查找数组中第三小的索引

Question

I'm trying to find the indexes of the three smallest elements in an array. My comments are in Norwegian, I can translate them if that's necessary!

It's an assignment, and I have to do the following: - return an array with the indexes of the three smallest values - have three help variables for index, and three for their values - use the method "indekssortering" to give the indexes a start value

This method is trying to find the indexes. I have m1, m2 and m3 as start-indexes, but they will be changed in the for-loop if we find a smaller number. m1_verdi, m2_verdi and m3_verdi is the values. They base themselves on the indexes from m1, m2 and m3.

//oppgave 9
    public static int[] tredjeMin(int[] a){
        if (a.length < 3) {
            throw new java.util.NoSuchElementException("Arrayet har lengden " + a.length + ", skal ha lengde  >= 3!");
        }

        int[] tre = Arrays.copyOfRange(a, 0, 3); //Kopierer de tre forste tallene i arrayet a
        int[] in = indekssortering(tre); //Indekssorterer de tre forste tallene

        int m1 = in[0]; // STARTVERDI: index til minste verdi
        int m2 = in[1]; // STARTVERDI: index til nest minste verdi
        int m3 = in[2]; // STARTVERDI: index til nest, nest minste verdi

        int m1_verdi = a[m1]; //  STARTVERDI: minste verdi
        int m2_verdi = a[m2]; // STARTVERDI: nest minste verdi
        int m3_verdi = a[m3]; // STARTVERDI: nest, nest minste verdi

        for (int i = 0; i < a.length; i++) { // Looper gjennom arrayet a
            if (a[i] < m3_verdi) {
                if (a[i] < m2_verdi) {
                    if (a[i] < m1_verdi) {
                        m2 = m1;
                        m2_verdi = m1_verdi; // Ny nest minst

                        m1 = i;
                        m1_verdi = a[m1]; // Ny minst
                    } else {
                        m3 = m2;
                        m3_verdi = m2_verdi; // ny nest, nest minst

                        m2 = i;
                        m2_verdi = a[m2]; // Ny nest minst
                    }
                } else {
                    m3 = i;
                    m3_verdi = a[m3]; // Ny nest, nest minst
                }
            }
        }

        return new int[] {m1, m2, m3};
    }

The method is calling the method "indekssortering" (index sorting), which is this one:

//oppgave 8
    public static int [] indekssortering(int[] a){
        int[] indeks = new int[a.length]; //Oppretter tomt array med samme lengde som a
        int[] kopi = Arrays.copyOf(a, a.length); // Oppretter kopi av a
        for (int i = 0; i < kopi.length; i++) { // Bubble sort av kopi:
            for (int j = 0; j < kopi.length-1; j++) {
                if (kopi[j] > kopi[j+1]) {
                    int temp = kopi[j];
                    kopi[j] = kopi[j+1];
                    kopi[j+1] = temp;
                }
            }
        }

        int i = 0;
        while (i < kopi.length) {
            for (int j = 0; j < kopi.length; j++) { // Per tall i sortert kopi, loop gjennom hele arrayet
                if (kopi[i] == a[j]) { // Sjekker om valgt tall matcher et tall fra original arrayet a
                    indeks[i] = j; // Setter indeksen til original array a som innhold i arrayet indeks
                    i++; // Oker indeksen
                    if (i == kopi.length) { // Om den valgte indeksen er storre enn arrayets lengde; break
                        break;
                    }
                }
            }
        }
        return indeks; // Returnerer arrayet indeks
    }

This method takes in an array, copies it, sorts the copy and matches the values in the copied array to the original, then takes those indexes and puts them in a new array. The output is the original indexes for the original array, as if the array has been sorted.

I have a long test file that i've not written, but it basically shows that the "tredjeMin" method returns the wrong indexes. The "indekssortering" method works and returns what it's supposed to. I dont have a main method, but when I tested it on my own, I did something like this:

int[] a = {2, 0, 8, 12, 4, 0, 13, 3, 0, 0};

        Oblig1 tester = new Oblig1();
        int[] z = tester.indekssortering(a);
        int[] u = tester.tredjeMin(a);

        System.out.println(Arrays.toString(a));
        System.out.println(Arrays.toString(z));
        System.out.println(Arrays.toString(u));

The array u is basically supposed to match the three first values inn z.

I have worked on this all day, and I'm basically blind to whats wrong. Thank you so much in advance!!

Edits: adding info

Answer 1

I'm having a little bit of a hard time understanding your code since the variables/methods/comments are in norwegian, but I think your method for find the three smallest indexes could be simplified a lot by following the below pseudocode. By simplifying it, it will be easier to debug.

The first difference I made is, rather than trying to keep the three min indexes sorted the entire time, just sort them at the end.

The second is to just track the indexes instead of the indexes and the values. To get the value, just check the array at that index.

for (int i = 0; i < a.length(); i++) {
    if (a[i] < max(a[m1], a[m2], a[m3])) { // you have to implement max()
        // replace max of m1, m2, m3 with i
    }
}

int[] result = new int[] {m1, m2, m3)

// sort the array of 3

Answer 2

Based on your comments - You don't need the tredjeMin method at all.

You can just do following to validate -

int[] a ={2, 0, 8, 12, 4, 0, 13, 3, 0, 0};
int sortedIndex[] = indekssortering(a);
System.out.println("Min 3 Indexes - " + sortedIndex[0]+" , "+ sortedIndex[1]+" , "+ sortedIndex[2]);