Array inside an array: iteration doesn't recognize strings inside of it

Question

I have trouble reading the elements inside a 2d array.

To contextualize, the user delivers an input like this:

Where the first digit is the number of next inputs (n in my code). And every next first digit is a course, and the digits that follow it are it's requirements (courses too). For example, course 3 needs courses 1 and 2 approved.

I need to count the occurrence of a String inside a 2D array but not considering the first element of each array (list1[i][0]). Also the first row list1[0] is not filled to make index call easier (that's why my function starts with i=1). The problem is that it won't recognize the items that are inside the 2d array, so the count returns always 0. This is my function

static int count(String[][] list, String s, int n) { //n is the number of rows
    int x=0;
    for (int i=1;i<=n;i++) {
        for (int j=1;j<list[i].length;j++) {
            if (list[i][j]==s) {
                x++;
            }
        }
    }
    return x;
}

The function actually works for an array defined like this:

String[][] list1= {{null},{"1"},{"2"},{"3","1","2"},{"4","3"},{"5","4"},{"6","3"},{"7","5","6"}};

Where, for example, count(list1,"1",7) returns 1 and count(list1, "3",7) returns 2.

But for the array where the inputs are saved as strings (list2), supposedly just like list1, the count returns 0.

The way I initialized the 2D array list2:

 String[][] list2 = new String[n+1][];
    String[] L1 = new String[n+1]; //just an auxiliary array
    for (int j=1; j<=n; j++) {
                list2[j] = new String[n+1];
                L1[j] = sc.nextLine();
                list2[j] = L1[j].split(" ");
            }

The weird thing is that if a call an array or an element of list2, it returns what is supposed to (for example, using the input above, list2[3]=["3","1","2"]) but the function just doesn't work. I had the same problem with another homework, and it was related to adding arrays to another array, I couldn't solve it so I changed the way the inputs were saved, but now due to restrictions I have no other choice.

Any help will be appreciated

One noticeable problem is that you are comparing strings with `==` instead of `.equals()`. — 0xCursor, May 06 '19 at 00:03
thanks a lot! i spent many hours trying to figure it out but never thought it was the way I was comparing the Strings — Francisca Suárez, May 06 '19 at 05:08

score 2 · Accepted Answer · answered May 06 '19 at 00:22

At Java == means exact same object for object comparison. For String object there is method equals, overrides the Object class equals() method implementation.

equals method returns true if object is a String, not null and represents the same sequence of characters.

static int count(String[][] list, String s, int n) { //n is the number of rows
    int x=0;
    for (int i=1;i<=n;i++) {
        for (int j=1;j<list[i].length;j++) {
            if (list[i][j].equals(s)) {
                x++;
            }
        }
    }
    return x;
}

If equality is not case sensitive use if (list[i][j].equalsIgnoreCase(s))

score 1 · Answer 2 · answered May 06 '19 at 00:17

1

2 problems:

1: arrays are 0-indexed. Start your loops at 0, not 1
2: use .equals() to compare strings. Never use == for objects unless you know exactly what you're doing

answered May 06 '19 at 00:17

Benjamin Urquhart

1,626
12
18

score 0 · Answer 3 · answered May 06 '19 at 00:14

0

Try using .equals() to compare the value in the matrix to the string s instead of the == operator

answered May 06 '19 at 00:14

ivan volkov

54
3

Array inside an array: iteration doesn't recognize strings inside of it

3 Answers3