combinations

useful_scripts/combinations.py

@@ -0,0 +1,71 @@
+#!/usr/bin/env python
+
+# Sebastian Raschka 2014
+# Functions to calculate factorial, combinations, and permutations
+# bundled in an simple command line interface.
+
+def factorial(n):
+    if n == 0:
+        return 1
+    else:
+        return n * factorial(n-1)
+    
+def combinations(n, r):
+    numerator = factorial(n)
+    denominator = factorial(r) * factorial(n-r)
+    return int(numerator/denominator)
+    
+def permutations(n, r):
+    numerator = factorial(n)
+    denominator = factorial(n-r)
+    return int(numerator/denominator)
+
+assert(factorial(3) == 6)
+assert(combinations(20, 8) == 125970)
+assert(permutations(30, 3) == 24360)
+
+
+
+
+if __name__ == '__main__':
+    
+    import argparse
+    parser = argparse.ArgumentParser(
+        description='Script to calculate the number of combinations or permutations ("n choose r")',
+        formatter_class=argparse.RawTextHelpFormatter,
+        
+        prog='Combinations',
+        epilog='Example: ./combinations.py -c 20 3'
+        )
+
+    parser.add_argument('-c', '--combinations', type=int, metavar='NUMBER', nargs=2,
+            help='Combinations: Number of ways to combine n items with sequence length r where the item order does not matter.')
+    
+    parser.add_argument('-p', '--permutations', type=int, metavar='NUMBER', nargs=2, 
+            help='Permutations: Number of ways to combine n items with sequence length r where the item order does not matter.')
+            
+    parser.add_argument('-f', '--factorial', type=int, metavar='NUMBER', help='n! e.g., 5! = 5*4*3*2*1 = 120.')
+    
+    parser.add_argument('--version', action='version', version='%(prog)s 1.0')
+
+    args = parser.parse_args()
+    
+    if not any((args.combinations, args.permutations, args.factorial)):
+        parser.print_help()
+        quit()
+    
+    if args.factorial:
+        print(factorial(args.factorial))
+            
+    if args.combinations:
+        print(combinations(args.combinations[0], args.combinations[1]))
+
+    if args.permutations:
+        print(permutations(args.permutations[0], args.permutations[1]))
+            
+    if args.factorial:
+        print(factorial(args.factorial))
+            
+    
+
+