How can I convert string to double in C

Question

How can I convert string to double in C    I want a function that returns 0 when the string is not numerical

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I think atof is exactly what you want  This function parses a string and converts it into a double  If the string does not start with a number  non-numerical  a 0 0 is returned   However  it does try to parse as much of the string as it can  In other words  the string  3abc  would be interpreted as 3 0  If you want a function that will return 0 0 in these cases  you will need to write a small wrapper yourself   Also  this function works with the C-style string of a null terminated array of characters  If you re using a string object  it will need to be converted to a char  before you use this function

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If it is a c-string  null-terminated array of type char   you can do something like    include  lt stdlib h gt  char str      3 14159   double num   atof str     If it is a C   string  just use the c str   method   double num   atof  cppstr c str        atof   will convert the string to a double  returning 0 on failure  The function is documented here  http   www cplusplus com reference clibrary cstdlib atof html

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I think atof is exactly what you want  This function parses a string and converts it into a double  If the string does not start with a number  non-numerical  a 0 0 is returned   However  it does try to parse as much of the string as it can  In other words  the string  3abc  would be interpreted as 3 0  If you want a function that will return 0 0 in these cases  you will need to write a small wrapper yourself   Also  this function works with the C-style string of a null terminated array of characters  If you re using a string object  it will need to be converted to a char  before you use this function

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Most simple way is to use boost  lexical cast   double value  try       value   boost  lexical cast lt double gt  my string     catch  boost  bad lexical cast const amp         value   0

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atof and strtod do what you want but are very forgiving  If you don t want to accept strings like  32asd  as valid you need to wrap strtod in a function such as this    include  lt stdlib h gt  double strict str2double char  str        char  endptr      double value   strtod str   amp endptr       if   endptr  return 0      return value

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One of the most elegant solution to this problem is to use boost  lexical cast as  Evgeny Lazin mentioned

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If it is a c-string  null-terminated array of type char   you can do something like    include  lt stdlib h gt  char str      3 14159   double num   atof str     If it is a C   string  just use the c str   method   double num   atof  cppstr c str        atof   will convert the string to a double  returning 0 on failure  The function is documented here  http   www cplusplus com reference clibrary cstdlib atof html

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Sample C program to convert string to double in C   include  lt iostream gt   include  lt comutil h gt   include  lt iomanip gt   bool ParseNumberToDouble const std  wstring amp  strInput  double  amp  dResult        bool bSucceeded   false      dResult   0      if   strInput empty    amp  amp  dResult                 variant t varIn   strInput c str            if  SUCCEEDED VariantChangeTypeEx  amp varIn   amp varIn  GetThreadLocale    0  VT R8                          dResult   varIn dblVal              bSucceeded   true                      return bSucceeded     int main         std  wstring strInput   L quot 1000 quot       double dResult   0       ParseNumberToDouble strInput  dResult        return 0

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One of the most elegant solution to this problem is to use boost  lexical cast as  Evgeny Lazin mentioned

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There is not a single function that will do that  because 0 is a valid number and you need to be able to catch when the string is not a valid number   You will need to check the string first  probably with a regular expression  to see if it contains only numbers and numerical punctuation   You can then decide to return 0 if that is what your application needs or convert it to a double   After looking up atof   and strtod   I should rephrase my statement to  there shouldn t be  instead of  there is not      hehe

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Most simple way is to use boost  lexical cast   double value  try       value   boost  lexical cast lt double gt  my string     catch  boost  bad lexical cast const amp         value   0

