100 doors exercise code using C#

Question

Can you please check my code if it is correct for the problem 100 doors?

This is the problem:

You have 100 doors in a row that are all initially closed. You make 100 passes by the doors. The first time through, you visit every door and toggle the door (if the door is closed, you open it; if it is open, you close it). The second time you only visit every 2nd door (door #2, #4, #6, ...). The third time, every 3rd door (door #3, #6, #9, ...), etc, until you only visit the 100th door. Question: What state are the doors in after the last pass? Which are open, which are closed?

static void Main(string[] args)
    {
        bool[] doors = new bool[100];

        bool test = true;
        bool temp1 = false;
        bool temp2 = true;
        bool temp;

        for (int i = 1; i <= 100; i++) {

            if (i == 3) {
                temp = temp1;
                temp1 = temp2;
                temp2 = temp;
            }

            if (test == true) {

                for (int a = 1; a <= 100; a++) {

                    if (a % i == 0) {

                        doors[a - 1] = temp1;
                    }
                    else if(a % i != 0)
                        doors[a - 1] = temp2;
                }
            }
            else if (test == false) {

                for (int c = 0; c < 100; c++) {

                    doors[c] = true;

                }
                test = true;
            }            
        }
        for (int d = 0; d < 100; d++) {
            Console.WriteLine(doors[d]);
        }

            Console.ReadKey();
    }
}

Answer 1

I have no idea what this does:

if (i == 3) {
    temp = temp1;
    temp1 = temp2;
    temp2 = temp;
}

Please don't say if (test == true) or if (test == false)

but just if (test) or if (!test)

and you can just say else instead else if (test == false)

But your code in overly complex anyway. How about just this:

    bool[] doors = new bool[101];
    for (int step = 1; step <= 100; ++step) {
        for (int door = step; door <= 100; door += step) {
            doors[door] = !doors[door];
        }
    }

    for (int door = 1; door <= 100; ++door) {
        if (doors[door]) Console.WriteLine(door);
    }

You'll see that the output is all the square numbers: 1, 4, 9, 16, 25, 36, 49, 64, 81 and 100

Answer 2

A construct like

if (bool == true)  

can be simplified to

if (bool)  

and in addition if you have a if..else if like

if (bool == true)
{


}
else if (bool == false)
{

}  

you can omit the else if and replace it with a else.

---

You should name your variables in a meaningful way, so that you or Sam the Maintainer will know at first glance what they are about.

bool test = true;
bool temp1 = false;
bool temp2 = true;
bool temp;  

---

Placing this whole code inside main() can be considered as bad practice. You should separate the responsibilities like toggling doors and printing the result.

---

For toggling a boolean value, you can simply do booleanVariable = !booleanVariable;

---

You have some unneeded vertical space (new lines) in your code which should be removed for readability.

---

Also there is no convention for where to place an opening brace { you should at least stick to your style. Right now you are mixing where you place the brace.

---

After applying these points your code will look like

class Program
{
    static void Main(string[] args)
    {
        Program p = new Program();
        bool[] toggledDoors = p.SolveDoorExcercise(100);
        String output = p.FormatOutput(toggledDoors,true);
        Console.Write(output);
        Console.ReadKey();
    }

    private bool[] SolveDoorExcercise(int numberOfDoors)
    {
        bool[] doors = new bool[numberOfDoors];
        for (int step = 1; step <= numberOfDoors; ++step)
        {
            for (int i = step - 1; i < numberOfDoors; i += step)
            {
                doors[i] = !doors[i];
            }
        }
        return doors;
    }

    private string FormatOutput(bool[] doors, bool opened)
    {
        StringBuilder sb = new StringBuilder(doors.Length * 2);
        for (int i = 0; i < doors.Length; i++)
        {
            if (doors[i] == opened)
            {
                sb.AppendLine((i + 1).ToString());
            }
        }
        return sb.ToString();
    }
}