Why does this function print what it does?

Question

If f and g are defined like this:

void f(int[] x, int[] y, int p, int q) {
    for (int i = 0; i < x.length; ++i) {
        x[i] = 2 * x[i];
    }
    x = new int[5];
    for (int i = 0; i < 5; ++i) {
        x[i] = 0;
    }
    y[3] = 5;
    p = q + 1;
}

void g() {
    int[] x = {1, 2, 3, 4};
    int[] y = {9, 7, 6, 5};
    f(y, x, y[0], x[0]);
    for (int p : x) {
        System.out.print("" + p + " ");
    }
    for (int q : y) {
        System.out.print("" + q + " ");
    }
    System.out.println();
}

then if you run g(), it prints 1 2 3 5 18 14 12 10, and I don't know why.

How do p and q even come into the equation?

p and q are just variables. What the for loop is doing is saying for every item in x do something — Martin Lund, Nov 08 '18 at 07:10
Step through the program, either on paper or with a debugger. — Andy Turner, Nov 08 '18 at 07:18
That is a really overly complicated example for a beginner but if you want to understand all of it right now you may find it helpful to google the following (with a java attached to each): "immutability", "reference types", "loops", "function calls", probably best in reverse order -.- — Wolfone, Nov 08 '18 at 07:36

score 0 · Answer 1 · answered Nov 08 '18 at 07:16

p and q are just variables which are used in for loop. Here the code uses "for each loop" which a variant of "for loop".

In **for each loop ** instead of declaring and initializing a loop counter variable, we declare a variable that is the same data type as the base type of the array or collection, followed by a semi-colon, which is then followed by the array or collection name.

to understand how for each loop work check out this answer-> How does the Java 'for each' loop work?

I think the question is about the `p` and `q` parameters of method `f`, not about the loop iterator variables. — Andreas, Nov 08 '18 at 07:31

Andreas · Accepted Answer · 2018-11-08T07:46:03.423

How do p and q even come into the equation?

Assuming you're asking about the parameters to method f:
p = q + 1; is a meaningless statement, since Java is "pass-by-value".

Also, the following code is meaningless for the same reason, since x is reassigned to a new array, the changes to x doesn't affect the values in the caller.

x = new int[5]; // caller is unaffected by this
for (int i = 0; i < 5; ++i) {
    x[i] = 0; // meaningless, since new int[5] is pre-initialized to all zeroes
}

Last part of confusion is caused by the swap of x and y. Method f(x, y) is called as f(y, x) in method g, so f's x is really g's y.

To help differentiate them, let's rename parameters of f to a and b. The new code (removing meaningless statements and parameters) is then:

void f(int[] a, int[] b) {
    for (int i = 0; i < a.length; ++i) {
        a[i] = 2 * a[i];
    }
    b[3] = 5;
}

As you can see, it doubles the values of first argument, and changes 4th (index 3) value of second argument to 5.

Which means that method call f(y, x) in method g will double values of y and change x[3] to 5.

x = {1, 2, 3, 4} becomes x = {1, 2, 3, 5}
y = {9, 7, 6, 5} becomes y = {18, 14, 12, 10}

Which leads to printed output: 1 2 3 5 18 14 12 10

Thankyou so much, it was the sneaky swap of x and y that was throwing me off. — user431587, Nov 08 '18 at 08:00

Why does this function print what it does?

2 Answers2