drag in all of boost  go with strtod 3  from  lt cstdlib gt  - it already returns a double    include  lt iostream gt   include  lt string gt   include  lt cstring gt   include  lt cstdlib gt   using namespace std   int main          std  string  num    0 6       double temp     strtod num c str    0        cout  lt  lt  num  lt  lt       lt  lt  temp  lt  lt  endl      return 0      Outputs     g   -o s s cc     s 0 6 0 6     Why atof   doesn t work     what platform compiler are you on

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As to why atof   isn t working in the original question  the fact that it s cast to double makes me suspicious  The code shouldn t compile without  include  lt stdlib h gt   but if the cast was added to solve a compile warning  then atof   is not correctly declared  If the compiler assumes atof   returns an int  casting it will solve the conversion warning  but it will not cause the return value to be recognized as a double    include  lt stdlib h gt   include  lt string gt          std  string num    0 6     double temp   atof num c str       should work without warnings

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My Problem    Locale independent string to double  decimal separator always      Error detection if string conversion fails   My solution  uses the Windows function  wcstod l       string to convert  Note  decimal seperator is     here std  wstring str   L 1 101       Use this for error detection wchar t  stopString      Create a locale for  C   Thus a     is expected as decimal separator double dbl    wcstod l str c str     amp stopString   create locale LC ALL   C       if  wcslen stopString     0               error handling     we ll run into this because of the separator     HTH     took me pretty long to get to this solution  And I still have the feeling that I don t know enough about string localization and stuff

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This answer is backing up litb in your comments  I have profound suspicions you are just not displaying the result properly    I had the exact same thing happen to me once  I spent a whole day trying to figure out why I was getting a bad value into a 64-bit int  only to discover that printf was ignoring the second byte  You can t just pass a 64-bit value into printf like its an int

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I had the same problem in Linux  double s2f string str     istringstream buffer str    double temp   buffer  gt  gt  temp   return temp      it works

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You can use std  stringstream       include  lt sstream gt      include  lt string gt     template lt typename T gt     T StringToNumber const std  string amp  numberAsString             T valor         std  stringstream stream numberAsString         stream  gt  gt  valor        if  stream fail               std  runtime error e numberAsString            throw e                return valor         Usage   double number  StringToNumber lt double gt   0 6

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You don t want Boost lexical cast for string  lt -  floating point anyway  That subset of use cases is the only set for which boost consistently is worse than the older functions- and they basically concentrated all their failure there  because their own performance results show a 20-25X SLOWER performance than using sscanf and printf for such conversions   Google it yourself  boost  lexical cast can handle something like 50 conversions and if you exclude the ones involving floating point  s its as good or better as the obvious alternatives  with the added advantage of being having a single API for all those operations   But bring in floats and its like the Titanic hitting an iceberg in terms of performance   The old  dedicated str- double functions can all do 10000 parses in something like 30 ms  or better   lexical cast takes something like 650 ms to do the same job

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Rather than dragging Boost into the equation  you could keep your string  temporarily  as a char   and use sprintf     But of course if you re using Boost anyway  it s really not too much of an issue

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With C  17  you can use std  from chars  which is a lighter weight faster alternative to std  stof and std  stod  It doesn t involve any memory allocation or look at the locale  and it is non-throwing  The std  from chars function returns a value of type from chars result  which is basically a struct with two fields  struct from chars result       const char  ptr      std  errc ec      By inspecting ec we can tell if the conversion was successful   include  lt iostream gt   include  lt charconv gt   int main         const std  string str    quot 12345678901234 123456 quot         double value   0 0      auto  p  ec    std  from chars str data    str data     str size    value       if  ec    std  errc              std  cout  lt  lt   quot Couldn t convert value quot                  return 0

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Yes  with a lexical cast  Use a stringstream and the  lt  lt  operator  or use Boost  they ve already implemented it   Your own version could look like   template lt typename to  typename from gt to lexical cast from const  amp x      std  stringstream os    to ret     os  lt  lt  x    os  gt  gt  ret     return ret

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double myAtof   string  amp num         double tmp        sscanf   num c str      lf     amp tmp         return tmp

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Lexical cast is very nice    include  lt boost lexical cast hpp gt   include  lt iostream gt   include  lt string gt   using std  endl  using std  cout  using std  string  using boost  lexical cast   int main         string str    0 6       double dub   lexical cast lt double gt  str       cout  lt  lt  dub  lt  lt  endl

[c++] std::string to float or double

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