Add Factorial in Nim (#5187)

Author: allenres

archive/n/nim/factorial.nim

@@ -0,0 +1,42 @@
+#[ 
+    Factorial in Nim
+    This program calculates the factorial of a non-negative integer.
+]#
+
+import os
+import strutils
+
+const UsageMessage = "Usage: please input a non-negative integer"
+
+proc factorial(n: int): int =
+  # Calculates the factorial of n (n!).
+  if n == 0:
+    return 1
+  
+  var result = 1
+  for i in 1..n:
+    result *= i
+  
+  return result
+
+# The main execution block:
+block:
+  # Check 1: No input argument provided.
+  if paramCount() == 0:
+    quit(UsageMessage, 1)
+
+  let inputStr = paramStr(1)
+
+  # Check 2: Invalid Input (Empty String or Not a Number)
+  var n: int
+  try:
+    n = parseInt(inputStr)
+  except:
+    quit(UsageMessage, 1)
+
+  # Check 3: Invalid Input (Negative Number)
+  if n < 0:
+    quit(UsageMessage, 1)
+
+  # Print the calculated result.
+  echo factorial(n)