Normally I would display more than two approaches, but shuffling is deceptively easy to do almost properly, and the Fisher-Yates algorithm is both the canonical solution and asymptotically optimal. A few notes on randomness are necessary before beginning - both approaches displayed below assume that the languages' pseudorandom number generators PRNGs are sufficiently random.

The sample code uses the simplest techniques available for getting pseudorandom numbers, but for each possible permutation of the array to be truly equally likely, more care must be taken. For example, an array of length n n n has n!

## Shuffle a given array using Fisher–Yates shuffle Algorithm

If we put each number in a "hat" and draw them out at random, the order in which we draw them will define a random ordering. The brute force algorithm essentially puts each number in the aforementioned "hat", and draws them at random without replacement until there are none left.

Mechanically, this is performed by copying the contents of array into a second auxiliary array named aux before overwriting each element of array with a randomly selected one from aux.

After selecting each random element, it is removed from aux to prevent duplicate draws. The implementation of reset is simple, as we just store the original state of nums on construction. The correctness of the algorithm follows from the fact that an element without loss of generality is equally likely to be selected during all iterations of the for loop. Given that the array to be shuffled has n n n elements, this probability is more concretely stated as the following:.

When expanded and rearrangedit looks like this for sufficiently large k k k :. The quadratic time complexity arises from the calls to list. Space complexity : O n O n O n.

Because the problem also asks us to implement resetwe must use linear additional space to store the original array. Otherwise, it would be lost upon the first call to shuffle.

We can cut down the time and space complexities of shuffle with a bit of cleverness - namely, by swapping elements around within the array itself, we can avoid the linear space cost of the auxiliary array and the linear time cost of list modification.

The Fisher-Yates algorithm is remarkably similar to the brute force solution. On each iteration of the algorithm, we generate a random integer between the current index and the last index of the array. Then, we swap the elements at the current index and the chosen index - this simulates drawing and removing the element from the hat, as the next range from which we select a random index will not include the most recently processed one.

One small, yet important detail is that it is possible to swap an element with itself - otherwise, some array permutations would be more likely than others. To see this illustrated more clearly, consider the animation below:. Time complexity : O n O n O n. The Fisher-Yates algorithm runs in linear time, as generating a random index and swapping two values can be done in constant time. Although we managed to avoid using linear space on the auxiliary array from the brute force approach, we still need it for resetso we're stuck with linear space complexity.

Don't abuse fancy words. Both solutions are Fisher-Yates Algorithm. One original is used by pen and paper, and one is improved for computer use. In reset method ; why we can't just return original? In the second solution for reset, shouldn't we return array instead of original?

We did that in the first solution, but why differently in second solution? In the Python code For anyone wondering, the utility derived in creating a new List object with each assignment is to create a 'Deep Copy' of the array.Given an array, write a program to generate a random permutation of array elements. Here shuffle means that every permutation of array element should equally likely.

Let the given array be arr[]. A simple solution is to create an auxiliary array temp[] which is initially a copy of arr[]. Randomly select an element from temp[]copy the randomly selected element to arr[0] and remove the selected element from temp[]. Repeat the same process n times and keep copying elements to arr[1], arr[2], …. Fisher—Yates shuffle Algorithm works in O n time complexity.

The assumption here is, we are given a function rand that generates random number in O 1 time. The idea is to start from the last element, swap it with a randomly selected element from the whole array including last. Now consider the array from 0 to n-2 size reduced by 1and repeat the process till we hit the first element. How does this work?

### Shuffle an Array using STL in C++

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. Writing code in comment? Please use ide. We don't need to run for. We don't. Python Program to shuffle a given array. A function to generate a random permutation of arr[]. Start from the last element and swap one by one. Pick a random index from 0 to i.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. This is my code. If yes, I would like to learn how to do it. You need to pass pointers to a[0] and a[10]not the elements themselves:. Just changing the arr to a pointer does not solve the solution. This will make the array swap to one type of permutation. This means that if you rerun the program, your array will be shuffled into the exact same way as it did in the previous run.

To fix this - the function offers a third parameter which acts as a seed. So the correct implementation of the function is as follows. Learn more. Ask Question.

Asked 7 years, 2 months ago. Active 2 years, 8 months ago. Viewed 53k times. What does my error mean? Computernerd Computernerd 5, 15 15 gold badges 55 55 silver badges 82 82 bronze badges.

Active Oldest Votes. Mankarse Computernerd, Pointers are random-access iterators, and it takes two iterators. ALL another qn: This method shuffles all the number when it is called once, is there any method which swaps 2 number only once when called??Posts Latest Activity. Page of 1. Filtered by:. Previous template Next. My code: Code:. Still haven't got the 'official' shuffle working but this: Code:. Comment Post Cancel. UE4 doesn't play nice with Standard Library calls. You should try to use UE4 calls and avoid anything that is standard lib as much as possible.

