Question

I am trying to run my code so it prints cyclic permutations, though I can only get it to do the first one at the moment. It runs correctly up to the point which I have marked but I can’t see what is going wrong. I think it has no break in the while loop, but I’m not sure. Really could do with some help here.

package permutation;

public class Permutation {
static int DEFAULT = 100;

public static void main(String[] args) {
    int n = DEFAULT;
    if (args.length > 0)
        n = Integer.parseInt(args[0]);

    int[] OA = new int[n];
    for (int i = 0; i < n; i++)
        OA[i] = i + 1;

    System.out.println("The original array is:");
    for (int i = 0; i < OA.length; i++)
        System.out.print(OA[i] + " ");
    System.out.println();

    System.out.println("A permutation of the original array is:");
    OA = generateRandomPermutation(n);
    printArray(OA);
    printPemutation(OA);
}

static int[] generateRandomPermutation(int n)// (a)
{
    int[] A = new int[n];
    for (int i = 0; i < n; i++)
        A[i] = i + 1;

    for (int i = 0; i < n; i++) {
        int r = (int) (Math.random() * (n));
        int swap = A[r];
        A[r] = A[i];
        A[i] = swap;
    }
    return A;
}

static void printArray(int A[]) {
    for (int i = 0; i < A.length; i++)
        System.out.print(A[i] + " ");
    System.out.println();
}

static void printPemutation(int p[])// (b)
{
    System.out
            .println("The permutation is represented by the cyclic notation:");
    int[] B = new int[p.length];
    int m = 0;
    while (m < p.length)// this is the point at which my code screws up
    {
        if (!check(B, m)) {
            B = parenthesis(p, m);
            printParenthesis(B);
            m++;
        } else
            m++;
    }// if not there are then repeat
}

static int[] parenthesis(int p[], int i) {
    int[] B = new int[p.length];
    for (int a = p[i], j = 0; a != B[0]; a = p[a - 1], j++) {
        B[j] = a;
    }
    return B;
}

static void printParenthesis(int B[]) {
    System.out.print("( ");
    for (int i = 0; i < B.length && B[i] != 0; i++)
        System.out.print(B[i] + " ");
    System.out.print(")");
}

static boolean check(int B[], int m) {
    int i = 0;
    boolean a = false;
    while (i < B.length || !a) {
        if ((ispresent(m, B, i))){
            a = true;
            break;
        }
        else
            i++;
    }
    return a;
}

static boolean ispresent(int m, int B[], int i) {
    return m == B[i] && m < B.length;
}
}

Answer 1

Among others you should check p[m] in check(B, p[m]) instead of m:

in static void printPemutation(int p[]):

while (m < p.length){
    if (!check(B, p[m])) {
         B = parenthesis(p, m);
         printParenthesis(B);
    }
    m++;
}

then

static boolean check(int B[], int m) {
    int i = 0;
    while (i < B.length) {
        if (m == B[i]) {
            return true;
        }
        i++;
    }
    return false;
}

this does somehow more what you want, but not always i fear…