When the order doesn't matter, it is a Combination. When the order does matter it is a Permutation.
There are 2 types of Permutation, repetition is allowed and no repetition allowed.
Permutations with Repetition = n ^ pow(r) = n * n * n * ... (r times)
Here n is the number of things to choose from, and we choose r of them, repetition is allowed, and order matters.
Permutations without Repetition = n! / (n − r)!
Here n is the number of things to choose from, and we choose r of them, no repetitions and order matters.
Similarly there are 2 types of Combination, repetition is allowed and no repetition allowed.
Combinations without Repetition = n! / ( r! * (n − r)! )
Combinations with Repetition = (r + n - 1)! / ( r! * (n − 1)! )
REMEMBER: (a + b) / 2 = a + (b - a) / 2 (Used as (a+b) could overflow Integer's range)
No comments:
Post a Comment