How to determine if a string is a number with C

Question

I ve had quite a bit of trouble trying to write a function that checks if a string is a number   For a game I am writing I just need to check if a line from the file I am reading is a number or not  I will know if it is a parameter this way    I wrote the below function which I believe was working smoothly  or I accidentally edited to stop it or I m schizophrenic or Windows is schizophrenic    bool isParam  string line        if  isdigit atoi line c str              return true       return false

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A solution based on a comment by kbjorklu is:

bool isNumber(const std::string& s)
{
   return !s.empty() && s.find_first_not_of("-.0123456789") == std::string::npos;
}

As with David Rector's answer it is not robust to strings with multiple dots or minus signs, but you can remove those characters to just check for integers.

However, I am partial to a solution, based on Ben Voigt's solution, using strtod in cstdlib to look decimal values, scientific/engineering notation, hexidecimal notation (C++11), or even INF/INFINITY/NAN (C++11) is:

bool isNumberC(const std::string& s)
{
    char* p;
    strtod(s.c_str(), &p);
    return *p == 0;
}

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include  lt string gt    For Validating Doubles   bool validateDouble const std  string  amp  input    int decimals   std  count input begin    input end             The number of decimals in the string int negativeSigns   std  count input begin    input end     -       The number of negative signs in the string  if  input size      decimals   negativeSigns     Consists of only decimals and negatives or is empty     return false  else if  1  lt  decimals    1  lt  negativeSigns     More than 1 decimal or negative sign     return false  else if  1    negativeSigns  amp  amp  input 0      -      The negative sign  if there is one  is not the first character     return false  else if  strspn input c str     - 0123456789      input size       The string contains a character that isn t in  - 0123456789      return false  return true       For Validating Ints  With Negatives   bool validateInt const std  string  amp  input    int negativeSigns   std  count input begin    input end     -       The number of negative signs in the string  if  input size      negativeSigns     Consists of only negatives or is empty     return false  else if  1  lt  negativeSigns     More than 1 negative sign     return false  else if  1    negativeSigns  amp  amp  input 0      -      The negative sign  if there is one  is not the first character     return false  else if  strspn input c str     -0123456789      input size       The string contains a character that isn t in  -0123456789      return false  return true       For Validating Unsigned Ints  bool validateUnsignedInt const std  string  amp  input    return  input size      0  amp  amp  strspn input c str     0123456789      input size        The string is not empty and contains characters only in  0123456789

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After consulting the documentation a bit more  I came up with an answer that supports my needs  but probably won t be as helpful for others   Here it is  without the annoying return true and return false statements  -     bool isNumber string line         return  atoi line c str

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Few months ago  I implemented a way to determine if any string is integer  hexadecimal or double   enum          STRING IS INVALID NUMBER 0          STRING IS HEXA          STRING IS INT          STRING IS DOUBLE     bool isDigit char c       return    0   lt   c   amp  amp   c lt   9        bool isHexaDigit char c       return     0   lt   c   amp  amp   c lt   9         tolower c  lt   a   amp  amp  tolower c  lt   f          char  ADVANCE DIGITS char  aux p        while CString  isDigit  aux p   aux p        return aux p     char  ADVANCE HEXADIGITS char  aux p        while CString  isHexaDigit  aux p   aux p        return aux p      int isNumber const string  amp  test str number       bool isHexa false      char  str    char   test str number c str         switch  str       case  -   str       is negative number                    break      case  0                  if tolower  str 1    x                        isHexa   true                    str  2                                break      default              break              char  start str   str     saves start position        if isHexa       candidate to hexa             str   ADVANCE HEXADIGITS str           if str    start str              return STRING IS INVALID NUMBER           if  str            str    0               return STRING IS HEXA        else     test if integer or float         str   ADVANCE DIGITS str           if  str            is candidate to double             str                str   ADVANCE DIGITS str               if  str            str    0                  return STRING IS DOUBLE               return STRING IS INVALID NUMBER                     if  str            str    0              return STRING IS INT              return STRING IS INVALID NUMBER        Then in your program you can easily convert the number in function its type if you do the following   string val     the string to check if number     switch isNumber val       case STRING IS HEXA         use strtol val c str    NULL  16   to convert it into conventional hexadecimal    break     case STRING IS INT         use  int strtol val c str    NULL  10   to convert it into conventional integer    break     case STRING IS DOUBLE        use atof val c str     to convert it into conventional float double    break      You can realise that the function will return a 0 if the number wasn t detected  The 0 it can be treated as false  like boolean

User · Answer

Try this   isNumber const std  string  amp str          return  str empty    amp  amp  str find first not of  0123456789      string  npos

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to check if a string is a number integer or floating point or so you could use      include  lt sstream gt       bool isNumber string str        double d      istringstream is str       is  gt  gt  d      return  is fail    amp  amp  is eof

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I just wanted to throw in this idea that uses iteration but some other code does that iteration    include  lt string h gt   bool is number const std  string amp  s        return  strspn  s c str     - 0123456789       s size          It s not robust like it should be when checking for a decimal point or minus sign since it allows there to be more than one of each and in any location  The good thing is that it s a single line of code and doesn t require a third-party library   Take out the     and  -  if positive integers are all that are allowed

