Question

I need to create a function that will generate 2 random numbers between x and y (e.g. x = 1, y = 20) which when added will not involve regrouping / carryover or which when subracted will not involve borrowing.

For example,

18 + 1 = good
14 + 5 = good
18-7 = good
29 - 8 = good

15 + 6 = bad
6 + 7 = bad
21 - 3 = bad
36 - 8 = bad etc.

I want to create a simple worksheet generator that will generate sample problems using the requirements above.

I guess I could always convert the number to string, get the right most digit for each of the 2 numbers, convert them back to integer, and test if one is greater than the other. Repeat for all the digit. Only thing is, that is so damn ugly (read inefficient). I am sure that there is a better way. Anyone have any suggestions? Thanks

Answer 1

using System;

class Sample {
    static void Main() {
        var rnd = new Random();
        var x = 1;
        var y = 20;
        var a = rnd.Next(x, y);
        var b = rnd.Next(x, y);
        var op = '+';
        Console.WriteLine("{0} {2} {1} = {3}", a, b, op , isValid(a, b, op)? "good":"bad");
        op = '-';
        Console.WriteLine("{0} {2} {1} = {3}", a, b, op , isValid(a, b, op)? "good":"bad");
    }
    static bool isValid(int x, int y, char op){
        int a = x % 10;
        int b = y % 10;
        switch (op){
        case '+':
            return a + b < 10;
        case '-':
            return x >= y && a - b >= 0;
        default:
            throw new Exception(String.Format("unknown operator '{0}'", op));
        }
    }
}