converting std::string to int using sstream

Question

I'm trying to convert a string of an arbitrary length to an int but so far it works only for strings of limited length. Code so far:

long long convertToInt (std::string x){
   long long number;
   std::istringstream ss(x);
   ss >> number;
   return number;}

for x=100000000000000000000000001 the function returns 0. Could someone explain why? Thanks.

even `long long` is too short for `100000000000000000000000001` — Marcus, May 08 '13 at 07:13
You might want to check out a library such as [GMP](http://gmplib.org/). — Some programmer dude, May 08 '13 at 07:16
Unless you're privy to a 128-bit extended unsigned long platform, I'm not seeing this happening. And those don't come easy, but if you have one your limit reaches 340282366920938463463374607431768211455. If not, time to break out a bignum lib. — WhozCraig, May 08 '13 at 07:21

hmjd · Answer 1 · 2013-05-08T07:20:14.963

5

The value "100000000000000000000000001" is to large to fit in a long long (or unsigned long long) so the extraction fails.

Use numeric_limits to determine the maximum value for types in your implementation:

#include <limits>

std::cout << std::numeric_limits<unsigned long long>::max() << "\n";
std::cout << std::numeric_limits<long long>::max() << "\n";
std::cout << "100000000000000000000000001\n";

prints:

18446744073709551615
9223372036854775807
100000000000000000000000001

Check the result of the extraction attempt to ensure extraction occurred:

if (ss >> number)
{
    return number;
}
// Report failure.

edited May 08 '13 at 07:20

answered May 08 '13 at 07:14

hmjd

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So what return type do I specify If I want the function to handle strings of arbitrary length? – maddy May 08 '13 at 07:17
@maddy, The one described in the link in the comment. – chris May 08 '13 at 07:17
@maddy, there might be some big integer libraries around that may support what your require but I am unfamiliar with them. – hmjd May 08 '13 at 07:18
http://gmplib.org/manual/C_002b_002b-Interface-Integers.html#C_002b_002b-Interface-Integers – kassak May 08 '13 at 07:34
@maddy that one is better ;) http://gmplib.org/manual/C_002b_002b-Interface-General.html#C_002b_002b-Interface-General – kassak May 08 '13 at 07:36

score 1 · Accepted Answer · answered May 08 '13 at 07:16

The inbuilt integer types your compiler provides are only guaranteed to be capable of storing numbers of a specific magnitude. It looks like your number is larger than that. You may find the C++ program is reporting the error - try...

if (ss >> number)
    return number;
throw std::runtime_error("conversion of string to int failed");

...or whatever other error handling you feel like.

If you must use larger number, you could try double, or if that doesn't suit your needs then check out "arbitrary precision" libraries like GMP - you'll find lots of stackoverflow questions about handling large numbers with answers suggesting, contrasting and illustrating GMP and alternatives.

converting std::string to int using sstream

2 Answers2