C string to double conversion

Question

Usually when I write anything in C   and I need to convert a char into an int I simply make a new int equal to the char    I used the code snippet     string word     openfile  gt  gt  word   double lol word    I receive the error that   Code1 cpp cannot convert  std  string  to  double  in initialization    What does the error mean exactly  The first word is the number 50  Thanks

User · Answer

If you are reading from a file then you should hear the advice given and just put it into a double.

On the other hand, if you do have, say, a string you could use boost's lexical_cast.

Here is a (very simple) example:

int Foo(std::string anInt)
{
   return lexical_cast<int>(anInt);
}

User · Answer

The problem is that C   is a statically-typed language  meaning that if something is declared as a string  it s a string  and if something is declared as a double  it s a double   Unlike other languages like JavaScript or PHP  there is no way to automatically convert from a string to a numeric value because the conversion might not be well-defined   For example  if you try converting the string  Hi there   to a double  there s no meaningful conversion   Sure  you could just set the double to 0 0 or NaN  but this would almost certainly be masking the fact that there s a problem in the code   To fix this  don t buffer the file contents into a string   Instead  just read directly into the double   double lol  openfile  gt  gt  lol    This reads the value directly as a real number  and if an error occurs will cause the stream s  fail   method to return true   For example   double lol  openfile  gt  gt  lol   if  openfile fail          cout  lt  lt   Couldn t read a double from the file    lt  lt  endl

User · Answer

include  lt iostream gt   include  lt string gt  using namespace std   int main         cout  lt  lt  stod    99 999      lt  lt  endl      Output  99 999  which is double  whitespace was automatically stripped   Since C  11 converting string to floating-point values  like double  is available with functions  stof - convert str to a float stod  - convert str to a double stold - convert str to a long double  As conversion of string to int was also mentioned in the question  there are the following functions in C  11  stoi - convert str to an int stol - convert str to a long stoul - convert str to an unsigned long stoll - convert str to a long long stoull - convert str to an unsigned long long

User · Answer

You can convert char to int and viceversa easily because for the machine an int and a char are the same  8 bits  the only difference comes when they have to be shown in screen  if the number is 65 and is saved as a char  then it will show  A   if it s saved as a int it will show 65   With other types things change  because they are stored differently in memory  There s standard function in C that allows you to convert from string to double easily  it s atof   You need to include stdlib h    include  lt stdlib h gt   int main         string word        openfile  gt  gt  word      double lol   atof word c str       c str is needed to convert string to const char                                       previously  the function requires it        return 0

User · Answer

Coversion from string to double can be achieved by using the  strtod    function from the library  stdlib h    include  lt iostream gt   include  lt stdlib h gt  int main           std  string data  20 9       double value   strtod data c str    NULL       std  cout lt  lt value lt  lt   n       return 0

User · Answer

The C   way of solving conversions  not the classical C  is illustrated with the program below   Note that the intent is to be able to use the same formatting facilities offered by iostream like precision  fill character  padding  hex  and the manipulators  etcetera   Compile and run this program  then study it   It is simple   include  iostream   include  iomanip   include  sstream  using namespace std   int main            Converting the content of a char array or a string to a double variable     double d      string S      S    4 5       istringstream S   gt  gt  d          cout  lt  lt    nThe value of the double variable d is    lt  lt  d  lt  lt  endl      istringstream  9 87654    gt  gt  d        cout  lt  lt    nNow the value of the double variable d is    lt  lt  d  lt  lt  endl            Converting a double to string with formatting restrictions     double D 3 771234567       ostringstream Q      Q fill           Q  lt  lt    lt  lt  lt    lt  lt  setprecision 6   lt  lt  setw 20   lt  lt  D  lt  lt    gt  gt  gt        S   Q str       formatted converted double is now in string       cout  lt  lt    nThe value of the string variable S is    lt  lt  S  lt  lt  endl      return 0      Prof  Martinez

User · Answer

include  lt string gt   include  lt cmath gt  double  string to double std  string s unsigned short radix   double n   0  for  unsigned short x   s size    y   0 x gt 0   if   s --x             if is equal n  pow 10 s size  -1-x   y   s size  -x  else     n     s x -48    pow 10 s size  -1-x - y     return n      or    In case you want to convert from different bases   include  lt string gt   include  lt iostream gt   include  lt cmath gt  double  string to double std  string s unsigned short radix   double n   0  for  unsigned short x   s size    y   0 x gt 0   if   s --x          n  pow radix s size  -1-x   y   s size  -x  else     n     s x -  s x  lt   9     0   0  7        pow radix s size  -1-x - y     return n    int main    std  cout lt  lt  string to double  10 A  16  lt  lt std  endl   Prints 16 625 std  cout lt  lt  string to double  1001 1  2  lt  lt std  endl   Prints 9 5 std  cout lt  lt  string to double  123 4  10  lt  lt std  endl   Prints 123 4 return 0

[c++] C++ string to double conversion

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