prepping for exams, need to clear some doubts

[half marlin]

I have my computer exam in a month, and as I study I might need to clear up some doubts.

-# Yes, I have a teacher, but she genuinely sucks.

[light lion]

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[half marlin]

idrk if this counts as java stuff but it must be related if its in our syllabus??? I think?????

but propositional logic

might be a stupid question idk but

if the inverse of an expression is the negation of the variables, is the inverse of a'→b' = a→b or a''→b''

I know they both mean the same thing but if you had to write the inverse what would you do????

[crisp idol]

Where did you get the inverse of an expression being the inverse of the variables?

Are you looking for the contrapositive? (a -> b)' <-> b' -> a'

[half marlin]

thats what weve got in the textbook and also what ive seen online though

[crisp idol]

ok ok

yeah I don't know the notation of calling that "inverse statement"

[crisp idol]

both are valid inverses of the expression, they are just different ways of saying the same thing

but typically you'd just remove double negation

[half marlin]

right okay

[crisp idol]

but if your prof wants you to write it down explicitly in a certain way, you should do that in exams

[half marlin]

thanks

[half marlin]

its a board exam and not in school so im going with whatevers generally acceptable

[light lion]

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[half marlin]

why isnt the object call working here???

[crisp idol]

What is ob?

[half marlin]

object of the class Automorphic

[crisp idol]

Can you show the whole class in a codeblock?

[half marlin]

it should be working cuz the main method calling void list does

[light lion]

Please format your code & make it more readable.
For the java programming language, it should look like this:

```java
public class Main {
public static void main(String[] args){
System.out.println("Hello World!");
}
```
• These are backticks, not quotes.

[half marlin]

import java.util.*;
public class Automorphic
{
    int l, u, count;
    public Automorphic(int l, int u)
    {
        Scanner in = new Scanner(System.in);
        System.out.println("Enter the lower limit of the range->");
        l = in.nextInt();
        System.out.println("Enter the upper limit of the range->");
        u = in.nextInt();
    }
    boolean check(int n)
    {
        int sq = (n*n)/10;
        if (sq==n)
            return true;
        else
            return false;
    }
    void list()
    {
        System.out.println("Number" + "\t" + "Square");
        for(int i = l; i<u; i++)
        {
            boolean auto = ob.check(i);
            if(auto==true)
            {
                System.out.println(i + "\t" + i*i);
                count=+1;
            }
        }
    }
    public static void main(String args[])
    {
        Automorphic ob = new Automorphic(0, 0);
        ob.list();
    }
}

[crisp idol]

oh I see

If you create an ob variable in main, it will only be valid in main

but you call ob.list (so you call list on ob) so list has access to ob using the keyword this

so you could do this.check(i)

or just check(i)

[half marlin]

this is so weird man

mostly because I didnt learn it but still

[crisp idol]

Whenever you are not in a static method, you have a "current" object (that is typically called the "receiver object")

and this just refers to that

main is static so there is no current/receiver object

there you create an Automorphic instance and call list on it so that instance is the receiver object within the list method

If you want to call another method on the current receiver object (in the same class), you can just do nameOfTheOtherMethod() or this.nameOfTheOtherMethod()

[half marlin]

does just nameOfTheOtherMethod() work in every instance where I might otherwise use this?

[crisp idol]

yes pretty much

[swift gate]

Hmmm, what happens, if i enter 33 for the lower and 2 for the upper bound?
And why do you need the count variable?

[crisp idol]

no numbers being checked?

[swift gate]

You know Java Danny?

[crisp idol]

sometimes

[swift gate]

Almost like i asked that question for Piglet to think about her code a little bit, bc in my exp that helps with learning 😁

[half marlin]

idk, they didnt ask to check if the numbers were valid in the question so I dont have to check it

besides theres something wrong with it anyway

im not sure why but it seems like the for loop in list wont execute

also they asked for a count variable so yeah

import java.util.*;
public class Automorphic
{
    int l, u, count;
    public Automorphic(int l, int u)
    {
        l=l;
        u=u;
        count=0;
    }
    boolean check(int n)
    {
        int sq = n*n;
        if (sq%10==n)
            return true;
        else
            return false;
    }
    void list()
    {
        System.out.println("Number" + "\t" + "Square");
        for(int i = l; i<u; i++)
        {
            System.out.println("y");
            if(check(i))
            {
                System.out.println(i + "\t" + i*i);
                count=+1;
            }
        }
        System.out.println("Frequency of Automorphic numbers between " + l + "and" + u + ":" + count);
    }
    public static void main(String args[])
    {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter the lower limit of the range->");
        int l = in.nextInt();
        System.out.print("Enter the upper limit of the range->");
        int u = in.nextInt();
        System.out.println();
        Automorphic ob = new Automorphic(l, u);
        ob.list();
    }
}

the print y thing was just to see if it was ever running

I dont get it man

like, the sample inputs we're supposed to try are 1 and 1000

so int i = 1; 1<1000; i++

why does that not work??

