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Prime Numbers Array in Java

I want to return an array that displays all the prime numbers within a certain range from 0 till whatever number I enter.
For the range from 0 to 5 I would like the array returned with [2,3,5]. Within the task I was told by my professor that I should fill the whole array with 0 before replacing those 0 wih prime numbers later.
Currently my code does not return the correct array as I do not seem to access the next location in the array but seem to always assign the value to the first location in the array.
My current result array is not [2,3,5] but [5,0,0,0,0].
Any help would be greatly appreciated.
public static int[] generierePrimzahlen(int bis){
int [] myAry = new int[bis];
Arrays.fill(myAry,0);
for(int i=0;i<myAry.length;i++){
for (int nextprime=1; nextprime < bis; nextprime++){
int counter = 0;
// System.out.println(nextprime);
if (istPrimzahl(nextprime)){
myAry[counter] = nextprime;
counter++;
}
}
System.out.print(myAry[i]+" ");
}
return myAry;
}
PS: I have a functioning method (istPrimzahl), which checks if a certain number is a prime number or not.

The problem is that your counter is in the wrong scope.
so instead of incrementing. on every iteration of the first for loop, you declare a new counter. so that it is 0 at the time u assign the prime number to the array.
public static int[] generierePrimzahlen(int bis){
int [] myAry = new int[bis];
// Arrays.fill(myAry,0); // no need, this is already been done at initialization
for(int i=0;i<myAry.length;i++){
int counter = 0;
// adding <= 'nextprime <= bis;' to check also the last number in the range
for (int nextprime=1; nextprime <= bis; nextprime++){
// int counter = 0; wrong scope
// System.out.println(nextprime);
if (istPrimzahl(nextprime)){
myAry[counter] = nextprime;
counter++;
}
}
if(myAry[0] != 0) // to get rid of displaying Zeros
System.out.print(myAry[i]+" ");
}
return myAry;
}

Put below line outside both for loop. That will work.
Reason for issue is - you are resetting the counter while entering the for loop.
int counter = 0;

ArrayList will be a better choice than array. However if using array is another school requirement, then what you have done:
int[] myAry = new int[size];
will already set all elements to zeroes.
There is also no need to use 2 loops for this. Just:
Loop through from 1 to n
if current number is prime, set it to array of current index
idx++
I do not seem to access the next location in the array but seem to always assign the value to the first location in the array.
That is because you are setting your counter variable back to zero in every iteration. You should declare it outside your loop.
Example:
int idx = 0; //place this outside the loop
for(int i=1; i<=n; i++)
if(isPrime(i))
myAry[idx++] = i;

How to represent uninitialized values in an int array in Java?

I have an int array that stores 5 integers. I intend to take 5 numbers as input and put them into the array in a sorted manner (ascending order). So basically, I'm putting the value, accepted at each step, at its position it should be in in ascending order. Assuming that no input is 0, I have this code which works:
System.out.println("Enter 5 integers...");
Scanner sc=new Scanner(System.in);
int[] arr = new int[5];
int c;
//assuming that no number entered is 0
for(int i=1; i<=5; i++){
c = sc.nextInt();
for(int j=0; j<5; j++){
if(arr[j] == 0){ //placing an input where there's 0
arr[j] = c;
break;
}
else if(c < arr[j]){
for(int k=4; k>j; k--)
arr[k] = arr[k-1];
arr[j] = c;
break;
}
}
}
System.out.println("The numbers in ascending order are\n"
+java.util.Arrays.toString(arr));
Basically I consider 0 as an invalid value and some other number (presumably non-zero) can be put in its place. I can take this for granted provided that the user does not enter 0. But, WHAT IF THE USER ENTERS 0 (or any number for that matter)? I want to be able to place the numbers correctly including 0. So I thought that I could use something else to represent uninitialized or empty spaces with something other than a number. I saw a post on SO where someone had a similar issue with a double array, so they could use Double.NaN to represent invalid or empty spaces. Unfortunately, JAVA doesn't have Integer.NaN so I couldn't use this trick.
So my question is, How can I represent empty/uninitialized spaces in an int array? Can something like NaN be used?

In Java you have an alternative to the "classical" way of bygone languages which would rely on a "magic value" to denote an uninitialised int (something like the largest or smallest possible value).
You can use Integer, rather than int, and use null for the "uninitialised" value:
Integer[] arr = new Integer[5];
Here, each element of arr is automatically initialised to null, which is nice.
But do then be aware of the dreaded == comparing references rather than values.

One way is to use some default value that you don't expect to be entered as a valid input (for example, a negative number such as -1 or Integer.MIN_VALUE can work if all valid inputs are non-negative). This is the only way to go if you want to use an array of primitive integers (i.e. int[]).
Another way is to use an array of Integer (Integer[]), whose elements are initialized to null by default. Using wrapper types instead of primitives is considered less efficient, so there's a tradeoff.