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See C   FAQ Lite How do I convert a std  string to a number   See C   Super-FAQ How do I convert a std  string to a number    Please note that with your requirements you can t distinguish all the the allowed string representations of zero from the non numerical strings       the requested function   include  lt sstream gt   double string to double  const std  string amp  s         std  istringstream i s      double x     if    i  gt  gt  x        return 0     return x           some tests   include  lt cassert gt   int main  int  char               simple case      assert  0 5    string to double   0 5                  blank space      assert  0 5    string to double   0 5              assert  0 5    string to double    0 5                 trailing non digit characters      assert  0 5    string to double   0 5a                 note that with your requirements you can t distinguish        all the the allowed string representation of zero from        the non numerical strings      assert  0    string to double   0                 assert  0    string to double   0                 assert  0    string to double   0 0               assert  0    string to double   0 00              assert  0    string to double   0 0e0             assert  0    string to double   0 0e-0            assert  0    string to double   0 0e 0            assert  0    string to double    0                assert  0    string to double    0                assert  0    string to double    0 0              assert  0    string to double    0 00             assert  0    string to double    0 0e0            assert  0    string to double    0 0e-0           assert  0    string to double    0 0e 0           assert  0    string to double   -0                assert  0    string to double   -0                assert  0    string to double   -0 0              assert  0    string to double   -0 00             assert  0    string to double   -0 0e0            assert  0    string to double   -0 0e-0           assert  0    string to double   -0 0e 0           assert  0    string to double   foobar            return 0

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atof and strtod do what you want but are very forgiving  If you don t want to accept strings like  32asd  as valid you need to wrap strtod in a function such as this    include  lt stdlib h gt  double strict str2double char  str        char  endptr      double value   strtod str   amp endptr       if   endptr  return 0      return value

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atof and strtod do what you want but are very forgiving  If you don t want to accept strings like  32asd  as valid you need to wrap strtod in a function such as this    include  lt stdlib h gt  double strict str2double char  str        char  endptr      double value   strtod str   amp endptr       if   endptr  return 0      return value

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Must say I agree with that the most elegant solution to this is using boost  lexical cast  You can then catch the bad lexical cast that might occure  and do something when it fails  instead of getting 0 0 which atof gives    include  lt boost lexical cast hpp gt   include  lt string gt   int main         std  string str    3 14       double strVal      try           strVal   boost  lexical cast lt double gt  str         catch bad lexical cast amp               Do your errormagic           return 0

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atof and strtod do what you want but are very forgiving  If you don t want to accept strings like  32asd  as valid you need to wrap strtod in a function such as this    include  lt stdlib h gt  double strict str2double char  str        char  endptr      double value   strtod str   amp endptr       if   endptr  return 0      return value

User · Answer

Must say I agree with that the most elegant solution to this is using boost  lexical cast  You can then catch the bad lexical cast that might occure  and do something when it fails  instead of getting 0 0 which atof gives    include  lt boost lexical cast hpp gt   include  lt string gt   int main         std  string str    3 14       double strVal      try           strVal   boost  lexical cast lt double gt  str         catch bad lexical cast amp               Do your errormagic           return 0

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I think atof is exactly what you want  This function parses a string and converts it into a double  If the string does not start with a number  non-numerical  a 0 0 is returned   However  it does try to parse as much of the string as it can  In other words  the string  3abc  would be interpreted as 3 0  If you want a function that will return 0 0 in these cases  you will need to write a small wrapper yourself   Also  this function works with the C-style string of a null terminated array of characters  If you re using a string object  it will need to be converted to a char  before you use this function

User · Answer

If it is a c-string  null-terminated array of type char   you can do something like    include  lt stdlib h gt  char str      3 14159   double num   atof str     If it is a C   string  just use the c str   method   double num   atof  cppstr c str        atof   will convert the string to a double  returning 0 on failure  The function is documented here  http   www cplusplus com reference clibrary cstdlib atof html

User · Answer

Must say I agree with that the most elegant solution to this is using boost  lexical cast  You can then catch the bad lexical cast that might occure  and do something when it fails  instead of getting 0 0 which atof gives    include  lt boost lexical cast hpp gt   include  lt string gt   int main         std  string str    3 14       double strVal      try           strVal   boost  lexical cast lt double gt  str         catch bad lexical cast amp               Do your errormagic           return 0