I thought maybe it was the same issue with ob as earlier and tried it with this but that didnt change anything

okay so I entered exactly int i = 1; 1<1000; i++ to check and that worked

but the print statement after the for loop for frequency also prints them as 0 for whatever reason so there must be something to that ig

I think maybe??? making list parameterized would work????? but I dont think im allowed to do that since its specified as a non-parameterized method

[crisp idol]

you have an int l local variable

and an int l field

these are different variables

The field is for non-static methods

the local variable is valid only in the main method

[half marlin]

but are the input values not getting assigned in the constructor

am I supposed to use this.l

ok no that didnt work either

[crisp idol]

oh I see

I didn't notice the constructor

you are doing l=l

so you are setting the parameter l to the parameter l

If you want to set the field, you need this.l

or name the variables differently

[swift gate]

Fair 👍

[swift gate]

But you already use i for that, no?^^

[half marlin]

eh no i is a looping variable to check each number within the range

[swift gate]

Ah fair

[swift gate]

If you want to debug why the loop doesn't work, you could just print l and u at the start of the list method ^^

[half marlin]

im gonna try doing another question now 🙏

[half marlin]

the first frequency thingy is within the constructor, the second is the one in the list method, I added this.l but that didnt change anything

oh eait wait wait

I tried another thing

instead of l = this.l I made it this.l = l and that seems to have worked

okay so now the essence of the program is working but its not enough because I have to check more than just one digit???

[swift gate]

Yeah, sry i didn't check the constructor.
If you have an input parameter "l" in a method and in general a field "l", then just "l" will always refer to the input parameter in the method. Therefore l=l means you asign the input parameter to the input parameter. l=this.l means you asign the value of the field to the input parameter. What you want to do is asign the value of the input parameter to the field so it gets stored in for later usage in other methods => this.l=l

Hope that helps^^

[half marlin]

what if I count the digits in the number, like 25 = 2, so then it could be a for loop to divide until that many digits are in the thing? I feel like that would work best with a recursive method but I dont think were allowed to add more methods? I thought otherwise like, maybe a while loop to multiply 10 by how many ever digits are there in the number???

okay yay im so smart I did it

im gonna actually fail and die if it takes me this long to do one program though

itll be a written exam, I cant go back and change too much or check for errors with a compiler

[swift gate]

Relax, if you code something for the first time it is pretty normal to take twice as long, as you later need in the exam 🙂

[swift gate]

Even under that circumstance^^

[light lion]

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[half marlin]

what am I supposed to do about this

import java.util.*;
class Anagram
{
    String s1, s2;
    int l1, l2;
    Anagram(String s1, String s2)
    {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter first word->");
        this.s1 = in.next().toLowerCase();
        System.out.println();
        System.out.print("Enter second word->");
        this.s2 = in.next().toLowerCase();
        System.out.println();
    }
    void sort(char c[])
    {
        char temp = ' ';
        for(int i = 0; i<this.s1.length(); i++)
        {
            for(int j = 0; j<this.s1.length(); j++)
            {
                if(c[i]>c[j])
                {
                    temp = c[i];
                    c[i] = c[j];
                    c[j] = temp;
                }
            }
        }
    }
    boolean areAnagram(char[] str1, char[] str2)
    {
        str1 = sort(str1);
        str2 = sort(str2);
        for(int i = 0; i<s1.length(); i++)
        {
            if(str1[i]!=str2[i])
                return false;
        }
    }
    void check()
    {
        char[] str1 = new char[s1.length()];
        for(int i=0; i<s1.length(); i++)
            str1[i] = s1.charAt(i);
        char[] str2 = new char[s2.length()];
        for(int i=0; i<s2.length(); i++)
            str1[i] = s1.charAt(i);
        boolean a = areAnagram(str1, str2);
        if(a==true)
            {
            System.out.println(s1 + " and " + s2 + "are anagram of each other");
            }
        else
            System.out.println(s1 + " and " + s2 + "are not anagrams of each other");
    }
    public static void main(String args[])
    {
        Anagram ob = new Anagram("first", "second");
        ob.check();
    }
}

[short solstice]

sort(char[]) sorts the given array in place and does not return anything.

