【DataStructure】Some useful methods for arrays

Last night it took me about two hours to learn arrays. For the sake of less time， I did not put emphaises on the practice question, just now when reading the book, I found that some methods referred to arrays are so beneficial to us. So in here make a simple summary.

Method 1: Check whether the array is sorted. 

  private static boolean isSorted(int[] a) {
    if (a.length < 2) {
      return true;
    }
    for (int i = 1; i < a.length; i++) {
      if (a[i] < a[i-1]) {
        return false;
      }
    }
    return true;
  }
}

     Method 2:  Use the start number and range to init the array

  public static void load(int[] a, int start, int range) {
    for (int i = 0; i < a.length; i++) {
      a[i] = start + random.nextInt(range);  // random 5-digit numbers
    }
  }

    Method 3:  Get the min number from the array

  private static int minimum(int[] a) {
    int min = a[0];
    for (int i = 1; i < a.length; i++) {
      if (a[i] < min) {
        min = a[i];
      }
    }
    return min;
  }

  Method 4： Remove the duplicate elements from object

  private static int[] withoutDuplicates(int[] a) {
    int n = a.length;
    if (n < 2) {
      return a;
    }
    for (int i = 0; i < n-1; i++) {
      for (int j = i+1; j < n; j++) {
        if (a[j] == a[i]) {
          --n;
          System.arraycopy(a, j+1, a, j, n-j);
          --j;
        }
      }
    }
    int[] aa = new int[n];
    System.arraycopy(a, 0, aa, 0, n);
    return aa;
  }

  Method 5: Finds the prime number according to certain range

  private static final int SIZE=1000;
  private static boolean[] isPrime = new boolean[SIZE];

  private static void initializeSieve() {
      for (int i = 2; i < SIZE; i++) {
        isPrime[i] = true;
      }
      for (int n = 2; 2*n < SIZE; n++) {
        if (isPrime[n]) {
          for (int m = n; m*n <SIZE; m++) {
            isPrime[m*n] = false;
          }
        }
      }
    }

Another way of implement the function of finding the prime number(Vector)

  private static final int SIZE=1000;
  private static Vector<Boolean> isPrime = new Vector<Boolean>(SIZE);
  
  private static void initializeSieve() {
    isPrime.add(false);  // 0 is not prime
    isPrime.add(false);  // 1 is not prime
    for (int i = 2; i < SIZE; i++) {
      isPrime.add(true);
    }
    for (int n = 2; 2*n < SIZE; n++) {
      if ((isPrime.get(n))) {
        for (int m = n; m*n < SIZE; m++) {
          isPrime.set(m*n, false);
        }
      }
    }
  }

Another way of implement the function of finding the prime number(BitSet)

  private static final int SIZE=1000;
  private static BitSet isPrime = new BitSet(SIZE);

  private static void initializeSieve() {
      for (int i = 2; i < SIZE; i++) {
        isPrime.set(i);
      }
      for (int n = 2; 2*n < SIZE; n++) {
        if (isPrime.get(n)) {
          for (int m = n; m*n <SIZE; m++) {
            isPrime.clear(m*n);
          }
        }
      }
    }

Method 6: Print out the result according to the certain format:

 public static void printSieve() {
    int n=0;
    for (int i = 0; i < SIZE; i++) {
      if (isPrime[i]) {
        System.out.printf("%5d%s", i, ++n%16==0?"\n":"");
      }
    }
    System.out.printf("%n%d primes less than %d%n", n, SIZE);
  }

Notes: There exists five spaces between each number, and it will change line when the length of char  % 6 is zero.

  2    3    5    7   11   13   17   19   23   29   31   37   41   43   47   53
   59   61   67   71   73   79   83   89   97  101  103  107  109  113  127  131
  137  139  149  151  157  163  167  173  179  181  191  193  197  199  211  223
  227  229  233  239  241  251  257  263  269  271  277  281  283  293  307  311
  313  317  331  337  347  349  353  359  367  373  379  383  389  397  401  409
  419  421  431  433  439  443  449  457  461  463  467  479  487  491  499  503
  509  521  523  541  547  557  563  569  571  577  587  593  599  601  607  613
  617  619  631  641  643  647  653  659  661  673  677  683  691  701  709  719
  727  733  739  743  751  757  761  769  773  787  797  809  811  821  823  827
  829  839  853  857  859  863  877  881  883  887  907  911  919  929  937  941
  947  953  967  971  977  983  991  997