Learning Ruby with Ninety-Nine Problems (1-10)

Hello everyone

I’ve recently started learning Ruby. After reading Ben Orenstein’s post On the fundamentals of programming, I decided to start with Dr. Werner Hett’s list of Ninety-Nine Prolog Problems.

My hope is that people with a wide range of talent can get something out of this blog. You are welcome to learn along with me, post your own solutions, discuss benefits and drawbacks, or even educate us on Ruby details that we may have missed.

On to the problems

A few things to remember:

• I am not trying to write a guide; I am just learning myself so your participation is essential.
• Don’t click the links until you have solved the problem on your own and are ready for spoilers.
• The problems are marked with the difficulty level. Detailed information on the difficulty ratings and the problems themselves can be found here
• I’ve modified these from the originals to use more specific Ruby terms and examples.
• Solutions are marked with initials and a number for easy reference while commenting.
• I will take solutions from the comments and put them in the post so they are formatted and easier to read

P01 (*) Find the last element of an array

# example:
def my_last(l)
	# your implementation here
end
list = [1, 3, 5, 8, 11, 13]
my_last list

=> 13

P02 (*) Find the last but one element of an array

# example:
def my_last_but_one(l)
	# your implementation here
end
list = [1, 3, 5, 8, 11, 13]
my_last_but_one list

=> 11

P03 (*) Find the K’th element of an array

# example:
def element_at(l, k)
	# your implementation here
end
list = [1, 3, 5, 8, 11, 13]
element_at list, 3

=> 5

P04 (*) Find the number of elements of an array

# example:
def count_elements(l)
	# your implementation here
end
list = [1, 3, 5, 8, 11, 13]
count_elements list

=> 6

P05 (*) Reverse an array

# example:
def my_reverse(l)
	# your implementation here
end
list = [1, 3, 5, 8, 11, 13]
my_reverse list

=> [13, 11, 8, 5, 3, 1]

P06 (*) Find out whether an array is a palindrome.

A palindrome can be read forward or backward; e.g. %w(x a m a x).

# example:
def is_palindrome(l)
	# your implementation here
end
list = %w(fall leaves after leaves fall)
is_palindrome list

=> true

P07 (**) Flatten a nested array structure

Transform an array, possibly holding arrays as elements into a "flat" array by replacing each array with its elements (recursively).

# example:
def my_flatten(l)
	# your implementation here
end
list = [1, [2, [3, 4], 5]]
my_flatten list

=> [1, 2, 3, 4, 5]

P08 (**) Eliminate consecutive duplicates of array elements.

If an array contains repeated elements, they should be replaced with a single copy of the element. The order of the elements should not be changed

# example:
def compress(l)
	# your implementation here
end
list = [1,1,1,2,3,3,1,1,4,5,5,5,5]
compress list

=> [1, 2, 3, 1, 4, 5]

P09 (**) Pack consecutive duplicates of array elements in sub arrays

If an array contains repeated elements they should be placed in separate sub arrays

# example:
def pack(l)
	# your implementation here
end
list = [1,1,1,2,3,3,1,1,4,5,5,5,5]
pack list

=> [[1, 1, 1], [2], [3, 3], [1, 1], [4], [5, 5, 5, 5]]

P10 (*) Run-length encoding of an array

Use the result of problem P09 to implement the so-called run-length encoding data compression method. Consecutive duplicates of elements are encoded as terms [N,E] where N is the number of duplicates of the element E.

# example:
def encode(l)
	# your implementation here
end
list = [1,1,1,2,3,3,1,1,4,5,5,5,5]
encode list

=> [[3, 1], [1, 2], [2, 3], [2, 1], [1, 4], [4, 5]]