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Using  lt regex gt   This code was tested   bool isNumber const std  string  amp token        return std  regex match token  std  regex       -   0-9         0-9

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You may test if a string is convertible to integer by using boost  lexical cast  If it throws bad lexical cast exception then string could not be converted  otherwise it can   See example of such a test program below    include  lt boost lexical cast hpp gt   include  lt iostream gt   int main int  char   argv            try                           int x   boost  lexical cast lt int gt  argv 1                    std  cout  lt  lt  x  lt  lt    YES n                     catch  boost  bad lexical cast const  amp                             std   cout  lt  lt   NO n                     return 0      Sample execution       a out 12 12 YES     a out 12 3 NO

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With this solution you can check everything from negative to positive numbers and even float numbers  When you change the type of num to integer you will get an error if the string contains a point    include lt iostream gt   include lt sstream gt  using namespace std    int main           string s         cin  gt  gt  s         stringstream ss        ss  lt  lt  s         float num   0         ss  gt  gt  num         if ss good                cerr  lt  lt   No Valid Number   lt  lt  endl                else if num    0  amp  amp  s 0      0               cerr  lt  lt   No Valid Number   lt  lt  endl                else             cout  lt  lt  num lt  lt  endl                           Prove  C   Program

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With C  11 compiler  for non-negative integers I would use something like this  note the    instead of std      bool is number const std  string  amp s      return  s empty    amp  amp  std  all of s begin    s end      isdigit       http   ideone com OjVJWh

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The most efficient way would be just to iterate over the string until you find a non-digit character   If there are any non-digit characters  you can consider the string not a number   bool is number const std  string amp  s        std  string  const iterator it   s begin        while  it    s end    amp  amp  std  isdigit  it     it      return  s empty    amp  amp  it    s end        Or if you want to do it the C  11 way   bool is number const std  string amp  s        return  s empty    amp  amp  std  find if s begin             s end       unsigned char c    return  std  isdigit c         s end        As pointed out in the comments below  this only works for positive integers   If you need to detect negative integers or fractions  you should go with a more robust library-based solution   Although  adding support for negative integers is pretty trivial

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I d suggest a regex approach  A full regex-match  for example  using boost  regex  with   -  0-9        0-9       would show whether the string is a number or not  This includes positive and negative numbers  integer as well as decimal   Other variations    0-9        0-9        only positive   -  0-9      only integer    0-9      only positive integer

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Brendan this  bool isNumber string line         return  atoi line c str           is almost ok   assuming any string starting with 0 is a number  Just add a check for this case  bool isNumber const string  amp line      if  line 0      0   return true   return  atoi line c str          ofc  123hello  will return true like Tony D noted

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I propose a simple convention   If conversion to ASCII is   0 or it starts with 0 then it is a number  It is not perfect but fast   Something like this   string token0   if  atoi token0 c str    gt 0    isdigit token0 c str   0         this is a value        do what you need to do

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I think this regular expression should handle almost all cases       -       0-9       0-9        so you can try the following function that can work with both  Unicode and ANSI   bool IsNumber CString Cs   Cs Trim      ifdef  UNICODE std  wstring sr    LPCWSTR Cs GetBuffer Cs GetLength     return std  regex match sr  std  wregex  T      -       0-9       0-9            else     std  string s    LPCSTR Cs GetBuffer    return std  regex match s  std  regex      -       0-9       0-9          endif

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Try this   bool checkDigit string str         int n str length        for int i 0     i    lt  n     i              if str i  lt  0     str i  gt  9          return false           return true

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As it was revealed to me in an answer to my related question  I feel you should use boost  conversion  try lexical convert

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Here s another way of doing it using the  lt regex gt  library   bool is integer const std  string  amp  s       return std  regex match s  std  regex    -      0-9

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bool isNumeric string s       if    s empty    amp  amp  s 0      -            s    0    s    prepend 0      string garbage       stringstream ss s        ss  gt  gt    auto ptr lt double gt  new double    gt  gt  garbage       the line above extracts the number into an anonymous variable  it could also be done like this  double x  ss  gt  gt  x  gt  gt  garbage           if there is no garbage return true or else return false     return garbage empty         how it works  the stringstream    overload can convert strings to various arithmetic types it does this by reading characters sequentially from the stringstream  ss in this case  until it runs out of characters OR the next character does not meet the criteria to be stored into the destination variable type   example1   stringstream ss  11    double my number  ss  gt  gt  my number    my number   11   example2   stringstream ss  011    double my number  ss  gt  gt  my number    my number   11   example3   stringstream ss  11ABCD    double my number  ss  gt  gt  my number    my number   11  even though there are letters after the 11    the  garbage  variable explanation    why not just check if extraction into my double has a valid value and then return true if it does   notice example3 above will still successfully read the number 11 into the my number variable even if the input string is  11ABCD   which is not a number    to handle this case we can do another extraction into a string variable which I named garbage  which can read anything that may have been left over in the string buffer after the initial extraction into the variable of type double   If anything is left over it will be read into  garbage  which means the full string passed in was not a number  it just begins with one   in this which case we d want to return false   the prepended  0  explanation    attempting to extract a single character into a double will fail returning 0 into our double  but will still move the string buffer position to after the character   In this case our garbage read will be empty which would cause the function to incorrectly return true  to get around this I prepended a 0 to the string so that if for example the string passed in was  a  it gets changed to  0a  so that the 0 will be extracted into the double and  a  gets extracted into garbage     prepending a 0 will not affect the value of the number so the number will still be correctly extracted into our double variable