You can keep a separate array of boolean of the same length, and each entry with the same index as in the int array indicates whether the value was initialized or not:
int[] arr = new int[5];
boolean[] initialized = new boolean[arr.length];
int c;
//assuming that no number entered is 0
for(int i=1; i<=5; i++){
c = sc.nextInt();
for(int j=0; j<5; j++){
if(!initialized[j]){ // use boolean array to find out if already initialized
arr[j] = c;
initialized[j] = true;
break;
}
else if(c < arr[j]){
for(int k=4; k>j; k--) {
arr[k] = arr[k-1];
initialized[k] = initialized[k-1];
}
arr[j] = c;
initialized[j] = true;
break;
}
}
}

WHAT IF THE USER ENTERS 0?
Then you need to:
init the array with a negative number
dont allow the user to give as
input negative number (validate)
another opütion canbe use the wrapper clas Integer instead of the primitives, since those can easily prove against a null reference...
in my opinion not necesary in your case....

Converting String elements from an array to as values into an integer Array

I'm not allowed to use methods from any class except String and IO Class
So my code snippet is:
String line = reader.readLine();
while (line != null) {
String[] elements = line.split(",");
// Array could be too big if there are multiple occurances of
// the same number
// Array length + 1 because I can't use the 0 and with a input line of
// 1,2,3 for example would be the length 3 but I would have the
// numbers 0,1,2 in the Array as my index.
String[][] placeholderMatrix = new String[elements.length+1][elements.length+1];
for(int i = 0; i < elements.length-1; i++){
placeholderMatrix[(int)elements[i]][(int)elements[i+1]] = 1;
}
line = reader.readLine();
}
In the File I'm getting are only numbers like that: 1,2,3,4,5,8,7,4
So in my splitted String Array are only Numbers but now if I want to use them as my index for my Matrix(placeholderMatrix)
My problem is in my for loop where I want to use them as my Index I can't use them because it is a String Array. Normally I would use Integer.parseInt but I'm not allowed to :/
Any ideas on how I can implement them as my Index? and any Idea how I can get the perfect length of my Matrix? Because If I get the following numbers: 1,2,2,2,3 My Matrix should only have the numbers:
0 1 2 3
1
2
3
But if I'm using elements.length+1 for the length of my Matrix I would get the numbers 0 1 2 3 4 5
Hope you could understand my problem. Sorry for my bad english and Thanks in advance.
Edit: SO i got another problem with that. If I implement the method(parseInt) of Dici and am using it in the line "placeholderMatrix[parse(elements[i])][parse(elements[i+1])] = 1;" I'm getting the error ArrayOutOfBounce because my defined Array is just the length of my splitted String Array elements. But if I define it with Integer.MAX_VALUE as my length I get a memory error because it is too big. Any ideas?
Edit2: My Task:
I have to take a row of Numbers seperated by ",". (I will split it with the String split method to get only the numbers) Now I have to create a Matrix(2 dimensional Array) and look for the number at the index i of my new String Array and the number at the index i + 1 and have to take the first Number as my column and th second as my row (or vice versa) and implement at that point a 1. Now are my Numbers I will get from 1 to Integer.MAX_VALUE so I would have to create such a big Matrix but this isn't possible because I get the MemoryError.
Error: java.lang.OutOfMemoryError: Requested array size exceeds VM limit
at Test.main(Test.java:29)
To understand what I have to do: http://de.wikipedia.org/wiki/Adjazenzmatrix the image at the right but for numbers from to Integer.MAX_VALUE so my 2D Array has to be defined with the length of Integer.MAX_VALUE?
Edit:
So Dici asked for an example:
My Sequence could be: 1,2,5,4
So my Matrix should be:
Hope this is what you wanted Dici
But the numbers I can get from the sequence are 1 to Integer.MAX_VALUE

For converting strings to integers, you can simply implement your own integer parser, it is not complicated. You can start with this and improve it if needed.
public int parseInt(String s) {
int n = 0;
int pow = 1;
for (int i=s.length() - 1 ; i>=0 ; i--) {
String si = String.valueOf(s.charAt(i);
if (si.matches("[0-9]")) {
n += pow*(s.charAt(i) - '0');
pow *= 10;
} else if (si.matches("+|-") && i == 0)
n *= s.charAt(i) == '+' ? 1 : -1;
else
throw new NumberFormatException();
}
return n;
}
Then, I'll handle the second part of your problem. If Integer.MAX_VALuE is one of your input values, you cannot possibly allocate an Integer.MAX_VALUE x Integer.MAX_VALUE matrix. What you need to do is assign contiguous ids to your input values and record the ids in a map so that you can access easily the index of the matrix corresponding to one node value. Here is an example to get you to understand :
public void someMethod() {
int id = 0;
Map<Integer,Integer> idMap = new HashMap<>();
String[] split = reader.readLine().split(",");
int [] nodes = new int[split.length];
for (int i=0 ; i<nodes.length ; i++) {
nodes[i] = parseInt(split[i]);
if (!idMap.containsKey(nodes[i]))
idMap.put(nodes[i],id++);
}
// the map is now constructed, it should probably be stored in an attribute
int[][] placeholderMatrix = new int[nodes.length][nodes.length];
for(int i = 0; i < nodes.length; i++){
if (i > 0) placeholderMatrix[idMap.get(nodes[i])][idMap.get(nodes[i-1])] = 1;
if (i < nodes.length-1) placeholderMatrix[idMap.get(nodes[i])][idMap.get(nodes[i+1])] = 1;
}
}
There are other ways to do it, let me know if this solution is ok