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See C   FAQ Lite How do I convert a std  string to a number   See C   Super-FAQ How do I convert a std  string to a number    Please note that with your requirements you can t distinguish all the the allowed string representations of zero from the non numerical strings       the requested function   include  lt sstream gt   double string to double  const std  string amp  s         std  istringstream i s      double x     if    i  gt  gt  x        return 0     return x           some tests   include  lt cassert gt   int main  int  char               simple case      assert  0 5    string to double   0 5                  blank space      assert  0 5    string to double   0 5              assert  0 5    string to double    0 5                 trailing non digit characters      assert  0 5    string to double   0 5a                 note that with your requirements you can t distinguish        all the the allowed string representation of zero from        the non numerical strings      assert  0    string to double   0                 assert  0    string to double   0                 assert  0    string to double   0 0               assert  0    string to double   0 00              assert  0    string to double   0 0e0             assert  0    string to double   0 0e-0            assert  0    string to double   0 0e 0            assert  0    string to double    0                assert  0    string to double    0                assert  0    string to double    0 0              assert  0    string to double    0 00             assert  0    string to double    0 0e0            assert  0    string to double    0 0e-0           assert  0    string to double    0 0e 0           assert  0    string to double   -0                assert  0    string to double   -0                assert  0    string to double   -0 0              assert  0    string to double   -0 00             assert  0    string to double   -0 0e0            assert  0    string to double   -0 0e-0           assert  0    string to double   -0 0e 0           assert  0    string to double   foobar            return 0

User · Answer

I think atof is exactly what you want  This function parses a string and converts it into a double  If the string does not start with a number  non-numerical  a 0 0 is returned   However  it does try to parse as much of the string as it can  In other words  the string  3abc  would be interpreted as 3 0  If you want a function that will return 0 0 in these cases  you will need to write a small wrapper yourself   Also  this function works with the C-style string of a null terminated array of characters  If you re using a string object  it will need to be converted to a char  before you use this function

User · Answer

There is not a single function that will do that  because 0 is a valid number and you need to be able to catch when the string is not a valid number   You will need to check the string first  probably with a regular expression  to see if it contains only numbers and numerical punctuation   You can then decide to return 0 if that is what your application needs or convert it to a double   After looking up atof   and strtod   I should rephrase my statement to  there shouldn t be  instead of  there is not      hehe

User · Answer

Must say I agree with that the most elegant solution to this is using boost  lexical cast  You can then catch the bad lexical cast that might occure  and do something when it fails  instead of getting 0 0 which atof gives    include  lt boost lexical cast hpp gt   include  lt string gt   int main         std  string str    3 14       double strVal      try           strVal   boost  lexical cast lt double gt  str         catch bad lexical cast amp               Do your errormagic           return 0

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Most simple way is to use boost  lexical cast   double value  try       value   boost  lexical cast lt double gt  my string     catch  boost  bad lexical cast const amp         value   0

User · Answer

There is not a single function that will do that  because 0 is a valid number and you need to be able to catch when the string is not a valid number   You will need to check the string first  probably with a regular expression  to see if it contains only numbers and numerical punctuation   You can then decide to return 0 if that is what your application needs or convert it to a double   After looking up atof   and strtod   I should rephrase my statement to  there shouldn t be  instead of  there is not      hehe

User · Answer

Sample C program to convert string to double in C   include  lt iostream gt   include  lt comutil h gt   include  lt iomanip gt   bool ParseNumberToDouble const std  wstring amp  strInput  double  amp  dResult        bool bSucceeded   false      dResult   0      if   strInput empty    amp  amp  dResult                 variant t varIn   strInput c str            if  SUCCEEDED VariantChangeTypeEx  amp varIn   amp varIn  GetThreadLocale    0  VT R8                          dResult   varIn dblVal              bSucceeded   true                      return bSucceeded     int main         std  wstring strInput   L quot 1000 quot       double dResult   0       ParseNumberToDouble strInput  dResult        return 0

User · Answer

One of the most elegant solution to this problem is to use boost  lexical cast as  Evgeny Lazin mentioned

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     I want a function that returns 0 when the string is not numerical    You can add try catch statement when stod throws an exception

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Most simple way is to use boost  lexical cast   double value  try       value   boost  lexical cast lt double gt  my string     catch  boost  bad lexical cast const amp         value   0

