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Method to copy primes from one array to another in Java

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I'm rather new to Java, and I'm trying to figure out a way to copy all primes inside of an array and copy those to another array.

To do so, I've implemented a separate isPrime() method to check whether the element is a prime, and another method that counts the number of primes in that array countPrimes(), such that I can determine the new array's size.

Here is where I'm kind of stuck:

public static int[] primesIn(int[] arr) 
 int primeHolder = countPrimes(arr);
 int[] copyArr = new int[primeHolder];
 for (int i = 0; i < arr.length; i++) 
 if (isPrime(arr[i]) == true) 
 copyArr[>Needs to start from 0<] = arr[i];
 
 
 return copyArr;


int[] arrayMan = 3,5,10,15,13;

At copyArr the position should be 0, followed by +1 everytime it finds a prime. If I were to give it i position, as in copyArr[i] = arr[i], then say the prime is at position 5, it would try to save the prime onto position 5of copyArr, which doesn't exist if there are only three primes in the original array, which would've given copyArr a length of only three.

Something tells me a different for loop, or maybe even an additional one would help, but I can't see how I should implement it. Help is greatly appreciated!

java for-loop primes

share|improve this question

asked Mar 27 at 21:21

TomTom

18488 bronze badges

add a comment | 

0

I'm rather new to Java, and I'm trying to figure out a way to copy all primes inside of an array and copy those to another array.

To do so, I've implemented a separate isPrime() method to check whether the element is a prime, and another method that counts the number of primes in that array countPrimes(), such that I can determine the new array's size.

Here is where I'm kind of stuck:

public static int[] primesIn(int[] arr) 
 int primeHolder = countPrimes(arr);
 int[] copyArr = new int[primeHolder];
 for (int i = 0; i < arr.length; i++) 
 if (isPrime(arr[i]) == true) 
 copyArr[>Needs to start from 0<] = arr[i];
 
 
 return copyArr;


int[] arrayMan = 3,5,10,15,13;

At copyArr the position should be 0, followed by +1 everytime it finds a prime. If I were to give it i position, as in copyArr[i] = arr[i], then say the prime is at position 5, it would try to save the prime onto position 5of copyArr, which doesn't exist if there are only three primes in the original array, which would've given copyArr a length of only three.

Something tells me a different for loop, or maybe even an additional one would help, but I can't see how I should implement it. Help is greatly appreciated!

java for-loop primes

share|improve this question

asked Mar 27 at 21:21

TomTom

18488 bronze badges

add a comment | 

I'm rather new to Java, and I'm trying to figure out a way to copy all primes inside of an array and copy those to another array.

To do so, I've implemented a separate isPrime() method to check whether the element is a prime, and another method that counts the number of primes in that array countPrimes(), such that I can determine the new array's size.

Here is where I'm kind of stuck:

public static int[] primesIn(int[] arr) 
 int primeHolder = countPrimes(arr);
 int[] copyArr = new int[primeHolder];
 for (int i = 0; i < arr.length; i++) 
 if (isPrime(arr[i]) == true) 
 copyArr[>Needs to start from 0<] = arr[i];
 
 
 return copyArr;


int[] arrayMan = 3,5,10,15,13;

At copyArr the position should be 0, followed by +1 everytime it finds a prime. If I were to give it i position, as in copyArr[i] = arr[i], then say the prime is at position 5, it would try to save the prime onto position 5of copyArr, which doesn't exist if there are only three primes in the original array, which would've given copyArr a length of only three.

Something tells me a different for loop, or maybe even an additional one would help, but I can't see how I should implement it. Help is greatly appreciated!

java for-loop primes

share|improve this question

asked Mar 27 at 21:21

TomTom

18488 bronze badges

public static int[] primesIn(int[] arr) 
 int primeHolder = countPrimes(arr);
 int[] copyArr = new int[primeHolder];
 for (int i = 0; i < arr.length; i++) 
 if (isPrime(arr[i]) == true) 
 copyArr[>Needs to start from 0<] = arr[i];
 
 
 return copyArr;


int[] arrayMan = 3,5,10,15,13;

share|improve this question

asked Mar 27 at 21:21

TomTom

18488 bronze badges

share|improve this question

asked Mar 27 at 21:21

TomTom

18488 bronze badges

asked Mar 27 at 21:21

TomTom

18488 bronze badges

asked Mar 27 at 21:21

TomTom

18488 bronze badges

2 Answers
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1

Have a second index variable int primeCount, and increment it whenever you find a prime. No need for a 2nd loop.

