factorial feature implementation

DIRECTORY.md

@@ -53,6 +53,7 @@
   * [Ceil](https://github.com/TheAlgorithms/Ruby/blob/master/maths/ceil.rb)
   * [Ceil Test](https://github.com/TheAlgorithms/Ruby/blob/master/maths/ceil_test.rb)
   * [Decimal To Binary](https://github.com/TheAlgorithms/Ruby/blob/master/maths/decimal_to_binary.rb)
+  * [Factorial](https://github.com/TheAlgorithms/Ruby/blob/master/maths/factorial.rb)
   * [Fibonacci](https://github.com/TheAlgorithms/Ruby/blob/master/maths/fibonacci.rb)
   * [Find Max](https://github.com/TheAlgorithms/Ruby/blob/master/maths/find_max.rb)
   * [Find Min](https://github.com/TheAlgorithms/Ruby/blob/master/maths/find_min.rb)

maths/factorial.rb

@@ -0,0 +1,39 @@
+# A ruby program to find factorial of a given integer
+# Factorial of a given integer is defined as the product of all the positive integers less than or equal to the given integer
+# Mathematical representation: n! = n * (n - 1) * (n - 2) * ... * 1
+
+def factorial(number)
+  input = number
+  factorial = 1
+  if number < 0
+    "Please check your input number! The given number is a negative number."
+  else
+    while number > 0
+      factorial *= number
+      number -= 1
+    end
+    "The factorial of #{input} is #{factorial}."
+  end
+  rescue
+    "Error: Please provide integer only!"
+end
+
+# Valid inputs
+puts factorial(0)
+# The factorial of 0 is 1.
+
+puts factorial(10)
+# The factorial of 10 is 3628800.
+
+puts factorial(1)
+# The factorial of 1 is 1.
+
+puts factorial(-5)
+# Please check your input number! The given number is a negative number.
+
+# Invalid inputs
+puts factorial("a")
+# Error: Please provide integer only!
+
+puts factorial("2")
+# Error: Please provide integer only!