[silent needle]

yes

so the array you provided is sorted

[swift gate]

Why are you using a Scanner in the constructor btw? Kinda sketchy

[half marlin]

sketchy? 😭

I feel like its convenient

[swift gate]

It really isn't.

Anagram(String s1, String s2) {
    this.s1 = s1;
    this.s2 = s2;
}

Btw, general question, why do you never add visibility modifiers to your fields and methods?
Did they not teach you private, package private, protected, public?

[half marlin]

1. the way our questions are given, we have to make our methods exactly as instructed
2. I also honestly have no real idea what most of those thingies do, like, maybe some vague understanding of it but not enough to think too hard on what the limits and uses are and to use them myself

[swift gate]

I also wonder, why you don't do the anagram check in the constructor, does the exercise directly discourage that?

[half marlin]

wait im just gonna show you the question

[swift gate]

1. I doubt that they tell you to write a constructor with two input parameters just to never use them but a scanner instead.
2. Your teachers seem to bad at their job.

[half marlin]

to 2, my last years teacher taught me enough to be able to code well enough for what we have to do (which is pretty great since she got me getting grades above that of my classmates who had studied it for 2 years prior while I was learning it for the first time) but my teacher this year...yeah she sucks

[swift gate]

There is nothing said about Scanners, but yeah, the check shouldn't be called in the constructor, my bad.
They really don't tought you visibility... man this sounds like a lousy course

[half marlin]

oh no its not the course thats bad

we do have it in our syllabus

I just need to catch up on that part

thing is since this is how all the programming questions are asked, I didnt need to know it cuz those specifiers at most come for like 1 mark questions

[swift gate]

The materials kinda imply a bad "teaching order". Or to be more direct: I think they should use C or Python to teach you the kind of stuff you asked so far^^

[swift gate]

Ahhhh, so they teach you the object oriented stuff later? Still a bit weird, but that would make sense

[half marlin]

nonono all of the stuff is there we have a couple of chapters on that

its just not given much weightage in the exam paper

[swift gate]

https://tenor.com/view/facepalm-really-stressed-mad-angry-gif-16109475

[half marlin]

like, theyd just ask maybe a 2m question like "what is the purpose of the keyword private" maybe

[half marlin]

perhaps, I wouldnt know tbh

[swift gate]

Yeah cool, but why do they not tell you to use those keywords, since no visibility is bad coding style xD

[swift gate]

Jup, i don't blame you. I just get annoyed at bad teachers.

[half marlin]

so valid ive spent all year complaining to any and all about these people

[half marlin]

is it? 👀

[swift gate]

Yes...

[half marlin]

do they make videos/guides on what good coding style is? like I know from the whole yandere simulator debacle that spaghetti code exists but besides just:

• weird logic
• low scope for adding further content or easily making changes
• being barely understandable to other coders
  idrk whats "bad"

[swift gate]

Heh...

• What do you mean by weird logic?
• It surely is easier if code doesn't contain, 30000 nested scope, but that is not a "hard" coding convention.
• That is why documentation exists.

[half marlin]

I forgor that scope was a java thing in and of itself

dont know what it is but its there

[swift gate]

Scope is a thing for every imperative programming language i know😅

[swift gate]

Scope is basically the space between { and }

[half marlin]

I think weird logic is like...excessively convoluted ways of achieving the programs desired output? like for example, you need to extract each digit from a number and youre converting it into a string and using a method something (String x, int y) to assign it to entirely seperate char variables (as opposed to using an array) and then converting it into integers to do whatever else you need to

sure it could work and im all for "if it works, it works" but afik coding logic is supposed to be simple and efficient above all

[swift gate]

Btw, did you know the String method toCharArray()

[half marlin]

no way

thats so cool

[half marlin]

ah okay

[swift gate]

Yeah, that is all fair, but isn't what i meant by "coding conventions". Dw about that now though, since the exam doesn't seem to give a shit either xD

[half marlin]

then what do you mean?

