Array manipulation: adding elements-CodePudding

I am new to programming, and I just learned about array. However, in one of the quiz questions, I can't figure out why my answer is wrong. I tried to explain my thinking.

The question is:

    int[] b = { 1, 2, 3 };
    int[] c = b;
    c[0]  = b[2];
    c[1]  = b[1];
    c[2]  = b[0];
    System.out.println(c[0]   c[1]   c[2]);

My thinking is although array is a reference type, int[] c = b can still be treated as

int [] c = { 1, 2, 3 }; // the same as int [] b

c[0]  = b[2] means c[0] = c[0]   b[2] = 1   3 = 4
c[1]  = b[1] means c[1] = c[1]   b[1] = 2   2 = 4
c[2]  = b[0] means c[2] = c[2]   b[0] = 3   1 = 4

So I thought that the output is 12. However, the answer is 15.

I am so sorry for asking such a basic question, but I was wondering if anyone can help me understand how to come up with 15. Thank you so much!

CodePudding user response：

actually java copies arrays by their references not values. so in the last line of your code, b[0] is changed to 4 so

c[2]  = b[0] means c[2] = c[2]   b[0] = 3   4 = 7

so your output is 15. If you want to copy an array without reference,you can do this:

int[] c = Arrays.copyOf(b, b.length);

now, your output should be 12.

CodePudding user response：

Since you set c = b, this means that whatever changes you make onto c will be reflected onto b and vice verca, since they reference the same array. If you wanted b and c to be separate arrays, you should use Arrays.copy().

For your problem, we can now go step by step to analyze how the values change during each step:

int[] b = { 1, 2, 3 };
int[] c = b;

c[0]  = b[2];
//c[0] = c[0]   b[2] = 1   3 = 4
//b[0] = 4
// b and c = {4, 2, 3}

c[1]  = b[1];
//c[1] = c[1]   b[1] = 2   2 = 4
//b[1] = 4
//b and c = {4, 4, 3}

c[2]  = b[0];
//c[2] = c[2]   b[0] = 3   4 = 7
//b[2] = 7
//b and c = {4, 4, 7}
System.out.println(c[0]   c[1]   c[2]);
//4   4   7 = 15 :)

I hope this helped! Please let me know if you need any further clarification or details :)

CodePudding user response：

As c and b refer to the same array, when c[0] is changed to 4, it means that b[0] is also 4. Thus, the result of c[2] = b[0] is 3 4 = 7, and the total sum of all elements is 4 4 7 = 15.

Assigning a reference to array int[] c = b; creates just an alias and it does NOT create a new instance of the array as in the case of its copying (using methods

then output will be 4 4 7 which is 15.