Assumed Knowledge
Learning Outcomes
  • Understand the difference between the value semantics of atomic data and the reference semantics of compound data
  • Understand the consequences for programming of reference semantics.
  • Understand how passing arrays to functions is a case of reference semantics

We discussed in the compound data topic how compound data can’t be stored directly in its memory slot. Instead, that slot holds a reference to some larger chunk of memory where the data is stored.

This different storage strategy leads to different behaviour for variables of these compound types. We call the old way (the behaviour for int, char, and any other value that fits in its memory slot directly) value semantics and we call the new behaviour (for arrays and objects) reference semantics.

There are no new rules to remember, but the existing rules result in things happening a little differently.

First, let’s be reminded of all the ways a variable can be assigned a value:

  • x = 6: the value 6 is put in the memory slot named x.
  • y = x: the value stored in the slot named x is copied to the slot named y.
  • foo(6): the value 6 is copied to the memory slot corresponding to the formal paramter of foo.
  • foo(x): the value stored in the slot named x is copied to the memory slot corresponding to the formal paramter of foo.

The new and interesting thing is what gets copied when the value stored in the slot is a reference?. Well, you can guess I am sure. The reference gets copied. Importantly, the larger chunk of data at the end of the reference does not get copied. So, when the value is in the slot and the value gets copied, we call it value semantics. When the reference is in the slot and the reference gets copied, we call it reference semantics.

The underlying concept is simple enough, now lets see some consequences that flow from it.

Arrays and functions

It’s very common to pass an array to a function. Generally, we operate on an array passed to a function, but sometimes it’s also useful to,

  1. modify the contents of the passed array inside the function
  2. return an array from the function that is copied into an array in the caller.

Operating on an array passed to a function

When you pass an array to a function, a shallow copy of the actual parameter is made into the formal parameter. For example, in the following code, data becomes a shallow copy of arr.

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void caller() {
    int[] arr = {10, 70, 50};
    int sum = total(arr);
}

int total(int[] data) {
    int result = 0;
    for(int i=0; i < data.length; i++) {
        result+=data[i];
    }
    return result;
}

Modifying contents of array passed

If you have a function that modifies the passed array, the contents of the actual parameter will also change.

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void caller() {
    int[] arr = {10, -70, 0};
    negate(arr);
}

void negate(int[] data) {
    for(int i=0; i < data.length; i++) {
        data[i]=data[i]*-1;
    }
}

Returning an array from a function

If a function returns an array (source) and the caller copies it into an array (destination), it’s a shallow copy as demonstrated in the following example.

If you have a function that modifies the passed array, the contents of the actual parameter will also change.

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void caller() {
    int[] data = getDiceOutcomes(5);
}

int[] getDiceOutcomes(int n) {
    int[] outcomes = new int[n];
    for(int i=0; i < outcomes.length; i++) {
        outcomes[i] = (int)random(1, 7);
    }
    return outcomes;
}

Multi-Dimensional Arrays

All this also means it is possible to have an array of arrays. These are called multi-dimensional arrays. For two dimensions, they are declared as

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int[][] multiDArray;

and when you do memory allocation you must give the size of the outer array and the size of all the inner arrays (in this case 2 elements in the outer array and 3 in each inner array)

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multiDArray = new int[2][3];

Resulting in the following occuring in memory

0 0 0 0 0 multiDArray 0

Note that each slot of the outer array contains everything you need for another array, the reference to the chunk of memory and the chunk in some other spot. The actual integers are stored in the secondary chunks. This means you need two slot numbers to get to any particular value. The first will get you the slot in the outer array and sthe second will get you the slot in that inner array. For example, here is what happens when I run

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multiDArray[1][2] = 4;
0 0 4 0 0 multiDArray 0

When you work over these arrays, you need nested loops. The outer will loop over the outer array and the inner loop will run all the way through for each inner array.

You can traverse a two-dimesional array arr as:

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for(int i=0; i < arr.length; i++) {
  //here you have access to sub-array at index i: arr[i]
  /since arr[i] is the array, arr[i].length gives number of items in that array
  for(int k=0; k < arr[i].length; k++) {
    //here you have access to item k inside arr[i], that is arr[i][k]
  }
}

Exercises

Exercise 1

Define a function that when passed an integer array, returns the sum of all even numbers in the array.

Exercise 2

Call the function defined in exercise 1 with an array of your choice and store the value returned in a variable result.

Exercise 3

Define a function that when passed two integer arrays, returns true if they are identical (same items in the same order), false otherwise.

Exercise 4

Define a function that when passed an integer array, returns true if all items in the array are positive, false otherwise.

Exercise 5

Define a function that when passed an integer array, returns true if there is any positive item in the array, false otherwise.

Exercise 6

Define a function that when passed a floating-point (double) array, returns an integer array containing the integer part of the items of the passed array. For example, if the array passed is {-3.6, 0.8, 7.2, -2.5}, the array returned should be {-3, 0, 7, -2}.

Exercise 7

Define a function that when passed an integer array, returns an array containing the positive items from the array. For example, if the array passed is {1, -3, 0, -5, 7, 0, 0, 0, 2, 9}, the array returned should be {1, 7, 2, 9}.

Exercise 8 (Advanced)

Define a function that when passed two integer arrays, returns true if they are the same (same items but not necessarily in the same order), false otherwise.

Coding in the real world

SCENARIO 1

Define a function that when passed an array containing outcomes of a series of dice rolls (6-faced dice), returns the number of times a 6 was rolled.

SCENARIO 2

Define a function that when passed an array containing weekly salaries of a person, returns the amount of tax that should be withheld based on the following payroll department’s withholding rules,

  • income up to $300 a week is tax-free
  • income from $301 to $500 a week is taxed at 15%
  • income from $501 to $800 a week is taxed at 20%
  • income from $801 to $1000 a week is taxed at 25%
  • income from $1001 to $1500 a week is taxed at 30%
  • income over $1500 a week is a week taxed at 35%

For example, if the array holding salaries is {1200, 600}, the first week’s salary is taxed

0.15*200 + 0.2*300 + 0.25*200 + 0.3*200 = 200

and the second week’s salary is taxed

0.15*200 + 0.2*100 = 50

Total tax: 250.

SCENARIO 3

Time to time, we get a list of students and their marks. The list of students is in a String array, while the marks are in an integer array. Obviously, the size of both the arrays is the same. Also, the name of the first student is at index 0 of the names array and his/her marks are at index 0 in the marks array and so on.

Define a function that returns the name of the student with the highest mark. For the basic version, in case of a tie, return the name of the first student with the highest mark. For the advanced version, in case of a tie, return an array with the names of all the students who have the same highest mark.

SCENARIO 4

During an experiment, our data was corrupted. While the valid values should be between 0 and 100, some values were inserted that are outside this range and therefore invalid.

Define a function to which we can pass an array containing the unfiltered values and it returns an array with the filtered values. For example, if the array passed is {4, -1, 0, 89, 105, 67, 100}, the method should return the array {4, 0, 89, 67, 100}.

SCENARIO 5

Define a function that returns the number of distinct values in the array passed. For example, if the array passed is {3, 7, 3, 3, 3, 20, 9, 20}, return 4 as there are four distinct values (3, 7, 20 and 9).

SCENARIO 6

Define a function that when passed an integer (store the value passed in the formal parameter n), returns an array that holds the first n prime numbers (a number is prime if it is more than or equal to 2 and is divisible by only 1 and itself).