Android studio shuffle array

Shuffle an Array or a List — Algorithm in Java — Tutorial

This article describes how to shuffle the content of an array or a list in Java. After the shuffle the elements in the array or the list are randomly sorted.

1. Shuffle an array with the Collections framework

An array or an java.util.List data structure contains a sorted list of values. Shuffling an array or a list means that you are randomly re-arranging the content of that structure. Have you wondered how you could shuffle an array or a list without the collection framework? This article demonstrates how the shuffling works so that you can learn how the standard libraries might do this.

If you are only interested in using shuffling for the elements in a data structure, you can use Collections.shuffle(list) to shuffle a list with the standard Java library or Collections.shuffle(Arrays.asList(a)) to shuffle the entries in an array .

The approach works independent of the content of the array or the list.

The shuffle is random as the algorithm by selecting uniformly an element which has not been selected. For example if the element at position 2 is selected it can be exchanged with all elements at position 2 until position n-1 (as the list /array has 0 — n-1 positions).

2. Implementation in Java

Create a Java project «de.vogella.algorithms.shuffle». Create the following program for sorting arrays.

Create the following program for sorting list.

Why does this shuffle all values randomly? The value at position 0 is exchanged with a randomly selected value (including the original value at position 0). This means that after the first loop, position 0 has a random value with an equal likelihood of any value. We continue this with position 1 but we do not need to consider position 0 again as this position already has a random value assigned. After the loop it is equally likely that each value is at any position.

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Shuffle() in Java

By Priya Pedamkar

Introduction to Shuffle() in Java

Java has many built-in functions to perform different operations on collections or other data types, and one of them is shuffle. Shuffle function is available in many other languages like Python.

• The shuffle function is used to shuffle the collection elements.
• It randomly permutes the list elements passed in parameters.
• There are two methods to shuffle in Java one is using the collections shuffle method, and another is by using random class.
• The collection shuffle function can also be called in two ways, one with a random parameter to specify randomness and another without parameter.
• shuffle(
• )
• shuffle(
• , )
• You need to pass the list so that we convert the array to the list, then pass it to a collection.shuffle and again convert the result to an array.

Syntax for Shuffle() in Java

Declaration for shuffle method:

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public static void shuffle(List list)

public static void shuffle(List list, Random random)

Parameters:

• List: The list which you will pass will be shuffled.
• Random: It’s the random function passed with a seed value that will become the source of randomness.

Returns:

• The shuffle function doesn’t return any value; it just shuffles the list.

Examples of Shuffle() in Java

In the example below, we created a list from an array with some alphabets and used the shuffle method to shuffle the array. Every time you run, you would get a different shuffled list.

Example #1

Code:

import java.util.*;
public class CollectionsShuffleExampleWithoutRandom <
public static void main(String[] args) <
List list = Arrays.asList(«R», «A», «H», «U», «L»);
System.out.println(«Before Shuffle : «+list);
Collections.shuffle(list);
System.out.println(«After shuffle : «+list);
>
>

Output:

Example #2

In the example below, we create a linked list of integers and add some integers to it. But here we passed another argument that is the Random function which will become the source of Randomness. Then we passed the Random function with seed value 5. This is another flavor, or we can say the method of using shuffle function with Randomness.

Code:

import java.util.*;
public class CollectionsShuffleWithRandom <
public static void main(String[] args) <
//Create linked list object
LinkedList list = new LinkedList ();
//Add values
list.add(90);
list.add(100);
list.add(1);
list.add(10);
list.add(20);
System.out.println(«Before Shuffle = «+list);
//Random() to shuffle the given list.
Collections.shuffle(list, new Random());
System.out.println(«Shuffled with Random() = «+list);
//Random(5) to shuffle the given list.
Collections.shuffle(list, new Random(5));
System.out.println(«Shuffled with Random(5) = «+list);
>
>

Output:

Shuffling without Shuffle Method

If you want more control over shuffle, then you could write your own method to shuffle the list with the random method and another approach to shuffle the list. This method is more flexible and easy to fit in any application. You can actually understand how shuffle works inside Java’s built-in method.

Input: An int array

Output: Shuffled array(in a randomized order)

Example:

The above function where you just need to pass an array integer, and it will return a shuffled array. Inside the function, you can see we are iterating the array till its length and generating a random number, and it will be treated as an array index, which will be swapped with another array. This is how the elements will be swapped inside an array. The resulting array will be a swapped one.

From the above function, we can get a basic concept of the shuffle function where a list of values will be sent, and a random number will be generated each time while iterating the elements in the array. The element will be swapped with another element in the same list with the index randomly generated from a random function.

Important Points for Shuffle Function

• This method works on randomly permuting the list elements and shuffling them.
• Each time executed, the result can be different.
• The function doesn’t take much time and runs in linear time.
• If you provide a list that doesn’t implement the RandomAccess interface, then shuffle will first copy the list into an array, shuffle the array copy, and then copies it into a list of the result and return it.
• Shuffle traverses the list backwards – the last element up to the second element repeatedly.
• While traversing, it randomly selects elements and swaps them with the current position.
• The randomly selected element is from the portion of the list from the first element to the current element.

Exceptions:

• UnsupportedOperationException: if the passed list or list-iterator does not support a set operation.

