CODEWITHSUNDEEP
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sam
sam

Reputation: 3

Changing variables outside of Scope C#

I'm a beginner C# programmer, and to improve my skills I decided to give Project Euler a try. The first problem on the site asks you to find the sum of all the multiples of 3 and 5 under 1000. Since I'm essentially doing the same thing twice, I made a method to multiply a base number incrementally, and add the sum of all the answers togethor. 

public static int SumOfMultiplication(int Base, int limit)
    {
        bool Escape = false;
        for (int mult = 1; Escape == true; mult++)
        {
            int Number = 0;
            int iSum = 0;
            Number = Base * mult;
            if (Number > limit)
                return iSum;
            else
                iSum = iSum + Number;

        }

regardless of what I put in for both parameters, it ALWAYS returns zero. I'm 99% sure it has something to do with the scope of the variables, but I have no clue how to fix it. All help is appreciated.

Thanks in advance,

Sam

Upvotes: 0

Views: 869

Answers (4)

Eric Lippert
Eric Lippert

Reputation: 660455

As others have pointed out, the problem is that the control flow does not do what you think it does. This is a common beginner problem.

My suggestion to you is learn how to use your debugger. Beginners often have this strange idea that they're not allowed to use tools to solve their coding problems; that rather, they have to reason out the defect in the program by simply reading it. Once the programs become more than a page long, that becomes impossible for humans. The debugger is your best friend, so get to know its features really well.

In this case if you'd stepped through the code in the debugger you'd see that the loop condition was being evaluated and then the loop was being skipped. At that point you wouldn't be asking "why does this return zero?", you'd be asking "why is the loop body always skipped?" Clearly that is a much more productive question to ask since that is actually the problem here.

Don't write any code without stepping through it in the debugger. Watch every variable, watch how it changes value (the debugger highlights variables in the watch windows right after they change value, by the way) and make sure that the control flow and the variable changes are exactly as you'd expect. Pay attention to quiet doubts; if anything seems out of the ordinary, track it down, and either learn why it is correct, or fix it until it is.

Regarding the actual problem: remember that 15, 30, 45, 60... are all multiples of both three and five, but you only want to add them to the sum once. My advice when solving Project Euler problems is to write code that is as like what you are trying to solve as is possible. Try writing the problem out in "pseudocode" first. I'd pseudocode this as:

sum = 0
for each positive number under 1000:
    if number is multiple of three or five then:
        add number to sum

Once you have that pseudocode you can notice its subtleties. Like, is 1000 included? Does the problem say "under 1000" or "up to 1000"? Make sure your loop condition considers that. And so on.

The closer the program reads like the problem actually being solved, the more likely it is to be correct.

Upvotes: 3

Darkhydro
Darkhydro

Reputation: 2082

There are several problems with this code. The first, and most important, is that you are using the Escape variable only once. It is never set to false within your for loop, so it serves no purpose whatsoever. It should be removed. Second, isum is declared within your for loop, which means it will keep being re-initialized to 0 every time the loop executes. This means you will only get the last multiple, not the addition of all multiples. Here is a corrected code sample: 

int iSum = 0;
for(int mult = 1; true; mult++)
{
    int Number = Base * mult;
    if(Number > limit)
        return iSum;
    else
        iSum += Number;
}

Upvotes: 0

Luka Rahne
Luka Rahne

Reputation: 10487

It does not enter for loop because for condition is false.

Escape == true

returns false

Advice:

  • Using for loop is much simpler if you use condition as limit for breaking loop

    for (int mult = 1; something < limit; mult++)

    This way in most cases you do not need to check condition in loop

  • Most programming languages have have operator modulo division. http://en.wikipedia.org/wiki/Modulo_operation

    It might come handy whit this problem.

Upvotes: 0

Ed Swangren
Ed Swangren

Reputation: 124770

Your loop never actually executes:

bool Escape = false;
for (int mult = 1; Escape == true; mult++)

Escape is set to false initially, so the first test fails (Escape == true returns false) and the body of the loop is skipped.

The compiler would have told you if you were trying to access variables outside of their defined scope, so that's not the problem. You are also missing a return statement, but that is probably a typo.

I would also note that your code never checks if the number to be added to the sum is actually a multiple of 3 or 5. There are other issues as well (for example, iSum is declared inside of the loop and initialized to 0 after each iteration), but I'll let you work that one out since this is practice. The debugger is your friend in cases like these :)

EDIT: If you need help with the actual logic I'll be happy to help, but I figure you want to work it out on your own if possible.

Upvotes: 6

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