Compare two string as numeric value - c++

Question

How should I compare two string representing numbers in C++? I thought of converting to number of long long type but the problem is the numerical value represented by string can exceed the long long MAX limit.
It is guaranteed that the string represents a numerical value.
There is a similar question in Java compare two numeric String values.
but that makes use of the BigInteger Library that we don't have in C++.

Answer 1

Compare them digit by digit:
a = "3254353245423345432423133423421"
b = "3254353245423345432443133423421"
for(int i = 0; i < a.length(); ++i):
if ((a[i] - '0') < (b[i] - '0'))
{
std::cout << "b is larger!"
}
I'm sure you can take it from here if you want to find out whether b is larger than a, or if they are equal. Alternatively, if they are different lengths, the larger one wins! (Check for zeros at the beginning, i.e. "000443342") Don't forget to consider negative numbers.

Answer 2

If you think about it a bit, its not hard. For simplicity we will assume two positive numbers with no leading zeros. If they have leading zeroes, discard them.
Now consider two numbers:
123456
23456 Its apparent that the first one is larger, because it is longer. This allows us to quickly solve most comparisons. Now, if they have equal length, just compare them from the beginning. The number with smaller leading digit is smaller. If they are equal, take the next digit.
Now what about other cases? Well, one positive and one negative number is easy, the negative one is smaller, period. If you have two negative numbers, then you have to do the same thing as you would when comparing two positive numbers, but this time the number with larger leading digit is smaller.
As was pointed to me in the comments, std::string already implements lexicographical comparison, which means that you only have to sanitize the strings into valid numbers, call std::string::compare and decide whether -1 means smaller (positive numbers) or larger (negative numbers).

Answer 3

If you are comparing two strings as integers,
chances are you are going to find yourself wanting to do other math as well.
Use GMP and save yourself the headaches.
#include <iostream>
#include <gmpxx.h>
int main(){
mpz_class x("12323423434534234234234");
mpz_class y("9994828945090011626439");
std::cout << std::boolalpha;
std::cout << (x < y) << '\n';
}
//example compilation: g++ mycxxprog.cc -lgmpxx -lgmp
This may be overkill for your particular problem, as it increases the project dependencies.
Be sure to consider other options.

Answer 4

Takes 2 strings as input, returns 1 if 2nd(b in this case) is larger, 0 otherwise.
int find(char a[],char b[])
{
int i = 0;
int flag = 0;
if(a[0] == '-' && b[0] !='-')
{
printf("%s is larger: %s is -ve",b,a);
return 1;
}
else if(b[0] == '-' && a[0] !='-')
{
printf("%s is larger: %s is -ve",a,b);
return 0;
}
for(i = 0; i < strlen(a); ++i)
{
if(a[i] == '.' && b[i] != '.')
{
printf("%s is larger: %s is .",b,a);
return 1;
}
else if(b[i] == '.' && a[i] != '.')
{
printf("%s is larger: %s is .",a,b);
return 0;
}
else if(a[i] == '.' && b[i] == '.')
{
printf("passed\n");
continue;
}
if ((a[i] - '0') < (b[i] - '0'))
{
flag = 1;
//break;
}
}
if(flag == 0)
return 0;
else
return 1;
}