Question

How do I generate 30 random numbers between 1-9, that all add up to 200 (or some arbitrary N), in C#?

I’m trying to generate a string of digits that can add together to be N.

Answer 1

I’m not sure what the statistics are on this but, the issue here is that you don’t want to randomly select a number that makes it impossible to sum N with M number of entries either by overshooting or undershooting. Here’s how I would do it:

static void Main() {     int count = 30;     int[] numbers = getNumbers(count, 155);     for (int index = 0; index < count; index++)     {         Console.Write(numbers[index]);         if ((index + 1) % 10 == 0)             Console.WriteLine('');         else if (index != count - 1)             Console.Write(',');     }     Console.ReadKey(); } static int[] getNumbers(int count, int total) {     const int LOWERBOUND = 1;     const int UPPERBOUND = 9;      int[] result = new int[count];     int currentsum = 0;     int low, high, calc;      if((UPPERBOUND * count) < total ||         (LOWERBOUND * count) > total ||         UPPERBOUND < LOWERBOUND)         throw new Exception('Not possible.');      Random rnd = new Random();      for (int index = 0; index < count; index++)     {         calc = (total - currentsum) - (UPPERBOUND * (count - 1 - index));         low = calc < LOWERBOUND ? LOWERBOUND : calc;         calc = (total - currentsum) - (LOWERBOUND * (count - 1 - index));         high = calc > UPPERBOUND ? UPPERBOUND : calc;          result[index] = rnd.Next(low, high + 1);          currentsum += result[index];     }      // The tail numbers will tend to drift higher or lower so we should shuffle to compensate somewhat.      int shuffleCount = rnd.Next(count * 5, count * 10);     while (shuffleCount-- > 0)         swap(ref result[rnd.Next(0, count)], ref result[rnd.Next(0, count)]);      return result; } public static void swap(ref int item1, ref int item2) {     int temp = item1;     item1 = item2;     item2 = temp; } 

I didn’t have a lot of time to test this so apologies if there’s a flaw in my logic somewhere.

EDIT:

I did some testing and everything seems solid. If you want a nice pretty spread it looks like you want something along the lines of Total = Count * ((UPPER + LOWER) / 2). Although I’m fairly certain that as the difference between UPPER and LOWER increases the more flexible this becomes.