Is there a quick way to check if a string is numeric?

Question

Is it possible to check if a string variable is entirely numeric? I know you can iterate through the alphabets to check for a non-numeric character, but is there any other way?

Look up `strtol`. If the `endptr` points one past the last character, all characters were converted. — AbdullahC, Oct 07 '13 at 00:58
Is this **really** a bottleneck for you? If not just use something easy. If it is you can use various SSE tricks to process more than one character in your block at a time (I've seen this done for whitespace - but don't have an example handy). You can also split your chunks up and spread across multiple threads. But really these will be overkill for most situations. — Michael Anderson, Oct 07 '13 at 01:24

score 1 · Answer 1 · edited May 23 '17 at 12:29

The quickest way i can think of is to try to cast it with "strtol" or similar functions and see whether it can convert the entire string:

char* numberString = "100";
char* endptr;
long number = strtol(numberString, &endptr, 10);
if (*endptr) {
    // Cast failed
} else {
    // Cast succeeded
}

This topic is also discussed in this thread: How to determine if a string is a number with C++?

Hope this helps :)

0x499602D2 · Answer 2 · 2013-10-07T02:02:50.063

1

#include <iostream>
#include <string>
#include <locale>
#include <algorithm>

bool is_numeric(std::string str, std::locale loc = std::locale())
{
    return std::all_of(str.begin(), str.end(), std::isdigit);
}

int main()
{
    std::string str;
    std::cin >> str;

    std::cout << std::boolalpha << is_numeric(str); // true
}

edited Oct 07 '13 at 02:02

answered Oct 07 '13 at 01:22

0x499602D2

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Why not use `std::isdigit(c, loc)`? I think it's clearer and less typing. – Blastfurnace Oct 07 '13 at 01:31
@Blastfurnace Is `isdigit` a C function? I'm trying to keep this entirely C++ oriented. – 0x499602D2 Oct 07 '13 at 01:32
There is a C++ `std::locale` version. See: [std::isdigit](http://en.cppreference.com/w/cpp/locale/isdigit) – Blastfurnace Oct 07 '13 at 01:34

henderso · Answer 3 · 2013-10-07T01:08:52.363

You can use the isdigit function in the ctype library:

  #include <stdio.h>
  #include <stdlib.h>
  #include <ctype.h>
  int main ()
  {
    char mystr[]="56203";
    int the_number;
    if (isdigit(mystr[0]))
    {
      the_number = atoi (mystr);
      printf ("The following is an integer\n",the_number);
    }
   return 0;
  }

This example checks the first character only. If you want to check the whole string then you can use a loop, or if its a fixed length and small just combine isdigit() with &&.

Is there a quick way to check if a string is numeric?

3 Answers3