Added factorial iterative program

maths/factorial_iterative.rb

@@ -0,0 +1,28 @@
+=begin
+A ruby program calculate factorial of a given number.
+Mathematical Explanation: The factorial of a number is the product of all the integers from 1 to that number. 
+i.e: n! = n*(n-1)*(n-2)......*2*1
+=end
+
+def factorial(n)
+    if n < 0
+        return nil
+    end
+    
+    fac = 1
+    while n > 0
+        fac = fac * n
+        n -= 1
+    end
+    
+    return fac
+end
+
+puts '4! = ' + factorial(4).to_s
+# 4! = 24
+
+puts '0! = ' + factorial(0).to_s
+# 0! = 1
+
+puts '10! = ' + factorial(10).to_s
+# 10! = 3628800