Sphere online judge finds my code wrong although its working correct for hundreds of test cases.

Question

Question goes like this.. Input

n [the number of multiplications <= 1000]
l1 l2 [numbers to multiply (at most 10000 decimal digits each)]

Text grouped in [ ] does not appear in the input file.

Output

The results of multiplications.

My code..

#include<iostream>
#include<vector>
using namespace std;
int main()
{
    long int n,a,b,c,t;
    vector<long int> v(1000);
    vector<long int>::iterator p;
    cin>>n;
    t=n;
    p=v.begin();
    while(t>0)
    {
        cin>>a;
        cin>>b;
        *p=a;
        *(p+1)=b;
        p+=2;
        t--;
    }
    t=n;
    p=v.begin();
    while(t>0)
    {
        c= (*p) * (*(p+1));
        cout<<"\n"<<c;
        p+=2;
        t--;
    }
  return 0;
}

Answer 1

long int isn't going to hold anywhere near 10,000 decimal digits.

−9,223,372,036,854,775,808 to +9,223,372,036,854,775,807

Answer 2

Had to google your problem to understand what it was: here it is, please be considerate of your readers :x

Your code cannot actually work: long int is not long enough (and this is implementation dependent anyway)

You will have to read the integers 'char' by 'char' and roll your own implementation of BigInts which is the difficulty of the problem here I surmise.

Furthermore, you will have to deal with overflow: the result of int x int might not fit into an int. Truth to be told, you should be prepared to have a result that might take up to 20.000 digits

Answer 3

It's not surprising that you can find 100s of test cases that your program will pass.

On most 32-bit compilers, long int (also written as long) is a 32-bit number. It can hold around 4 billion unique values (so that's a 10 digit number). So if you've only found 100 test cases that your program passes, you aren't really trying!

Did you try asking your program to multiply 100108810983456785767868356745567566892348745 by 29387345245742687609834756790356568098645?