"The Trip" challenge from Programming Challenges Here is a problem from the book Programming Challenges by Skiena and Revilla:


  The Trip
  
  A number of students are members of a club that travels annually to
  exotic locations. Their destinations in the past have included
  Indianapolis, Phoenix, Nashville, Philadelphia, San Jose, and Atlanta.
  This spring they are planning a trip to Eindhoven. The group agrees in
  advance to share expenses equally, but it is not practical to have
  them share every expense as it occurs. So individuals in the group pay
  for particular things, like meals, hotels, taxi rides, plane tickets,
  etc. After the trip, each student's expenses are tallied and money is
  exchanged so that the net cost to each is the same, to within one
  cent. In the past, this money exchange has been tedious and time
  consuming. Your job is to compute, from a list of expenses, the
  minimum amount of money that must change hands in order to equalize
  (within a cent) all the students' costs.
  
  The Input
  
  Standard input will contain the information for several trips. The
  information for each trip consists of a line containing a positive
  integer, n, the number of students on the trip, followed by n lines of
  input, each containing the amount, in dollars and cents, spent by a
  student. There are no more than 1000 students and no student spent
  more than $10,000.00. A single line containing 0 follows the
  information for the last trip.
  
  The Output
  
  For each trip, output a line stating the total amount of money, in
  dollars and cents, that must be exchanged to equalize the students'
  costs.
  
  Sample Input

3
10.00
20.00
30.00
4
15.00
15.01
3.00
3.01
0

  
  Sample Output

$10.00 
$11.99



Here is my solution which gives correct answer for all inputs I have tried so far, but there must be some case when it does not work because the judge says so. Can anybody help me to understand why?

int main (int argc, char * argv[]) {
    int64_t a;
    float paid[1000];
    std::vector&lt;float&gt; results;
    while (std::cin &gt;&gt; a &amp;&amp; a != 0) {

        int sum = 0;
        for (int i = 0; i &lt; a; i++) {
            std::cin &gt;&gt; paid[i];
            paid[i] *= 100;
            sum += paid[i];
        }
        int result = 0;
        int avg = (std::ceil)((float)sum/a);

        int mod = sum % a;
        int c = 0;
        if (mod &gt; 0) {
            for (int i = 0; i &lt; a; i++) {
                if (c &gt;= a - mod)
                    break;
                if (paid[i] &lt; avg) {
                    paid[i]++;
                    c++;
                }
            }
            for (int i = 0; i &lt; a; i++) {
                if (c &gt;= a - mod)
                    break;
                if (paid[i] &gt; avg) {
                    paid[i] += a - mod - c;
                    break;
                }
            }
        }

        for (int i = 0; i &lt; a; i++) {
            if (paid[i] &gt; avg) {
                result += (paid[i] - avg);
            }
        }

        results.push_back((float)result/100);
    }
    std::cout &lt;&lt; std::fixed;
    std::cout &lt;&lt; std::setprecision(2);
    for (std::vector&lt;float&gt;::iterator it = results.begin(); it != results.end(); ++it) {
        std::cout &lt;&lt; "$" &lt;&lt; *it &lt;&lt; std::endl;
    }
    return 0;
}
Your algorithm seems to be correct though a bit overcomplicated.
But the code is not correct. consider:

9999.1
9999.1
9999.0
9999.1

$0.05


And the correct answer is $0.07

This is due to lack of precision in float implementation. It does not mean you should use double, it mean you should check your casts and try to avoid using float at money computations at all.

Also, i suggest to check your inputs.