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     I want a function that returns 0 when the string is not numerical    You can add try catch statement when stod throws an exception

User · Answer

See C   FAQ Lite How do I convert a std  string to a number   See C   Super-FAQ How do I convert a std  string to a number    Please note that with your requirements you can t distinguish all the the allowed string representations of zero from the non numerical strings       the requested function   include  lt sstream gt   double string to double  const std  string amp  s         std  istringstream i s      double x     if    i  gt  gt  x        return 0     return x           some tests   include  lt cassert gt   int main  int  char               simple case      assert  0 5    string to double   0 5                  blank space      assert  0 5    string to double   0 5              assert  0 5    string to double    0 5                 trailing non digit characters      assert  0 5    string to double   0 5a                 note that with your requirements you can t distinguish        all the the allowed string representation of zero from        the non numerical strings      assert  0    string to double   0                 assert  0    string to double   0                 assert  0    string to double   0 0               assert  0    string to double   0 00              assert  0    string to double   0 0e0             assert  0    string to double   0 0e-0            assert  0    string to double   0 0e 0            assert  0    string to double    0                assert  0    string to double    0                assert  0    string to double    0 0              assert  0    string to double    0 00             assert  0    string to double    0 0e0            assert  0    string to double    0 0e-0           assert  0    string to double    0 0e 0           assert  0    string to double   -0                assert  0    string to double   -0                assert  0    string to double   -0 0              assert  0    string to double   -0 00             assert  0    string to double   -0 0e0            assert  0    string to double   -0 0e-0           assert  0    string to double   -0 0e 0           assert  0    string to double   foobar            return 0

User · Answer

See C   FAQ Lite How do I convert a std  string to a number   See C   Super-FAQ How do I convert a std  string to a number    Please note that with your requirements you can t distinguish all the the allowed string representations of zero from the non numerical strings       the requested function   include  lt sstream gt   double string to double  const std  string amp  s         std  istringstream i s      double x     if    i  gt  gt  x        return 0     return x           some tests   include  lt cassert gt   int main  int  char               simple case      assert  0 5    string to double   0 5                  blank space      assert  0 5    string to double   0 5              assert  0 5    string to double    0 5                 trailing non digit characters      assert  0 5    string to double   0 5a                 note that with your requirements you can t distinguish        all the the allowed string representation of zero from        the non numerical strings      assert  0    string to double   0                 assert  0    string to double   0                 assert  0    string to double   0 0               assert  0    string to double   0 00              assert  0    string to double   0 0e0             assert  0    string to double   0 0e-0            assert  0    string to double   0 0e 0            assert  0    string to double    0                assert  0    string to double    0                assert  0    string to double    0 0              assert  0    string to double    0 00             assert  0    string to double    0 0e0            assert  0    string to double    0 0e-0           assert  0    string to double    0 0e 0           assert  0    string to double   -0                assert  0    string to double   -0                assert  0    string to double   -0 0              assert  0    string to double   -0 00             assert  0    string to double   -0 0e0            assert  0    string to double   -0 0e-0           assert  0    string to double   -0 0e 0           assert  0    string to double   foobar            return 0

User · Answer

One of the most elegant solution to this problem is to use boost  lexical cast as  Evgeny Lazin mentioned

User · Answer

If it is a c-string  null-terminated array of type char   you can do something like    include  lt stdlib h gt  char str      3 14159   double num   atof str     If it is a C   string  just use the c str   method   double num   atof  cppstr c str        atof   will convert the string to a double  returning 0 on failure  The function is documented here  http   www cplusplus com reference clibrary cstdlib atof html

User · Answer

There is not a single function that will do that  because 0 is a valid number and you need to be able to catch when the string is not a valid number   You will need to check the string first  probably with a regular expression  to see if it contains only numbers and numerical punctuation   You can then decide to return 0 if that is what your application needs or convert it to a double   After looking up atof   and strtod   I should rephrase my statement to  there shouldn t be  instead of  there is not      hehe

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