armstrong number feature implementation

DIRECTORY.md

@@ -72,6 +72,7 @@
   * [Add Digits](https://github.com/TheAlgorithms/Ruby/blob/master/maths/add_digits.rb)
   * [Aliquot Sum](https://github.com/TheAlgorithms/Ruby/blob/master/maths/aliquot_sum.rb)
   * [Aliquot Sum Test](https://github.com/TheAlgorithms/Ruby/blob/master/maths/aliquot_sum_test.rb)
+  * [Armstrong Number](https://github.com/TheAlgorithms/Ruby/blob/master/maths/armstrong_number.rb)
   * [Average Mean](https://github.com/TheAlgorithms/Ruby/blob/master/maths/average_mean.rb)
   * [Average Median](https://github.com/TheAlgorithms/Ruby/blob/master/maths/average_median.rb)
   * [Binary To Decimal](https://github.com/TheAlgorithms/Ruby/blob/master/maths/binary_to_decimal.rb)

maths/armstrong_number.rb

@@ -0,0 +1,57 @@
+# A ruby program to find a given number is armstrong number or not
+# Wiki url: https://en.wikipedia.org/wiki/Narcissistic_number
+# other resources: https://pages.mtu.edu/~shene/COURSES/cs201/NOTES/chap04/arms.html
+
+#
+# Examples: -> 153 = (1 * 1 * 1) + (5 * 5 * 5) + (3 * 3 * 3) [Armstrong number] and -> 125 != (1 * 1 * 1) + (2 * 2 * 2) + (5 * 5 * 5)
+# -> 1634 = (1 * 1 * 1 * 1) + (6 * 6 * 6 * 6) + (3 * 3 * 3 * 3) + (4 * 4 * 4 * 4)
+#
+
+def armstrong_number(number)
+  num = number
+  sum = 0
+  len = number.digits.count
+  while(number != 0)
+    remainder = number % 10
+    sum += remainder ** len
+    number /= 10
+  end
+
+  if num == sum
+    "The number #{num} is an Armstrong number."
+  else
+    "The number #{num} is not an Armstrong number."
+  end
+  rescue
+    "Error: Please provide number only!"
+end
+
+#
+# Valid inputs
+#
+puts armstrong_number(153)
+# The number 153 is an Armstrong number.
+
+puts armstrong_number(0)
+# The number 0 is an Armstrong number.
+
+puts armstrong_number(370)
+# The number 370 is an Armstrong number.
+
+puts armstrong_number(10)
+# The number 10 is not an Armstrong number.
+
+puts armstrong_number(1634)
+# The number 1634 is an Armstrong number.
+
+puts armstrong_number(123)
+# The number 123 is not an Armstrong number.
+
+#
+# Invalid inputs
+#
+puts armstrong_number("153")
+# Error: Please provide number only!
+
+puts armstrong_number("a")
+# Error: Please provide number only!