In modern days of abundant memory, things are usually not done like this. If you don't have some extra hard requirements, you could just use a resizable ArrayList<Integer>, and add() stuff in there. (and convert it back to int[] at the end if needed). This is also better in this case, because typically your countPrimes call will run much slower than ArrayList reallocations.

share|improve this answer

edited Mar 27 at 21:36

answered Mar 27 at 21:33

battlmonstrbattlmonstr

2,96711 gold badge1212 silver badges2525 bronze badges

add a comment | 

1

Read your words carefully:

At copyArr the position should be 0, followed by +1 everytime it
finds a prime.

That means that index in a new array does not depend on its position in the old array.
Create a counter. And each time you place a prime number into a new array, increment it by 1. Thus you can always know where to put a new number.

share|improve this answer

answered Mar 27 at 21:35

Pavel SmirnovPavel Smirnov

3,35233 gold badges99 silver badges2222 bronze badges

add a comment | 

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1

Have a second index variable int primeCount, and increment it whenever you find a prime. No need for a 2nd loop.

In modern days of abundant memory, things are usually not done like this. If you don't have some extra hard requirements, you could just use a resizable ArrayList<Integer>, and add() stuff in there. (and convert it back to int[] at the end if needed). This is also better in this case, because typically your countPrimes call will run much slower than ArrayList reallocations.

share|improve this answer

edited Mar 27 at 21:36

answered Mar 27 at 21:33

battlmonstrbattlmonstr

2,96711 gold badge1212 silver badges2525 bronze badges

add a comment | 

1

Have a second index variable int primeCount, and increment it whenever you find a prime. No need for a 2nd loop.

In modern days of abundant memory, things are usually not done like this. If you don't have some extra hard requirements, you could just use a resizable ArrayList<Integer>, and add() stuff in there. (and convert it back to int[] at the end if needed). This is also better in this case, because typically your countPrimes call will run much slower than ArrayList reallocations.

share|improve this answer

edited Mar 27 at 21:36

answered Mar 27 at 21:33

battlmonstrbattlmonstr

2,96711 gold badge1212 silver badges2525 bronze badges

add a comment | 

Have a second index variable int primeCount, and increment it whenever you find a prime. No need for a 2nd loop.

In modern days of abundant memory, things are usually not done like this. If you don't have some extra hard requirements, you could just use a resizable ArrayList<Integer>, and add() stuff in there. (and convert it back to int[] at the end if needed). This is also better in this case, because typically your countPrimes call will run much slower than ArrayList reallocations.

share|improve this answer

edited Mar 27 at 21:36

answered Mar 27 at 21:33

battlmonstrbattlmonstr

2,96711 gold badge1212 silver badges2525 bronze badges

share|improve this answer

edited Mar 27 at 21:36

answered Mar 27 at 21:33

battlmonstrbattlmonstr

2,96711 gold badge1212 silver badges2525 bronze badges

answered Mar 27 at 21:33

battlmonstrbattlmonstr

2,96711 gold badge1212 silver badges2525 bronze badges

answered Mar 27 at 21:33

battlmonstrbattlmonstr

2,96711 gold badge1212 silver badges2525 bronze badges

1

Read your words carefully:

At copyArr the position should be 0, followed by +1 everytime it
finds a prime.

That means that index in a new array does not depend on its position in the old array.
Create a counter. And each time you place a prime number into a new array, increment it by 1. Thus you can always know where to put a new number.

share|improve this answer

answered Mar 27 at 21:35

Pavel SmirnovPavel Smirnov

3,35233 gold badges99 silver badges2222 bronze badges

add a comment | 

1

Read your words carefully:

At copyArr the position should be 0, followed by +1 everytime it
finds a prime.

That means that index in a new array does not depend on its position in the old array.
Create a counter. And each time you place a prime number into a new array, increment it by 1. Thus you can always know where to put a new number.

share|improve this answer

answered Mar 27 at 21:35

Pavel SmirnovPavel Smirnov

3,35233 gold badges99 silver badges2222 bronze badges

add a comment | 

Read your words carefully:

At copyArr the position should be 0, followed by +1 everytime it
finds a prime.

That means that index in a new array does not depend on its position in the old array.
Create a counter. And each time you place a prime number into a new array, increment it by 1. Thus you can always know where to put a new number.

share|improve this answer

answered Mar 27 at 21:35

Pavel SmirnovPavel Smirnov

3,35233 gold badges99 silver badges2222 bronze badges

share|improve this answer

answered Mar 27 at 21:35

Pavel SmirnovPavel Smirnov

3,35233 gold badges99 silver badges2222 bronze badges

answered Mar 27 at 21:35

Pavel SmirnovPavel Smirnov

3,35233 gold badges99 silver badges2222 bronze badges

answered Mar 27 at 21:35

Pavel SmirnovPavel Smirnov

3,35233 gold badges99 silver badges2222 bronze badges