Inheritance/ IO

[vapid crane]

        String [] xy = new String[2];

        Scanner scan = new Scanner(name);
        for (int i = 0; i < 2; i++) {
            xy[i] = scan.next();
        }
        rows = Integer.parseInt(xy[0]);
        columns = Integer.parseInt(xy[1]);
        
        double [][] tempArray = new double[rows][columns];
        
        while(scan.hasNext()) {
            scan.next(scan.nextLine());
        }

        for(int i = 0; i < rows; i++) {
            for (int j = 0; j < columns; j++) {
                tempArray[i][j] = Double.parseDouble(scan.next());

            }
        }

        super(tempArray);


    }

    protected double[][] array;
    protected int rows;
    protected int columns;

    //Creates an empty matrix.
    public Matrix(int row, int column) {
        double[][] matrix = new double[row][column];

        this.array = matrix;
        this.rows = row;
        this.columns = column;

    }
    //Creates a matrix using an array.
    public Matrix(double[][] matrix) {
        double [][] tempArray = new double[matrix.length][matrix[0].length];
        for(int i = 0; i < matrix.length; i++) {
            for(int j = 0; j < matrix[i].length; j++) {
                tempArray[i][j] = matrix[i][j];
            }
        }
        this.rows = matrix.length;
        this.columns = matrix[0].length;
        this.array = tempArray;
    }

Umm.. so basically for an assignment I have in school I have to read and write an array from a file. I have certain requirements that I must follow. The first bit of code is under a child class "FileMatrix" and the second bit is the parent class "Matrix". I'm not fully sure if what i wrote even works. It is supposed to do as follows: read the first two digits from the file, set those as the rows and column length, then read the rest of the file and insert those values into the new found array. I can't even test this though because my "super (tempArray);" doesn't work. It says "Constructor call must be the first statement in the constructor."

If anyone could help me that would be great, im just really lost haha

Thanks!

[ruby ibex]

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[fervent gorge]

sounds like you would use a static from for this, rather than a constructor

[vapid crane]

sadly it has to be a constructor...

i'm not even sure how to do the rest of it, but im tryna start somewhere haha

I've done other parts to this assignment, however this is like the core of the assignment

[fervent gorge]

you'd probably use the Matrix(int, int) constructor then

you can get the first 2 ints immediately for super

the FileMatrix(File) would use the FileMatrix(Scanner) constructor, like it says, by making a Scanner from the File

[vapid crane]

how would you suppose I make the FileMatrix(File) use the FileMatrix(Scanner) , i guess im unsure how to actually make 1 method use the other

I just tried changing the super(tempArray); => super(rows, columns); but its the same message

Also I appreciate your help 🙂 it means a lot

I also just realized something... super() has to be the first statement in the constructor... i finally understand haha, but how am I supposed to find the values of "rows" and "columns" before super(rows, columns) if super has to be first?

[fervent gorge]

you can use this to reference another constructor just like super

[fervent gorge]

it just has to be the first statement, it doesn't have to be the first expression or function call. with a scanner, you can do super(input.nextInt(), input.nextInt())

that's why the FileMatrix(File) constructor uses the FIleMatrix(Scanner) constructor

[vapid crane]

but if I start my super like that, the scanner wouldn't be open already though? so would the input.nextInt even work?

[fervent gorge]

idk what you mean by that

[vapid crane]

one moment ill ss

[fervent gorge]

this super call needs to be in FileMatrix(Scanner), where a Scanner already exists

the FileMatrix(File) constructor creates a Scanner, and possibly populating the matrix; you could populate the matrix in either constructor

[vapid crane]

I gotchu

that makes sense

Im sorry i feel like you're just giving it all to me but i appreciate it. Under FileMatrix(File); are you saying that in my initial "super()" I would try and make it access the FileMatrix(Scanner)? Like do I open a scanner within the super() under FileMatrix(File)

because FileMatrix(File) needs "super()" as well

[fervent gorge]

you wouldn't have a super call in FileMatrix(File)

[vapid crane]

I thought super was required

under all child class constructors

[fervent gorge]

you have to delegate to a super constructor somehow, and that can be through another constructor of the same class

[vapid crane]

ohhhh thats important to know

haha

[fervent gorge]

so each constructor needs to have either this or super call (unless there exists a 0-arg constructor in the parent)
a super call is just delegating to a super constructor directly
a this call is delegating to another constructor, which in turn must delegate to a super constructor itself

to clarify;
this refers to the current class
super refers to the parent class
this is usually a superset of super, having more stuff, due to inheritance, and super can be used to exclude overridden things to get the parent's definition
however, constructors are not inherited, so this() and super() don't overlap

[vapid crane]

Okay okay its starting to make sense

hm I see that "this()" also needs to be the first statement however,...

    public FileMatrix(Scanner sc) {
        super(sc.nextInt(), sc.nextInt());


    }

    private FileMatrix(File name) {
        
        Scanner scan = new Scanner(name);
        this(scan);

        double [][] tempArray = new double[rows][columns];
        
        while(scan.hasNext()) {
            scan.next(scan.nextLine());
        }

        for(int i = 0; i < rows; i++) {
            for (int j = 0; j < columns; j++) {
                tempArray[i][j] = Double.parseDouble(scan.next());

            }
        }

So my this(scan) doesnt work haha

also I tried created a scanner inside of this() and didnt work

[fervent gorge]

how come?

