Expert: Ralph McArdell - 8/26/2008

Question
Hi Ralph,

Is there any alternative way to validate an input whether it is numeric or not? I tried to use isdigit(), but it shows that 123abc is true, and -123 is false. the input is not from cin, it is read from the command line. i'm asked to make a cpp program to calculate something like:
- task.exe 50 -3 (the operation wll be 5 * -3, which the result will be -150)
- task.exe 50 3ab (it will show an error)

please help, thank you.

Answer
Yes. Many.


----------------------------------------------------------------------------------------------------------------
   Note:   I am going to assume you are using a reasonably ISO standard C++
           language and library implementation.
           This is reasonable; the ISO C++ standard was published about 10 years
           ago so most C++ implementations should be fairly compliant by now.
----------------------------------------------------------------------------------------------------------------


First I would advise you to go and read the small print of isdigit - it works with characters not strings (C-strings as in argv[] entries nor C++ std::string objects).

Another point about isdigit is that it checks for _just_ that - whether a character is a digit, not a valid number. Digit characters would normally be the set of character values: ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'].

A better way would be to convert them using IOStreams - as we might if using std::cin or reading from a file where we can check if the conversion worked by checking the stream state after the read. However in this case we will be reading from a string using a std::istringstream which reads input from a std::string.

We could use a std::istrstream object instead, which in this case is possible a more obvious choice as it works with char * (i.e. character arrays). However std::istrstream and std::osstrstream are listed as compatibility features in the ISO C++ standard for backwards compatibility with pre-standard C++ library implementations and code using them. They are documented as "deprecated features" and the standard goes on to say

       "where deprecated is defined as: Normative for the current edition of the
        Standard, but not guaranteed to be part of the Standard in future revisions."

(the term "Normative" means it has to be included in ISO standard C++ compliant C++ implementations)

To use a std::istringstream we create an object of that type passing it the string to use as input. If we pass a C-style zero terminated array of char then it will be converted to a std::string. We have to include <sstream>. Thus we could write:

   #include <sstream>    // for std::istringstream

   // ...

   std::istringstream  iStrStrm( argv[1] );

We then using it like so:

   #include <iostream>
   #include <sstream>    // for std::istringstream
   
   int main( int argc, char * argv[] )
   {
       if ( argc < 2 )
       {
         std::cerr << "Error: require at least one numeric command line argument."
                   << std::endl;
         return 1;
       }

       std::istringstream  iStrStrm( argv[1] );

       int n(0);
       iStrStrm >> n;

       std::cout << "n=" << n
                 << "\nstd::cin "
                 ;
       if ( iStrStrm.good() )
       {
           std::cout << "is good\n";
       }
       else
       {
           if ( iStrStrm.bad() )
           {
               std::cout << "is fatally bad ";
           }
           else if ( iStrStrm.fail() )
           {
               std::cout << "has formatting failure ";
           }
           if ( iStrStrm.eof() )
           {
               std::cout << "is at EOF ";
           }
           std::cout << std::endl;
       }
       
       return 0;   
   }

Now if we build and run it we should see something like the following:

For:

   theProgram 1000

The output should be something like:

   n=1000
   std::cin is at EOF

For:

   theProgram -1000

The output should be something like:

   n=-1000
   std::cin is at EOF

For:

   theProgram abcd

The output should be something like:

   n=0
   std::cin has formatting failure
   
And for the tricky case you sight:

   theProgram 3ab

The output should be something like:

   n=3
   std::cin is good


You will note that for fully valid arguments we not only have no failure of format conversion (iStrStrm.fail() is false) but also that the end of file flag is set (iStrStrm.eof() is true). This is because there is no whitespace around command line arguments so for a fully valid numeric argument the stream will consume and convert all the characters in the argument string and then find the end of the string and interpret it to be the end of file condition.

For the two bad cases either the fail bit is set (iStrStrm.fail() is true) or the EOF bit is not set (iStrStrm.eof() is false) indicating that there were un-processed characters in the argument string.

I hope the above example program will help you form the basis of a solution for your problem. There are other ways to do this sort of thing but the above method seems the most obvious to me!