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Why reinvent the wheel   The C standard library  available in C   as well  has a function that does exactly this   char  p  long converted   strtol s   amp p  10   if   p           conversion failed because the input wasn t a number   else          use converted     If you want to handle fractions or scientific notation  go with strtod instead  you ll get a double result    If you want to allow hexadecimal and octal constants in C C   style   0xABC    then make the last parameter 0 instead   Your function then can be written as  bool isParam string line        char  p      strtol line c str     amp p  10       return  p    0

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We may use  a stringstream  class           bool isNumeric string str               stringstream stream                            double number          stream lt  lt str         stream gt  gt number          return stream eof

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I ve found the following code to be the most robust  c  11   It catches both integers and floats     include  lt regex gt  bool isNumber  std  string token         return std  regex match  token  std  regex           -     digit              digit

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C C   style for unsigned integers  using range based for C  11   int isdigits const std  string  amp  s        for  char c   s  if   isdigit c   return  0       return  1

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Could you simply use sscanf s return code to determine if it s an int   bool is number const std  string amp  s        int value      int result   sscanf valueStr c str      d    amp value       return  result    EOF  amp  amp  readResult    0

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My solution using C  11 regex   include  lt regex gt    it can be used for more precise check  like unsigned int  double etc   static const std  regex INT TYPE    -   0-9      static const std  regex UNSIGNED INT TYPE       0-9      static const std  regex DOUBLE TYPE    -   0-9       0-9      static const std  regex UNSIGNED DOUBLE TYPE       0-9       0-9       bool isIntegerType const std  string amp  str       return std  regex match str   INT TYPE      bool isUnsignedIntegerType const std  string amp  str       return std  regex match str   UNSIGNED INT TYPE      bool isDoubleType const std  string amp  str       return std  regex match str   DOUBLE TYPE      bool isUnsignedDoubleType const std  string amp  str       return std  regex match str   UNSIGNED DOUBLE TYPE       You can find this code at http   ideone com lyDtfi  this can be easily modified to meet the requirements

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bool is number const string amp  s  bool is signed        if  s empty    return false       if  is signed  amp  amp   s front             s front       -                  return is number s substr 1  s length   - 1   false              auto non digit   std  find if s begin    s end       const char amp  c    return  std  isdigit c           return non digit    s end

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The simplest I can think of in c    bool isNumber string s        if s size    0  return false      for int i 0 i lt s size   i              if  s i  gt   0   amp  amp  s i  lt   9    false                return false                      return true      Working code sample  https   ideone com nRX51Y

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You can do it the C   way with boost  lexical cast   If you really insist on not using boost you can just examine what it does and do that   It s pretty simple   try      double x   boost  lexical cast lt double gt  str      double could be anything with  gt  gt  operator    catch        oops  not a number

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Yet another answer  that uses stold  though you could also use stof stod if you don t require the precision    bool isNumeric const std  string amp  string        std  size t pos      long double value   0 0       try               value   std  stold string   amp pos             catch std  invalid argument amp                 return false            catch std  out of range amp                 return false             return pos    string size    amp  amp   std  isnan value

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This function takes care of all the possible cases   bool AppUtilities  checkStringIsNumber std  string s         Eliminate obvious irritants that could spoil the party       Handle special cases here  e g  return true for       -      if they are acceptable as numbers to you     if  s          s           s           s     -     s            s     -    return false         Remove leading   trailing spaces   IF   they are acceptable to you     while  s size    gt  0  amp  amp  s 0          s   s substr 1  s size   - 1       while  s size    gt  0  amp  amp  s s size   - 1          s   s substr 0  s size   - 1           Remove any leading   or - sign     if  s 0            s 0      -           s   s substr 1  s size   - 1          Remove decimal points     long prevLength   s size         size t start pos   0      while  start pos   s find      start pos      std  string  npos           s replace start pos  1              If the string had more than 2 decimal points  return false      if  prevLength  gt  s size     1  return false         Check that you are left with numbers only         Courtesy selected answer by Charles Salvia above     std  string  const iterator it   s begin        while  it    s end    amp  amp  std  isdigit  it     it      return  s empty    amp  amp  it    s end           Tada

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Here is a solution for checking positive integers   bool isPositiveInteger const std  string amp  s        return  s empty    amp  amp               std  count if s begin    s end    std  isdigit     s size

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