[swift gate]

Optimization comes after the initial development. First get your stuff running, then think about making it better.

[swift gate]

For example:

• Formatting guidelines
• Using the correct keywords explicitly
• Catching exceptions
  Stuff like that^^

[half marlin]

ahh okay

[swift gate]

There was a typo in your check btw

Btw, why are you using s1.length() in sort, when you wanna sort the c

And your areAnagram method is vunerable, bcs it doesn't check, whether the input parameters are of equal length

[swift gate]

Man this exercise is also so flawed from an object oriented design perspective, but whatever...

[half marlin]

theyre both the same size tho

[half marlin]

I know, I was wondering if I should check that

[half marlin]

dont know what you mean but okay lol

[swift gate]

Lemme write you a main that disproves that^^

[swift gate]

Ye, check it

[swift gate]

public static void main(String[] args) {
    Anagram a = new Anagram("hello", "elloh");
    a.sort("cba".toCharArray());
}

Execute that and see what happens

[half marlin]

I will but wait

I never solved the issue in the first place

[swift gate]

🤷‍♂️ 😂

[half marlin]

^

yeah that didnt work 😭

wait a second

I dont need to assign values

that works

[swift gate]

It would have if you changed the void in void sort.

[swift gate]

Jup

That is the other option

[half marlin]

okay now theres a logic error -_- ill try to fix this

oh shit I forgot to use the instance variables in the question

[swift gate]

No, sets and test are indeed not anagrams

[half marlin]

OH

right

okay but it doesnt work still

there can only be an issue where im comparing the words after sorting

[silent needle]

skibidi

[half marlin]

okay ah. some weird shit is happening to the second array

[silent needle]

wait can u show the code

[half marlin]

wow I have never seen that thing before

[silent needle]

oof cant open on mobike

mobile

[half marlin]

ah okay wait

[silent needle]

i think the sorting part doesnt work

[swift gate]

Yes the sorting is cooked

j should be i + 1 instead of 0

Initially

[half marlin]

still doesnt change that null error

[swift gate]

Yeah, bcs you still have the typo in check

[half marlin]

I fixed that

I need to make assigning it to each string possible somehow

[swift gate]

This seems over complicated, but as long as it works 🤷‍♂️

[half marlin]

its so sad this program had to take so long 😔

the one after this took me only like 15mins or something

and it was actually fully functional

[swift gate]

Happens :/

[light lion]

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[crisp idol]

either make sort return the sorted array or just do sort(str1) without the str1 =

[half marlin]

yeah but then the next part doesnt work

[crisp idol]

hm?

[half marlin]

thinking about this made me realize that there would probably be a reverse function, turning a character array into a string and I looked it up and apparently you can do this: String str1 = new String(ch_arr);, which I think would work out

[half marlin]

idk that was how the code was before and it didnt work so I changed it to that

wait no

hgh whatever

@swift gate imma try the code you sent me now

[light lion]

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[half marlin]

okay so apparently acc to a quick google search you can use multiple constructors but why??? I thought that maybe it was a mistake but its in the answer key too so im not sure

[short solstice]

In the end constructors are also just functions with an implicit return value.

[half marlin]

ugh okay theyre doing something here I dont know yet and im gonna just ask my friend to teach it to me later

[crisp idol]

If you declare multiple constructors, you can choose one of them when creating an object of the class

[swift gate]

You can also do that with every other method. There can be like a bazillion "print" methods for example, as long as they have different input parameters

So called Polymorphism

[half marlin]

OHHHHH we have that thing in our syllabus

thats good to know cuz I dont know it besides having learnt the basic concept of it and earlier I was slightly worried my friend might not be familiar with the thing either

[swift gate]

The thing is Polymorphism is more than that

Interfaces and inheritance can be interpreted a polymorphic concept aswell

[half marlin]

yeah ik the general idea and vaguely remember the syntax and all but not enough to use it

[vale coral]

Good luck for your exams!

Anything I could answer too, I was looking to test myself by trying to explain stuff if I knew it

[light lion]

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[half marlin]

if theres an instance variable whose value im changing in a recursive void method when I use it in another void method will I get the original or changed value

[crisp idol]

If both are called on the same instance, they share the fields/instance variables

[crisp idol]

If a non-static method calls another non-static method in the same class without explicitly specifying a receiver object/instance (so when you are doing someMethod() instead of someObject.someMethod()), that is the case

[half marlin]

right okay thanks