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How to do permutations12/14/2023 The output of the above program, with repeated elements, is, as below. Next_permutation() also works for arrays and containers with repeated elements. While(next_permutation(vi.begin(),vi.end())) I didn’t bother to write examples on how to use prev_permutation() because its usage is similar to that of next_permutation(). It is advisable to sort your array or container, in descending order, before calling your first prev_permutation() in that way, you know you get all your permutations when prev_permutation() returns false, without calculating the total number of permutations, which translates into total number minus 1 of prev_permutation() iterative calls you must call.īelow are examples of using next_permutation() on array of integers. It must be noted that, even when the return value is false, the previous permutation is still generated. Prev_permutation() returns false when it encounters a sequence in ascending order. It is advisable to sort your array or container, in ascending order, before calling your first next_permutation() in that way, you know you get all your permutations when next_permutation() returns false, without calculating the total number of permutations, which translates into total number minus 1 of next_permutation() iterative calls you must call. It must be noted that, even when the return value is false, the next permutation is still generated. Next_permutation() returns false when it encounters a sequence in descending order. next_permutation() finds the next permutation whereas prev_permutation(), as its name implies, finds the previous permutation. First and Last are the first iterator and the one past the last iterator, respectively. The parameters are even simpler than the recursive one that I coded. Std::vector::const_iterator it=now.begin() įor(int cnt1=0 cnt1::const_iterator it1=now.begin() įor(vector::iterator it= perm.begin() it!=perm.end() ++it)įortunately for C++ programmers, the C++ Standard Template Library provides a next_permutation() template function and a corresponding prev_permutation() function defined in the header.īool next_permutation(BidIt First,BidIt Last) īool prev_permutation(BidIt First,BidIt Last) You need not know how vector_permutation() internally works you just know that, whenever a new permutation is generated, it calls func and passes func the ‘next’ vector you just need to define func() to process the permutation.īelow is an example of using vector_permutation() The solution is a function pointer that takes in a parameter of the type std::vector. If the permutation function finds permutations recursively, a way must exist that the user can process each permutation. func is a callback function that you define. Vector, next, contains the next permutation. Void vector_permutation(std::vector& now, Void string_permutation( std::string& orig, std::string& perm ) Orig is the original permutation and perm is the permutated string.īelow is an example on how to use this string_permutation() function. Examples of using it can be found in string_perm_example.cpp. I had written a recursive function, string_permutation(). By now, you should be able to figure the inherent pattern. When the second column is 1, 2, 3, or 4, the third column is also in ascending order.Īlso, notice the leftmost column is always in ascending order. Do you notice that the numbers in the second column are in ascending order?Įven for permutations where the leftmost column is 2, 3, or 4, the second column is always in ascending order. Look again at the permutations of which the first (leftmost) column is 1. ExplanationĪctually, finding permutations of a small group of numbers by yourself is not difficult, even without the help of computers. The formula for finding the total number of permutations is factorial of number of elements. I will also explain how to use the STL template function next_permutation(). This article explains the technique of finding permutations and provides source code for the recursive implementation.
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