You could do something like:
String keyword = "1,2,3,4,5,8,7,4";//input line from file
String replacedKeyword = keyword.replaceAll("[^\\d]", "");//except numbers replace all. Assuming one digit numbers only.
String[][] placeholderMatrix = new String[replacedKeyword.length()+1][replacedKeyword.length()+1];
char keys[] = replacedKeyword.toCharArray();
for (int i = 0; i<keys.length - 1; i++) {
placeholderMatrix[keys[i] - '0'][keys[i + 1] -'0'] = "1";
}

I couldn't really understand what you want exactly. but, if that going to help a simple method to convert String number to int:
int toInt(String number) {
int num = 0;
for (int i=0; i<number.length(); i++) {
num = num*10 + (number.charAt(i)-'0');
}
return num;
}

finding the number of pairs of numbers in an array that add up to a number

I am trying to come up with a program that will search inside of an array that is given a length by the user that picks out whether there is a pair of numbers that sum to 7. The idea is that if there is k amount of dice being thrown, how many pairs of numbers out of those dice thrown add up to 7. So far this is all that I could come up with but I am very stuck.
This is the driver class for the program. I have to write a class that will make this driver function properly.
import java.util.Scanner;
public class SevenDriver{
public static void main(String[] args){
System.out.println("Enter number of dice to toss");
Scanner s = new Scanner(System.in);
int diceCount = s.nextInt();
SevenTally t = new SevenTally(diceCount);
int experiments = 1000000;
int wins = 0;
for(int j = 0; j < experiments; j++)
if(t.experiment()) wins++;
System.out.println((double)wins/experiments);
}
}
This is what I have so far. It does not currently work or compile. I am just looking for some ideas to get me going. Thanks!
public class SevenTally{
private int diceCount;
public SevenTally(int die){
diceCount = die;
}
public int genDice(){
return 1 + (int)(Math.random()*6);
}
public boolean experiment(){
boolean[] nums = new boolean[diceCount];
int ranNum;
int sum = 7;
for(int i = 0; i < nums.length; i++){
ranNum = genDice();
if (nums[ranNum] == sum){
return true;
}
}
int left = 0;
int right = nums.length - 1;
while(left<right){
int tempSum = nums[left] + nums[right];
if(tempSum == 7){
return true;
}
else if(tempSum>7){
right--;
}
return false;
}
}

First populate your array of length k with random int in [1;6]
The number of possible pairs in an array of length k is the number of 2-combinations in the array, which is (k-1)*k/2 (http://en.wikipedia.org/wiki/Combination)
You can test all the possible pairs (i,j) in your array like so:
int win = 0;
int tally = 7;
for(int i=0; i<k-1; i++){
for(int j=i+1; j<k; j++){
if(array[i]+array[j] == tally){
win++;
}
}
}
What this does is that it sets the first element of the pair to be the first element of the array, and sums it with the other elements one after the other.
It pairs array[0] with array[1] to array[k-1] at the first pass of the i for loop, that's k pairs.
Then k-1 pairs at second pass, and so on.
You end up with (k)+(k-1)+(k-2)+...+1 pairs, and that's exactly (k-1)*k/2 pairs.
done =]
edit: sorry, haven't read the whole thing. the method experiment() is supposed to return a boolean. you can return win>0?true:false; for example...

This Wiki page has some algorithms to do that. Its not a trivial problem...

You're generating a random number in ranNum, and then using it as an index into the array nums. Meanwhile, nums never gets filled, so no matter which box you index into, it never contains a 7.
What you want to do, if I understand your problem correctly, is fill each space in the array with the result of a die roll, then compare every two positions (rolls) to see if they sum to seven. You can do that using a nested for loop.
Essentially, you want to do this: (written in pseudocode as I'm not a java programmer)
int[] results[numrolls]
for (count = 0 to numrolls-1) { results[numrolls]=dieRoller() }
for (outer = 0 to numrolls-2)
for (inner = outer+1 to numrolls-1)
if (results[outer] + results[inner] == 7) return true
return false;
However, in this case there's an even easier way. You know that the only ways to get a sum of 7 on 2d6 are (1,6),(2,5),(3,4),(4,3),(5,2),(6,1). Set up a 6-length boolean array, roll your dice, and after each roll set res[result] to true. Then return (1-based array used for simplicity) ( (res[1] && res[6]) || (res[2] && res[5]) || (res[3] && res[4]) ).

ArrayIndexOutOfBoundsException means you are trying to access an element of the array that hasn't been allocated.
In your code, you create a new array d of length diceCount, but then you genDice() on always 6 elements.

ArrayIndexOutOfBoundsException when looping

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.