Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

How do I check if a C++ string is an int?

When I use getline, I would input a bunch of strings or numbers, but I only want the while loop to output the "word" if it is not a number. So is there any way to check if "word" is a number or not? I know I could use atoi() for C-strings but how about for strings of the string class?

int main () {   stringstream ss (stringstream::in | stringstream::out);   string word;   string str;   getline(cin,str);   ss<<str;   while(ss>>word)     {       //if(    )         cout<<word<<endl;     } } 
like image 627
user342231 Avatar asked May 16 '10 17:05

user342231


People also ask

How do you check if a string is an integer or int?

We can use the isdigit() function to check if the string is an integer or not in Python. The isdigit() method returns True if all characters in a string are digits. Otherwise, it returns False.

How do you check if an element of a string is a number?

Use the Number() Function to Check Whether a Given String Is a Number or Not in JavaScript. The Number() function converts the argument to a number representing the object's value. If it fails to convert the value to a number, it returns NaN.


2 Answers

Another version...

Use strtol, wrapping it inside a simple function to hide its complexity :

inline bool isInteger(const std::string & s) {    if(s.empty() || ((!isdigit(s[0])) && (s[0] != '-') && (s[0] != '+'))) return false;     char * p;    strtol(s.c_str(), &p, 10);     return (*p == 0); } 

Why strtol ?

As far as I love C++, sometimes the C API is the best answer as far as I am concerned:

  • using exceptions is overkill for a test that is authorized to fail
  • the temporary stream object creation by the lexical cast is overkill and over-inefficient when the C standard library has a little known dedicated function that does the job.

How does it work ?

strtol seems quite raw at first glance, so an explanation will make the code simpler to read :

strtol will parse the string, stopping at the first character that cannot be considered part of an integer. If you provide p (as I did above), it sets p right at this first non-integer character.

My reasoning is that if p is not set to the end of the string (the 0 character), then there is a non-integer character in the string s, meaning s is not a correct integer.

The first tests are there to eliminate corner cases (leading spaces, empty string, etc.).

This function should be, of course, customized to your needs (are leading spaces an error? etc.).

Sources :

See the description of strtol at: http://en.cppreference.com/w/cpp/string/byte/strtol.

See, too, the description of strtol's sister functions (strtod, strtoul, etc.).

like image 108
paercebal Avatar answered Sep 18 '22 15:09

paercebal


The accepted answer will give a false positive if the input is a number plus text, because "stol" will convert the firsts digits and ignore the rest.

I like the following version the most, since it's a nice one-liner that doesn't need to define a function and you can just copy and paste wherever you need it.

#include <string>  ...  std::string s;  bool has_only_digits = (s.find_first_not_of( "0123456789" ) == std::string::npos); 

EDIT: if you like this implementation but you do want to use it as a function, then this should do:

bool has_only_digits(const string s){   return s.find_first_not_of( "0123456789" ) == string::npos; } 
like image 20
Ferdinando Randisi Avatar answered Sep 17 '22 15:09

Ferdinando Randisi