This is the last problem I could not solve:
You are given an array {1,2,3,4,5,6,7,8,9}
Create a function that creates a random permutation without using a temporary list, in Θ(n) time.
Couldn't I use Collections.shuffle function to do this once I get the int[] array? I'm not sure what exactly the problem is asking for. I could do a simple loop where I could randomize it using an iterative method and do a simple check but shuffle would be easier, no?
In pseudo code:
index1 = 0
index2 = 0
loop: when index1 < array.length
index2=random[index1,array.length]
print[array[index2]
swap[index1,index2]
index1++
In Java:
int x = 0;
int y = 0;
Random r = new Random();
while(x < array.length){
y = x + r.nextInt(array.length-x);
System.out.println(array[y]);
int temp = array[x];
array[x] = array[y];
array[y] = temp;
x++;
}
Related
So I am trying to make an Array with random numbers, but whenever I try Math.random or create a new Random object and use it, I always get the same number multiple times. My code is this one:
int[] Array = new int[size];
for (int Y : Array) {
Array[Y] = (int) (Math.random() * 10) + 3;
}
or this one:
int[] Array = new int[size];
for (int Y: Array) {
Array[Y] = rand.nextInt(30);
}
The output im getting is: [0][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3][3]
I haven't set a seed and I tried it outside the loop and inside but still only get the same number.
You are not referring to the index of the array, but to the specific element which remains the same. You have to use indexed loop.
Try with that (it is a good practice to use camelCase for variables, so 'Array' starting with small 'a')
int[] array = new int[size];
for(int i = 0; i < array.length; i++) {
array[i] = rand.nextInt(30);
}
I want to fill an array of size X with random integers from 0 to X with no duplicates. The catch is I must only use arrays to store the collections of int, no ArrayLists. How do I go about implementing this?
I don't understand why I can't seem to get this. But this is my most recent bit of code that fills the list but allows for duplicates.
System.out.print("Zero up to but excluding ");
int limit = scanner.nextInt();
// create index the size of the limit
int [] index = new int[limit];
for(int fill=0;fill<limit;fill+=1){
index[fill] = (limit);
}
int randomNumber = 0;
Random rand = new Random();
int [] randoms = new int[limit];
boolean flag = true;
// CODE TO NOT PRINT DOUBLES
for (int z=0;z<limit;z+=1){
randomNumber = rand.nextInt(limit);
int i=0;
while (i<limit){
if (index[i] == randomNumber){
flag = true;
}
else {
flag = false;
break;
}
i+=1;
}
if (flag == false){
randoms[z] = randomNumber;
index[z] = randomNumber;
}
}
System.out.println("Randoms: "+java.util.Arrays.toString(randoms));
Here's one way to do it:
Create an array of length N
Fill it from 0 to N-1
Run a for loop and swap randomly 2 indices
Code:
// Step 1
int N = 10;
int[] array = new int[N];
// Step 2
for(int i=0; i < N; i++)
array[i] = i;
// Step 3
for(int i=0; i < N; i++) {
int randIndex = (int) (Math.random() * N);
int tmp = array[i];
array[i] = array[randIndex];
array[randIndex] = tmp;
}
Why not rephrase the problem to shuffling an array of integers. First fill the array monotonically with the numbers 0 to X. Then use the Random() function to select one of the X numbers to exchange with the number in position 0. Repeat as many times as you may like. Done.
Here is your bug:
while (i<limit){
if (index[i] == randomNumber){
flag = true;
}
else {flag = false;break;} <--- rest of the array is skipped
i+=1;
}
after you generated a new number, you start to check for equality , however once you find that randomNumber!=index[i] (else statement) you break out of the while. look this: actual array is 3,4,5,1 your new number is 5, you compare it to 3 just to find out that they different so flag is set to false and break out happens.
Consider using another array filled with elements in order from 0 to X. Then, with this array, shuffle the elements around. How do you go about this? Use a loop to traverse through every single element of the array, and for each iteration, choose a random number from 0 to array.length - 1 and switch the elements at the index you're currently on and the random index. This is how it would look like,
In your main, you would have an array initialized by doing this,
int[] arr = new int[10];//10 can be interchangeable with any other number
for(int i = 0; i < arr.length; i++){
arr[i] = i;
}
shuffleArray(arr);
And the shuffle method would look like this,
public int[] shuffleArray(int[] arr){
Random rand = new Random();
for(int i = 0; i < arr.length; i++){
int r = rand.nextInt(arr.length);//generate a random number from 0 to X
int k = arr[i];
arr[i] = arr[r];
arr[r] = k;
}
}
I am working on a sudoku game and I am using the method of creating a solved puzzle and then "digging holes" to get the puzzle that is displayed to the user. For some reason, the testingPuzzle array is being reset to the numbers array every time it goes through the loop. It is true that the testingPuzzle array should be set to the numbers array originally, but it should be edited one spot at a time every time it goes through the loop and have a bunch of zeroes in it after a few iterations. Here is the loop itself:
do{
x = Math.abs(rand.nextInt() % 9);
y = Math.abs(rand.nextInt() % 9);
takeaway_num = testingPuzzle[x][y];
testingPuzzle[x][y] = 0;
} while (arraysEqual(solvePuzzle(testingPuzzle), numbers));
SolvePuzzle is a method that solves the given puzzle as parameters and returns that solved puzzle. So basically arraysEqual(solvePuzzle(testingPuzzle), numbers)checks to see if testingPuzzle is solvable
I am setting the testingPuzzle array equal to the numbers array before the do while loop. It looks like this:
int[][] testingPuzzle = new int[9][9];
for(int y = 0; y < 9; y++){
for(int x = 0; x < 9; x++){
testingPuzzle[x][y] = numbers[x][y];
}
}
Just so you know numbers is the sodoku answer that was generated in a previous method.
