Project Euler #1 in Ruby

Project Euler #1

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.

Find the sum of all the multiples of 3 or 5 below 1000.

The problem is fairly straightforward, and is the most correctly answered problem in the whole set.  I will be explaining a little bit of Ruby in this solution I give.

1. x = 0
2. sum = 0
3. while (x < 1000)
4.   if (x % 3 == 0 && x % 5 == 0)
5.     sum += x
6.   elsif (x % 3 == 0)
7.     sum += x
8.   elsif (x % 5 == 0)
9.     sum += x
10.   end
11.   x += 1
12. end
13. puts sum

1.  This is the declaration of a variable in Ruby, and you will notice you do not have to declare its type.  It is inferred from what the variable is set equal to when it is declared.  Handy!  This is what is meant by being a "dynamically typed language", other programming languages like C require you to declare the variable's type at declaration and forever afterward it is of that type.  This is referred to as a "statically typed language". 

2.  See line 1.

3.  While loops in Ruby are very similar to those seen in Java or C++, the difference being we don't use brackets, instead we get a great deal of use out of 'end' statements.  You will notice that everything within the while loop is tabbed out once (at least).  If you use NetBeans, or Notepad++, or some such fancy editor, it will automatically do this and help you see where your blocks of code are located.

What you need to know for 'while' loops, is that you need the keyword 'while' then a conditional, and to close the 'while' block, you must include the 'end' keyword, just like I did on line 12. 

For more information on while loops in Ruby, you may use Tutorials Point, or the slightly more concise Techotopia wiki.

4.  This is our first look at 'if' statements.  Again, very similar to C++, even in the modifiers '&&' or '||' which are 'AND' and 'OR' respectively, in how they modify logical statements. 

In our case, line 4 says "if x modulus 3 is equal to 0 and if x modulus 5 is equal to 0 then...", and that is all there is to it!  For those new to the game, modulus is just looking for the remainder after we divide.

For example:  6 % 5 = 1, because 6/5 = 1 R 1, where 'R' stands for 'remainder', and % stands for 'modulus' or 'mod' for short.

If you would like to learn more about the modulus, or the area of modular arithmetic in mathematics, you can look here, for a quick primer on the subject.

5.  Line 5 is just what it looks like for anyone that has used C or Java, the "+=" operator takes the left hand side and simply adds to it the right hand side.  For example:

1. x = 0   #=> 0
2. x += 3  #=> 3
3. x += 39 #=> 42

6.  An 'elsif' statement is just an "else" and "if" statement crammed together, and takes a conditional just like the 'if' statement in line 4.

10.  We will skip to line 10, as lines 7 through 9 are explained previously.  All there is to explain here is that when using 'if', you must also close the block with the 'end' keyword.  This is not true for an 'else' or 'elsif' as these are part of a larger 'if' block or statement.

13.  This is the last line in our code, and important if we want to know what the answer is!  "puts" stands for "put string", and all you have to do to use it is put a string after it.  Our sum happens to be of the 'Integer' class, but it has been written in such a way to know what to do when we use it in the 'puts' statement.  More on this later!



As for what we are trying to accomplish in this project Euler problem, we want to sum up all of the multiples of 3 and 5 below 1000.  Pretty straightforward.  There are a few places where one can trip up though:

1.  If you incorrectly sum up multiples of both 3 and 5.  For example, 15 is a multiple of both 3 and 5, and if you write your code incorrectly you will get a sum that is too large, because you have added twice every multiple of both 3 and 15.

2.  Another place is not noticing that the question states below 1000, not up to and including 1000.  So just be wary of that. 

If anyone has some other pitfalls to be wary of, please email me and I will be sure to include them here, for those that follow after!  Good luck all, and happy coding!