CodeGym/Pomoc z zadaniami Java/I fail to see the reason for throwing out the exception.

Poziom 20

Debica

10.08.2024
86widoki
10komentarze

I fail to see the reason for throwing out the exception.

Pytania dotyczące zadania

Rozwiązane

My idea was, like, to put read bytes to an array list. Then take the first element to memory variable, and then iterate through the whole array to count repeated times. And after iterating, when repeated counter has value 1. I wanted to assign value from variable "memory" to “not Repeated list”, and reset counter to take another element from the working list. But like you see, my first two element go well, and the third one go fail. It threw out an exception.

default: [49, 49, 51, 49, 49, 52, 49, 49]

external InMemory: 49(index 0)

internal loop: 49?49 is repeat 1
internal loop: 49?49 is repeat 2
internal loop: 49?49 is repeat 3
internal loop: 49?49 is repeat 4
internal loop: 49?49 is repeat 5
internal loop: 49?49 is repeat 6

external InMemory: 49(index 1)

internal loop: 49?49 is repeat 1
internal loop: 49?49 is repeat 2
internal loop: 49?49 is repeat 3
internal loop: 49?49 is repeat 4
internal loop: 49?49 is repeat 5
internal loop: 49?49 is repeat 6

external InMemory: 51(index 2)

internal loop: 51?51 is repeat 1
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 8, Size: 8
	at java.util.ArrayList.rangeCheck(ArrayList.java:657)
	at java.util.ArrayList.get(ArrayList.java:433)
	at pl.codegym.task.task18.task1804.Solution.main(Solution.java:56)

Process finished with exit code 1

package pl.codegym.task.task18.task1804; import java.io.FileInputStream; import java.util.ArrayList; import java.util.Collections; import java.util.Scanner; /* Najrzadziej występujące bajty */ public class Solution { public static void main(String[] args) throws Exception { Scanner scanner = new Scanner(System.in); // String fileName = scanner.nextLine(); String fileName = "C:\\Users\\PC\\Desktop\\CodeGym\\CodeGymTasks\\2.JavaCore\\src\\pl\\codegym\\task\\task18\\task1804\\readFile.txt"; FileInputStream myFileInput = new FileInputStream(fileName); ArrayList<Integer> byteTable = new ArrayList<>(); while (myFileInput.available() > 0) { byteTable.add(myFileInput.read()); } myFileInput.close(); // test print System.out.println("default: " + byteTable); // test print // System.out.println("table size: " + byteTable.size()); // test print // //Collections.sort(byteTable); // System.out.println("sorted: " + byteTable); // test print ArrayList<Integer> notRepeat = new ArrayList<>(); int i; int j; int memory = 0; int counter = 0; for (i = 0; i <= byteTable.size()-1 ; i++) { memory = byteTable.get(i); System.out.println("\nexternal InMemory: " + memory + "(index " + i + ")\n"); // test print for (j = 0; j <= byteTable.size()-1 ; j++) { if(memory == byteTable.get(j)) { counter++; System.out.println("internal loop: " + memory + "?" + byteTable.get(j) + " is repeat " + counter); // test print } } if(counter == 1) { notRepeat.add(memory); System.out.println(memory + "?" + byteTable.get(j) + "not repeat yes"); // test print } //reset counter counter = 0; } System.out.println("\nNot repated list: " + notRepeat); // test print } }

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Thomas

Poziom 41 , Bayreuth, Germany

10 sierpnia, 12:36

przydatny

The exception is because you don't handle your index correctly (and if possible I won't save it outside the loops definition -> here you might use a didicated variable or similar). What happens... you have nested loops and run through them till the end of the array (usually you express that using i = 0; i < byteTable.size()). Somewhere in that loop you think: 'wohoo, that's the index I need and want to use later, outside this loop'. But what do you do... nothing. The loop continues and the index variables count up. Somwhen that index vars are > byteTable.size()-1 and the loop stops. Remeber that index var that is now outside of the byteTable size you intend to use to access that byteTable and that of course will lead to an exception. So what to do... the simple way is to just break; when you found the index to use. The other way (as mentioned above), you use some dedicated variable that you explicitly assign a value to (when you found your result inside the loop). Some thought to your approach... What does it help if you know a byte is in your list repeatedly? In a long text they all may occur repeatedly. So not much help. What you need to know it how often a byte occurs in your text. So the first idea sould be to use a map whith the byte as key and the count as value. Then you could think about int array with a length of 128 (the max positive byte value is 127, so 0 to 127). The value of each array element is the count. The index represents the byte. E.g. a byte is 66 (for a B) then you increment the value of the array element 66 like: bytes[66]++;