I am using my own method for testing if the arrays are equal, called "arraysEqual because I thought .equals() was the problem originally. Here is the method I am using for this:
private static boolean arraysEqual(int[][] a, int[][] b){
for(int y = 0; y < 9; y++){
for(int x = 0; x < 9; x++){
if(a[x][y] != b[x][y]){
return false;
}
}
}
return true;
}
I'm not really sure why the testingPuzzle Array is being set to the same as the numbers array at the end of each loop. I think it could have something to do with passing the actual array vs. a copy of it to a method that takes in an array but I'm not sure how that works in java.
I'm having difficulty understand how to write this array. I need it to out-print 10x5 (50 elements total), and have the first 25 elements equal to the sqrt of the index that it is in, and the last 25 to equal 3 * the index. Yes, this is homework but I'm not asking for you to do it for me, I just need help! I'm getting errors when using Math saying that I cant use double and the double array together. Here is what I have so far:
public class snhu4 {
public static void main(String args[]) {
double alpha[][] = new double[10][5];
double[] sum, sum2;
for (int count=0; count<=25;count++) {
alpha[count]= Math.sqrt(count);
}
for (int count=26; count<=50;count++) {
alpha[count]= count *3;
}
for (int count=0; count<=50;count++) {
System.out.print(alpha[count]);
}
}
}
Because alpha is a multidimensional array, you can't refer to its elements like a normal array.
int myarray[][] = new int[2][2];
In the above example, the array myarray is multidimensional. If I wanted to access the second element in the first array, I would access it like this:
int myint = myarray[0][1];
You are trying to access a multidimensional array by using the access for a normal array. Change
alpha[count]
to
alpha[0][count]
or similar.
Read here for more information on multidimensional arrays.
you defined alpha as a 2D array with lets say 10 items in the first dimension and 5 in the second, and 5x10 is 50 elements.
When using your array to assign values to these elements, u must call upon the array using 2 indices, one for each dimension:
alpha[i][j] = /*double value*/; //with 0<=i<=9 and 0<=j<=4
So the first 25 elements going from left to right in dimension order is going to be:
[0to9][0] and [0to9][1] and [0to4][2]
the next 25 will be
[4to9][2] and [0to9][3] and [0to9][4]
from then on i cannot give you the answers to your homework, but the loops should look like this:
int j;
for(int i = 0; i<25; i++)
{
j=i/10; //integer division will return 0 for i<10, 1 for 10<i<20, etc..
alpha[i%10][j] = Math.sqrt(i);
}
and you can figure out the rest
The 10x5 appears to be an output constraint, not a design constraint.
You are using Java, so use Java constructs, not C-language constructs;
specifically store the values in a List not an array.
Here are some hints:
List<Integer> valuesList = new ArrayList<Integer>();
for (int index = 0; index < 25; ++index)
Integer currentValue = Math.sqrt(index);
valuesList.add(currentValue);
for (int index = 25; index < 50; ++index)
Integer currentValue = index * 3;
valuesList.add(currentValue)
int count = 1;
for (Integer current : valuesList)
if ((count % 5) == 0) // write a newline.
System.out.print(current);
++count
I'm pretty much a noob to programming but i have researched all over the place and cant find an answer. im using eclipse and every time i run my program it says:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1
at computer.guess(game1player2.java:24)
at game1player2.main(game1player2.java:39)
Here's my code:
import java.util.Scanner;
class computer{
int g = 0;
int[] compguess = new int[g];
void guess(){
int rand;
while(0 < 1){
int i;
rand = (int) Math.ceil(Math.random()*10);
for (i = 1; i < compguess.length; i++){
if(rand == compguess[i]){
break;
}
}
if(i > compguess.length){
g++;
rand = compguess[g];
System.out.println(compguess[compguess.length]);
}
}
}
}
public class game1player2 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
computer computer1 = new computer();
for(int a = 0; a < 2; a++){
computer1.guess();
for(int n = 0; n <= computer1.compguess.length; n++)
System.out.println(computer1.compguess[n]);
}
{
input.close();
}
}
}
i am now really confused, i am trying to make a computer generate a random number 1-10, but if it is already in the array generates another one.
int g = 0;
int[] compguess = new int[g];
Your array is size 0, so you have no valid entries.
Since you initialized g as zero, your array compguess has a length of zero. Next when you enter your for loop you assign 1 to i which will allow you to enter into the if condition at the end of guess which will try to access element compguess[1] but this cannot exist because the array is of size zero.
You will run into problems if you do not correct the following.
Change: for(int n = 0; n <= computer1.compguess.length; n++)
To: for(int n = 0; n < computer1.compguess.length; n++)
If your array length is 8 then the last item in the array will be index 7, but the <= tells the loop to grab item index 8.
Your compguess has a length of 0, and you are starting your for loop with i = 1, wich is already greater than 0.
compguess is a zero-length array. If you try to index it, you will fall out of the array and hence the ArrayIndexOutOfBoundsException
If your intent is to make the array longer and add a new item to the end of it, you can't do that. I'm guessing that this is what you were trying to do here:
rand = compguess[g];
First of all, if the language did allow it, you'd want to write it the other way:
compguess[g] = rand;
because you're trying to put a value into a new element of the array, not read from the array. This would actually work in some languages (JavaScript, Perl, others). In Java, however, when you create an array object with something like new int[], the size is fixed. You can't make it longer or shorter.
You probably want to use an ArrayList, which does let you create an array that you can make longer. See this tutorial.