Applications of Shuffle

There could be many situations where the shuffle function below is some applications:

• Shuffling a list of questions in a QA application.
• Shuffling list of quotes that you want to show to your app users every day.
• Lottery application where you want to assign a ticket number to the user.
• Generating unique transaction numbers for a payment field.
• Generation unique user id for different users can be prefixed to user id.
• It can also be used in cryptographic applications.

Conclusion

In the above article, we understood how shuffle works and how you can use it. There can be multiple use cases somewhere you would be using shuffle function with random parameter else without random parameter, and some applications might need a different flexible implementation where you can write your own shuffle function using Java’s Random function.

Recommended Articles

This is a guide to Shuffle() in Java. Here we discuss the Introduction and Important Points for Shuffle Function along with different examples and its code implementation. You may also have a look at the following articles to learn more –

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Android studio shuffle array

Write the function shuffle(array) that shuffles (randomly reorders) elements of the array.

Multiple runs of shuffle may lead to different orders of elements. For instance:

All element orders should have an equal probability. For instance, [1,2,3] can be reordered as [1,2,3] or [1,3,2] or [3,1,2] etc, with equal probability of each case.

The simple solution could be:

That somewhat works, because Math.random() — 0.5 is a random number that may be positive or negative, so the sorting function reorders elements randomly.

But because the sorting function is not meant to be used this way, not all permutations have the same probability.

For instance, consider the code below. It runs shuffle 1000000 times and counts appearances of all possible results:

An example result (depends on JS engine):

We can see the bias clearly: 123 and 213 appear much more often than others.

The result of the code may vary between JavaScript engines, but we can already see that the approach is unreliable.

Why it doesn’t work? Generally speaking, sort is a “black box”: we throw an array and a comparison function into it and expect the array to be sorted. But due to the utter randomness of the comparison the black box goes mad, and how exactly it goes mad depends on the concrete implementation that differs between engines.

There are other good ways to do the task. For instance, there’s a great algorithm called Fisher-Yates shuffle. The idea is to walk the array in the reverse order and swap each element with a random one before it:

Let’s test it the same way:

The example output:

Looks good now: all permutations appear with the same probability.

Also, performance-wise the Fisher-Yates algorithm is much better, there’s no “sorting” overhead.

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Learn Java for Android Development: Working with Arrays

This quick lesson shows you how to work with arrays in Java. This lesson is part of an ongoing series of tutorials for developers learning Java in order to develop Android applications.

What is an Array?

An array is a common data structure used to store an ordered list of items. The array elements are typed. For example, you could create an array of characters to represent the vowels in the alphabet:

Much like C or C++, Java arrays are indexed numerically on a 0-based system. This means the first element in the array (that is, ‘a’) is at index 0, the second (‘e’) is at index 1, and so on.

Java makes working with arrays easier than many other programming languages. The array itself is an object (of type array), with all the benefits thereof. For example, you can always check the size of an array using its length property:

What Can I Store In An Array?

You can store any object or primitive type in an array. For example, you can store integers in an array:

Or, you could store non-primitive types like Strings (or any other class) in an array:

Sometimes, you may want to store objects of different types in an array. You can always take advantage of inheritance and use a parent class for the array type. For example, the Object class is the mother of all classes… so you could store different types in a single array like this:

The elements of a Java object array are references (or handles) to objects, not actual instances of objects. An element value is null until it is assigned a valid instance of an object (that is, the array is initialized automatically but you are responsible for assigning its values).

Declaring Arrays

There are a number of ways to declare an array in Java. As you’ve seen, you can declare an array and immediately provide its elements using the C-style squiggly bracket syntax. For example, the following Java code declares an array of integers of length 3 and initializes the array all in one line:

You can also declare an array of a specific size and then assign the value of each element individually, like this:

This is equivalent to creating an array like this:

There are several other ways to create arrays. For example, you can create the array variable and assign it separately using the new keyword. You can also put the array brackets before the variable name, if you desire (this is a style issue). For example, the following Java code defines an array of String elements and then assigns them individually:

Modifying Array Content

As you have seen, you can assign array values by using the bracket syntax:

You can retrieve array values by index as well. For example, you could access the second element in the array called aStopLightColors (defined in the previous section) as follows:

Iterating Arrays

Finally, arrays are often used as an ordered list of objects. Therefore, you may find that you want to iterate through the array in order, accessing each element methodically.

There are a number of ways to do this in Java. Because you can always check the size of an array programmatically, you can use any of the typical for or while loop methods you may find familiar. For example, the following Java code declares a simple integer array of three numbers and uses a simple for-loop to iterate through the items:

Java also provides a very handy for-each loop to iterate through arrays in a friendly fashion. The for-each loop helps avoid silly programming mistakes so common in loops (off-by-one errors, etc.). To use the for-each loop syntax, you need to define your loop variable, then put a colon, and then specify the name of your array. For example, the following code provides the similar loop structure as the previous for-loop shown above:

As you can see, the for-each loop is slick. However, you no longer know the index while iterating. Thus, it can’t be used in all situations.

Conclusion

In this quick lesson you have learned about arrays in Java. Arrays are a fundamental data structure used for Java programming that store an ordered number of objects of a given type in an organized fashion. In Java, arrays are objects themselves and store references to objects, making assignment and use easier (but subtly different) than how arrays are employed in other programming languages.

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