C# Homework - control structures (for, if)

Question

I got a homework assignment today: "Create a program that calculates out how many ways you can add three numbers so that they equal 1000."

I think this code should work, but it doesn't write out anything.

using System;

namespace ConsoleApp02
{
    class Program
    {
        public static void Main(string[] args)
        {
            for(int a = 0; a < 1000; a++)
            {
                for(int b = 0; b < 1000; b++)
                {
                    for(int c = 0; c < 1000; c++)
                    {
                        for(int puls = a + b + c; puls < 1000; puls++)
                        {
                            if(puls == 1000)
                            {
                                Console.WriteLine("{0} + {1} + {2} = 1000", a, b, c);
                            }
                        }
                    }
                }
            }
            Console.ReadKey(true);
        }
    }
}

What am I doing wrong? Any tips or solution?

Answer 1

Your innermost loop (iterating the puls variable) doesn't really make any sense, and because of the condition on it (puls < 1000) Console.WriteLine never runs.

Perhaps you should test whether A + B + C is 1000, instead.

Also, you'll find that you might be missing a couple particular combinations of numbers because of the bounds on your loops (depending on the problem statement.)

Answer 2

You don't need the inner loop.

if (a+b+c == 1000)
   write

Answer 3

Once the inner most loop gets to 1000, it breaks out of the for loop and never even checks if it is 1000 in the 'if' statement.

Answer 4

Your homework so just a hint. What is going on in the final for loop? Is that really what you want to do?

Answer 5

That code will return no answer.

The interior for loop adds a + b + c and puts the result into puls. However, you stop the loop before puls can get to 1000, and then test inside the for statement to see if puls equals 1000. So, you won't get an answer.

Just test puls against 1000 in the inner loop. Why?

Answer 6

You can replace your innermost for loop with just a test for if (a+b+c == 1000) {...}. Then you can add optimizations like - once a sum has been found with a combination, there is no need to continue with the inner loop.

Answer 7

In your final inner loop, "int puls = a + b + c; puls < 1000; puls++" you're ensuring that puls never = 1000, if puls is not less than 1000, it kicks out of the loop. That is why you are getting no values. But rethink your logic some as well.

Answer 8

The code does not cover the following cases

• 1000 + 0 + 0
• 0 + 10000 + 0
• 0 + 0 + 10000

The inner most loop should be an if rather for.

You don't need {} if there is only one statement in the loop or if clause.

EDIT: Removed code answers

Answer 9

On a separate note, this particular implementation, while it'll work (with the modifications suggested by the other answers), is quite a performance hit, as the complexity of your algorithm is O(n^3). In other words, you are going through the innermost check one bilion times.

Here's a hint how you can optimize it to at least O(n^2) or just one milion iterations: for each pair of a and b generated by the two outer for loops, there's only one value for c that will result in 1000.

Answer 10

If you get this assignment as a computer science student, you'd probably want to solve this using Dynamic Programming.

Answer 11

Question doesn't specify that negative numbers are not allowed. Answer is infinite.