Now Available! The problem that needed to be handled was not the shuffle itself - Blueprints has a shuffle function that works fine if you don't need to replicate results. It's that we needed a shuffle that accepted a random seed for replicatable results. This is my first time hearing of a shuffle blueprint node, but from my understanding, it reorders the array in a random fashion. If so, I'd make a function that takes a TArray as a parameter, and second TArray a temp one inside that function.

Then, I would make a for-loop going from 0 to the length of the original array. If so, set the temporary TArray at index i to be a RandRange function using 'i' as the start and original TArray length as the end.

Otherwise, continue.

However, because you don't want the random function to potentially choose the same random number, you should use the TArray::RemoveAt function to remove the index the chosen index of the original TArray. Because, if all goes well, the size of the original array is 0 now. I would then use a second for-loop from 0 to the size of the TempArray which adds each entry from the temp array back into the original array. You should return that value as it would be the randomized original array now.

That being said, all of this is completely untested. Any version of shuffle should act as though you were shuffling a deck of cards. The version of shuffle I'm used to: Code:.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information.

This is my code. If yes, I would like to learn how to do it. You need to pass pointers to a[0] and a[10]not the elements themselves:.

Just changing the arr to a pointer does not solve the solution. This will make the array swap to one type of permutation. This means that if you rerun the program, your array will be shuffled into the exact same way as it did in the previous run. To fix this - the function offers a third parameter which acts as a seed.

So the correct implementation of the function is as follows. How are we doing? Please help us improve Stack Overflow. Take our short survey. Learn more. Ask Question. Asked 7 years, 2 months ago. Active 2 years, 8 months ago. Viewed 53k times. What does my error mean? Computernerd Computernerd 5, 15 15 gold badges 55 55 silver badges 82 82 bronze badges. Active Oldest Votes.

Mankarse Computernerd, Pointers are random-access iterators, and it takes two iterators. ALL another qn: This method shuffles all the number when it is called once, is there any method which swaps 2 number only once when called??

Computernerd, std::swap? It often swaps only two items, but may swap more, or none at all in which case it returns false. Mankarse Mankarse Theo Theo 46 2 2 bronze badges. Renato Aloi Renato Aloi 1 1 silver badge 5 5 bronze badges. For some reason this always shuffles the array in the same order. Or is this what shuffle is about? This will act as your seed. Grant Wilson Grant Wilson 1. Sign up or log in Sign up using Google.

Sign up using Facebook.STL contains two methods which can be used to get a shuffled array. This method rearranges the elements in the range [first, last randomly, using g as a uniform random number generator. It swaps the value of each element with that of some other randomly picked element. It determines the element picked by calling g. Note: Output may differ each time because of the random function used in the program.

This function randomly rearranges elements in the range [first, last. It swaps the value of each element with some other randomly picked element. When provided, the function gen determines which element is picked in every case. Otherwise, the function uses some unspecified source of randomness.

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Load Comments.Say, for example, this is my input file:. My problem is I want to randomly shuffle only the rows so I get row 1 [9 5 6 2] somewhere in maybe row 3 and row 3 is found somewhere in row 2 and so forth, where my columns are kept intact.

Most searches online give me how to shuffle the elements in a row - random shuffling of rows AND columns. However, below is what I've done so far to get the values that were repeating themselves but it was obvious from my output that shuffling of columns were done as well-which is what I do NOT want.

What I want to know is if I'm on the right track, then what am I doing wrong? Or where am I going wrong? Here's what my shuffling code looks like given my variables and array were defined and initialized and I've read from input file:.

First get your swap correct. You can't sort, shuffle or do anything with rows till your swap is correct. If rows are considered elements, then you can't split them up. That means that in order to swap, you have to either loop through the row one element at a time and swap the whole row or you can take advantage of the fact that rows are contiguous in memory and treat it like any other swap and swap the whole row at once using memcpy or memmove.

You are always swapping FOUR elements at a time and they'd better be from the same row. Line 13, unless it's purely for debugging purposes, should not be inside of this swapping routine. If the new random number already exists in the array then go back and generate another random number. Create a new temp array to hold the shuffled rows, then after the shuffling is complete copy the data in the temp array back into the original array.

When I view the outfile, it contained 63 rows, but with some rows duplicating themselves I guess it's because the rand function randomly selects and might pick the same number the next time around.

I don't want my program to do that, I just want it to reshuffle the rows all over so that I have exactly 63 unique rows being shuffled into their new positions. For example, I can have row 5 at row position 57, row 57 at position 42, row 42 at position 1, row 1 at position 32, and so forth. Right now, my program is swapping row 1 to row position 44, row 44 to position 1, row 2 to position 3, row 3 to position 2, Row 4 is at position 5 and row 5 at position 4 and so on.

Is there another way of shuffling, either than swapping rows as I've done above, and without duplicating a row quite a number of times? The red part needs to be clarified. Use it for debugging purposes, but don't expect it to not duplicate anything.

Your code actually looks just fine to me. The random shuffling part, that is. I'm not guaranteeing there are no problems, but it looks fine. I'll be more surprised if you have duplicate rows in memory once you're all done.

Again, I'm not guaranteeing. Shuffle, then output. That won't accidentally duplicate anything. See my last post.

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