[vapid crane]

how come I tried to make the scanner inside of this()?

or why wont it work?

[fervent gorge]

what's wrong with it

it should work

[vapid crane]

"Scanner cannot be resolved to a variable"

this(Scanner sc = new Scanner(name));

thats what i tried

name is the file name

[fervent gorge]

you can't have a declaration as an expression

Type name is a declaration, it creates the variable name

for local variables, they're exclusively statements, they can't be used as expressions

the value new Scanner(name) is an expression that evaluates to the Scanner instance we need

(also btw name isn't a good variable name for a File, you could use file like the description says; similarly for the Scanner, for consistency)

[vapid crane]

Im not sure I fully understand what you mean

or at all lol

[fervent gorge]

this is kinda getting deep into the specifications of java as a language so... yeah i wouldn't expect you to get it first try tbh, lol. don't worry about it

[vapid crane]

haha

"the value new Scanner(name) is an expression that evaluates to the Scanner instance we need" this line especially makes no sense to me

[fervent gorge]

basically there are 2 main pieces of code in any language, statements and expressions
statements do things, expressions are values
some pieces of code can be either statements or expressions depending on the context in which they're used
(some code won't fit into statement vs expression, like declarations for some languages, including java; statements and expressions are afaik universal though, while declarations are not)

statements do things, they're the verbs of code. in java, they always end with ;, except for flow and block statements (we can get into this later)
so statements are the main body of the code; things like super(); for example, as you've seen in the error, it has to be the first statement in a constructor

[vapid crane]

okok I gotchu, statements do things, expressions hold values

so are variables expressions then?

[fervent gorge]

expressions are nouns, they have values, things like 5, 1 + 1, new Scanner() are all expressions
these can be used as parts of statements, to form more complete sentences, in a way

although i suppose the statement is the sentence itself

[fervent gorge]

variables have a lot of forms, so they aren't one or the other.

[vapid crane]

kk

[fervent gorge]

the variable itself, say sc or tempArray, are expressions, they don't do anything so they can't be statements

but creating or assigning to the variable are indeed actions, so double[][] tempArray; or Scanner scan = new Scanner(name); are statements

[fervent gorge]

specifically those, are declarations, as mentioned here

[vapid crane]

kk

i see

[fervent gorge]

declarations, at least in java, can't be expressions.

but plain assignments, like line = sc.nextLine() maybe, can be either statements or expressions. in their expression form, they have the value of the thing being assigned, so for example x = 5 evaluates to 5, doing int x; System.out.println(x = 5); would print 5

[vapid crane]

alright

hmm

[fervent gorge]

since declarations are statements, and you can't have statements before super/this, you would have to just use the value itself, new Scanner(file), since you can't create a new variable.

oh also, with knowing statements and expressions, you can define the syntax for various statements in more formal terms, which can let you see the full capabilities sometimes
for example, declaring a variable is

Type name;
Type name = expr;
```(simplified, there's more than this)

[vapid crane]

Ok i wrote that and there's no errors. However im confused lol. everything you said makes sense, its just "new Scanner(file)" If I havent initialized Scanner, new Scanner does that I guess, but doesn't assign it a varibale name..

[fervent gorge]

you have initialized a Scanner, you just haven't put it into a local variable

[vapid crane]

which the other constrcutor would do then?

I initialized it, but Im allowing the other constructor to assign it the local variable "sc"

?

[fervent gorge]

just new Scanner(file); is also both a statement and an expression; it does a thing, it creates an instance of the class
when used as an expression, the value is the object that was created (or more accurately, a reference to the object)
so with Scanner sc = new Scanner(file);, that's doing 3 things;

• creating a variable sc of type Scanner
• creating a new Scanner instance
• assigning that instance to the variable sc

[fervent gorge]

you wouldn't use assignment for the scanner anymore. the FileMatrix(Scanner input) constructor has it as a parameter, so when you do this(new Scanner(file)); that input is assigned to the Scanner instance that was passed for you

[vapid crane]

yeah thats what I mean I think

[fervent gorge]

just to be clear, the process for parameters isn't "assignment", it just acts like it

it's generally just called "passing arguments"

[vapid crane]

this(new Scanner(file)) that goes over to the constructer FileMatrix(Scanner sc) which makes the "new Scanner(file) assigned to the variable name "sc"

[fervent gorge]

/run

class X {
  X() {
    this(1);
  }

  X(int i) {
    System.out.println(i);
  }

  public static void main(String[] args) {
    new X();
  }
}

[night crystal]

Here is your java(15.0.2) output @fervent gorge

1

[fervent gorge]

yep

[vapid crane]

see this makes perfect sense lol, I've been doing this all year essentially

It's just in school rn, we jumped from that^ to what im doing now

it was such a big jump, all my peers are so thrown off as well

everything you've taught me will be passed down to some class mate haha

well I have some writing to do now, and see if I can figure more out. Thanks a lot 🙂 im gonna see what i can do with this knowledge

bro you are blessed af

with your help Im now able to create the entire array

and print it

now I have other things to do in the assignment haha but you were blessed