Please point out my flaw/s ?

Question about the task

Playing Javarella

Java Syntax, Level 7, Lesson 9

Under discussion

Can't get it

1. Enter 20 numbers from the keyboard, save them in a list, and then sort them to three other lists:
Numbers divisible by 3 (x%3==0), numbers divisible by 2 (x%2==0), and all other numbers.
Numbers simultaneously divisible by 3 and 2 (for example 6) go into both lists.
The order in which the lists are declared is very important.
2. The printList method should display each list item on a new line.
3. Using the printList method, display these three lists. First, the list for x%3, then the list for x%2, and then the last list.

Requirements:

Declare and immediately initialize 4 ArrayList<Integer> variables. The first list will be the main one. The other lists will be supplementary.
Read 20 numbers from the keyboard and add them to the main list.
Add to the first supplementary list all numbers in the main list that are divisible by 3.
Add to the second supplementary list all numbers in the main list that are divisible by 2.
Add to the third supplementary list all the remaining numbers from the main list.
The printList method should display each element of the passed list on a new line.
The program should display the three supplementary lists using the printList method.

package com.codegym.task.task07.task0713; import java.io.BufferedReader; import java.io.InputStreamReader; import java.util.ArrayList; import java.util.List; /* Playing Javarella */ public class Solution { public static void main(String[] args) throws Exception { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); ArrayList<Integer> Main = new ArrayList<>(); ArrayList<Integer> By3 = new ArrayList<>(); ArrayList<Integer> By2 = new ArrayList<>(); ArrayList<Integer> ByBoth = new ArrayList<>(); for (int i = 0; i < 20; i++) { Main.add(Integer.parseInt(reader.readLine())); } for (int i = 0; i < Main.size(); i++) { Main.get(i); if (Main.get(i) % 3 == 0 || Main.get(i) % 2 == 0 && Main.get(i) % 3 == 0) { By3.add(Main.get(i)); } else { ByBoth.add(i); } } for (int i = 0; i < Main.size(); i++) { Main.get(i); if (Main.get(i) % 2 == 0 || Main.get(i) % 3 == 0 && Main.get(i) % 2 == 0) { By2.add(Main.get(i)); } else { ByBoth.add(i); } } System.out.println("Divisible by 3 \n"); printList(By2); System.out.println("Divisible by 2 \n"); printList(By3); System.out.println("Remaining \n"); printList(ByBoth); } public static void printList(List<Integer> list) { for (int j: list ) { System.out.println(j); } } }

Comments (7)

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Guadalupe Gagnon

Level 37 , Tampa, United States

25 January 2019, 16:54

Numbers are getting put in a whole bunch of weird places. Remove line 28 through 38 and replace it with the code ill post below. It is your code unchanged, but I added a bunch of sys.out statements so that there is visual of what is going on. Run it and once it is done go back through the output to see how your code is handling things. Here is the code:

for (int i = 0; i < Main.size(); i++)
{
    System.out.print("Loop " + i+": ");
    System.out.print("Checking: "+Main.get(i)+ " (is " + Main.get(i) + " mod 3 == 0? "+(Main.get(i) % 3 == 0));
    System.out.print(" OR is "+ Main.get(i) + " mod 2 == 0? "+(Main.get(i) % 2 == 0));
    System.out.print(" AND is " + Main.get(i) + " mod 3 == 0? "+(Main.get(i) % 3 == 0)+")? ");
    System.out.println((Main.get(i) % 3 == 0 || Main.get(i) % 2 == 0 && Main.get(i) % 3 == 0));
    if (Main.get(i) % 3 == 0 || Main.get(i) % 2 == 0 && Main.get(i) % 3 == 0) {
        System.out.println("adding to list By3");
        By3.add(Main.get(i));
    }
    else
    {
        System.out.println("adding to list Byboth");
        ByBoth.add(i);
    }
}

Use this to diagnose where your problem lies.

Joy Majumdar

Level 16 , Kolkata, India

31 January 2019, 05:00

package com.codegym.task.task07.task0713; import java.io.BufferedReader; import java.io.InputStreamReader; import java.util.ArrayList; import java.util.List; /* Playing Javarella */ public class Solution { public static void main(String[] args) throws Exception { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); ArrayList<Integer> Main = new ArrayList<>(); ArrayList<Integer> Divby3 = new ArrayList<>(); ArrayList<Integer> Divby2 = new ArrayList<>(); ArrayList<Integer> Rem = new ArrayList<>(); for (int i = 0; i < 20; i++) { Main.add(Integer.parseInt(reader.readLine())); if (Main.get(i) % 3 == 0 && Main.get(i) % 2 == 0 || Main.get(i) % 3 == 0) { Divby3.add(Main.get(i)); } else if (Main.get(i) % 2 == 0 && Main.get(i) % 2 == 0 || Main.get(i) % 3 == 0) { Divby2.add(Main.get(i)); } else { Rem.add(Main.get(i)); } } System.out.println("Divisible by 3 or both 3 & 2 : \n"); printList(Divby3); System.out.println(); System.out.println("Divisible by 2 or both 2 & 3 : \n"); System.out.println(); printList(Divby2); System.out.println("Remaining elements of the main list : \n"); System.out.println(); printList(Rem); } public static void printList(List<Integer> list) { for (int i: list ) { System.out.println(i); } } }

Joy Majumdar

Level 16 , Kolkata, India

31 January 2019, 05:01

Is the above code ok

Joy Majumdar

Level 16 , Kolkata, India

31 January 2019, 05:02

This new code also not verifying which I have now showing you

Guadalupe Gagnon

Level 37 , Tampa, United States

31 January 2019, 05:28

I'll check it in the morning. It's midnight where I am.

Pavlo Plynko Java Developer at CodeGym

31 January 2019, 07:18

At the same time, either "if" or "else" can be executed, so this code cannot add any number to both lists.

Guadalupe Gagnon

Level 37 , Tampa, United States

31 January 2019, 15:56

I agree with popeye. If you enter the number 6, which is both divisible by 2 and 3, it will only go into the list Divby3. This is how i would write your main loop:

for (int i = 0; i < 20; i++) {
   int num = Integer.parseInt(reader.readLine();
   Main.add(num);

   if (num % 3 == 0)
      Divby3.add(num);

   if (num % 2 == 0)
      Divby2.add(num);

   if((num % 3 != 0) && (num % 2 != 0))
      Rem.add(num);
}