Passing parameters for methods in Scala (alternatives to change variables values) - scala

Question

I need translate a Java code to Scala, but the compiler show me error. I understand that parameter input on methods are val type. Which alternative i can adopt if i need transform these values? I think to apply case class or class... Below the snippet code (in Scala):
def pmerge_FA(x: Pennant,y: Pennant): Pennant={
if(x == null && y == null && this.root == null){
return null
}else if(x == null && y == null){
return this
}else if(this.root == null && x == null){
return y
}else if(this.root == null && y == null){
return x
}else if(x == null){
y = y.pmerge(this) //error
null
}else if(this.root == null){
y = y.pmerge(x) //error
null
}else if (y == null){
y = this.pmerge(x) // error
null
}else{
y = y.pmerge(x)
this
}
}
Note that error is showed where y parameter is updated.
Thanks

Answer 1

Yes, the error is shown because you cannot reassign something to val, and parameters to methods in Scala are only sent as vals (immutables).
Because you don't provide the full definition of this, it's difficult to suggest an alternative solution, but:
In general, instead of if-else "Java" style, in Scala you can use pattern matching, and instead of null you can use Option, which is very powerful.
For Example, I suggest refactoring your method in this "Scala" Style (partial implementation)
def pmerge_FA(x: Pennant, y: Pennant): Option[Pennant] = {
(Option(x),Option(y), Option(this.root)) match {
case (None, None, None) => None
case (None, None, _) => Option("")
case (None, _, None) => Option(y)
case (_, None, None) => Option(x)
case (None, _, _) =>
....
}
}
Such that you will return the x, y as their new values, or create a case class like:
case class PennantCaseClass (x:Pennant, y:Pennant)
And returning it when needed.
Again, If you will provide some more info about Pennant class it will be easier to give a better alternative implementation for this method.

Answer 2

New values of y (namely y.pmerge(...)) are never used after assignments. So I guess all assignments y = y.pmerge(...) can be replaced with just invocations y.pmerge(...).
Does y.pmerge(...) do any side effects? Just in case, if not then values y.pmerge(...) are never used (only null or this is returned), so in such case lines y = y.pmerge(...) can be removed at all.
So the code can be either (if there are side effects)
def pmerge_FA(x: Pennant,y: Pennant): Pennant={
if(x == null && y == null && this.root == null){
null
}else if(x == null && y == null){
this
}else if(this.root == null && x == null){
y
}else if(this.root == null && y == null){
x
}else if(x == null){
y.pmerge(this)
null
}else if(this.root == null){
y.pmerge(x)
null
}else if (y == null){
this.pmerge(x)
null
}else{
y.pmerge(x)
this
}
}
or (if there are no side effects)
def pmerge_FA(x: Pennant,y: Pennant): Pennant={
if(x == null && y == null && this.root == null){
null
}else if(x == null && y == null){
this
}else if(this.root == null && x == null){
y
}else if(this.root == null && y == null){
x
}else if(x == null){
null
}else if(this.root == null){
null
}else if (y == null){
null
}else{
this
}
}

Answer 3

Oh, right! There are three classes to build a Bag data structure object, with nodes added in perfectly balanced tree. These methods works in that one. Below the complete code (in Java). The Pennant class build a forest using the node of graph object.
Node Class:
public class Node {
private Node left;
private Node right;
private int item;
public Node() {
left = null;
right = null;
item = 0;
}
public Node(int value) {
left = null;
right = null;
item = value;
}
public Node getLeft() {
return left;
}
public void setLeft(Node left) {
this.left = left;
}
public Node getRight() {
return right;
}
public void setRight(Node right) {
this.right = right;
}
public int getItem() {
return this.item;
}
public void setItem(int item) {
this.item = item;
}
}
public class Pennant{
private Node root;
public Pennant() {
this.root = null;
}
public Pennant(int value) {
this.root = new Node(value);
}
public void setRoot(Node root) {
this.root = root;
}
public Node getRoot() {
return this.root;
}
public Pennant pmerge(Pennant y) {
if(this.getRoot() == null) {
return y;
}else {
this.getRoot().setRight(y.getRoot().getLeft());
y.getRoot().setLeft(this.getRoot());
}
return y;
}
public Pennant pmerge_FA(Pennant x, Pennant y) {
if(x == null && y == null && this.getRoot() == null) {
return null;
}else if(x == null && y == null) {
return this;
}else if(this.getRoot() == null && x == null) {
return y;
}else if(this.getRoot() == null && y == null) {
return x;
}else if(x == null) {
y = y.pmerge(this);
return null;
}else if(this.getRoot() == null) {
y = y.pmerge(x);
return null;
}else if (y == null) {
y = this.pmerge(x);
return null;
}else {
y = y.pmerge(x);
return this;
}
}
public Pennant psplit() {
if(this.getRoot() != null && this.getRoot().getLeft() != null) {
Pennant y = new Pennant();
y.setRoot(this.getRoot().getLeft());
this.getRoot().setLeft(y.getRoot().getRight());
y.getRoot().setRight(null);
return y;
}
return null;
}
public void remove_all(Node node) {
if (node.getLeft() != null) {
remove_all(node.getLeft());
}
if(node.getRight() != null) {
remove_all(node.getRight());
}
node = null;
}
}