Robert

Poziom 20 , Debica, Poland

10 sierpnia, 14:11

przydatny

I'm put it after line: 50.

if (j == byteTable.size()-1) {
    break;
}

and this after 'counter' variable, line: 62:

if (i == byteTable.size()-1) {
    break;
}

And works great, I'm checked few times with different data. After comments 'test print lines' and add print out matches requirements, solution pass. 👍

for (int number : notRepeat){
        System.out.print(number + " ");
    }

If there will be similar task, I will test map for that, now I can't because I don't have that extended subscription for re-veryfing tasks. I wondering previous time how to do this during receiving data (in-way). It will be possible to do it using this suggested map in while(data.avaiable() > 0){...} block. ??? When during each pass in while loop, each single byte will be save to map and then when in next pass it will be repeated (that previous byte) then map will not save them. So, non-repeating bytes will be stored. Will the all data be read at once in single pass during while loop?

Thomas

Poziom 41 , Bayreuth, Germany

10 sierpnia, 15:30

przydatny

you also may want to consider the array solution

String fileName = new Scanner(System.in).nextLine();
        // the source file as byes
        byte[] source = Files.readAllBytes(Path.of(fileName));

        // alternative for that (CG probably won't accept it, either try Paths.of(fileName)) or:
/*
        byte[] source;
        try(FileInputStream in = new FileInputStream(fileName)) {
            source = in.readAllBytes();
        }
*/

        // the array holding the count values for all bytes 0 to 127, we assume there are no negative bytes inside
        // the file :) (-128 to -1) but then we'd just use new int[256] and counts[0] would represent -128
        int[] counts = new int[128];

        // if file is empty then we have nothing to do
        if (source.length == 0) {
            return;
        }

        // counting
        for (byte b : source) {
            counts[b]++;
        }

        // find the max count
        int max = counts[0];
        for (int i = 1; i < counts.length; i++) {
            if (counts[i] > max) {
                max = counts[i];
            }
        }
        System.out.printf("max count = %d%n", max);

        // output all that have a value of max
        for (int i = 0; i < counts.length; i++) {
            if (counts[i] == max) {
                System.out.printf("byte: %d, char: %c%n", i, i);
            }
        }
    }

Thomas

Poziom 41 , Bayreuth, Germany

10 sierpnia, 15:59

przydatny

if you want to check out the map approach... just till the map is filled

String fileName = new Scanner(System.in).nextLine();
byte[] source = Files.readAllBytes(Path.of(fileName));

Map<Character, Integer> counts = new HashMap<>();
for (byte b : source) {
    counts.put((char) b, counts.getOrDefault((char) b, 0) + 1);
}

counts.entrySet().forEach(System.out::println);

if you're more interested in a List of all Charactes of a certain count you can collect to a Map<Integer, List<Character>> and output the result like (replacement of line 9 in the code above):

counts.entrySet().stream()
         .collect(Collectors.groupingBy(
                 Map.Entry::getValue,
                 Collectors.mapping(Map.Entry::getKey, Collectors.toList())))
         .forEach((k, v) -> System.out.println(k + " -> " + v));

I admit the stream api may look weird ;) and this is nothing you need to know atm. Just a gimmick and nice to know that it is possible.

Robert

Poziom 20 , Debica, Poland

11 sierpnia, 13:48

I'm already started task1805 "Sort bytes". I took your point of view. I'm using ArrayList like before and I have lucky to discover 'TreeMap' it gives ascending order in Key during the print. The problem was when, I tried to use in diamond bracket Integer and Character, (TreeMap<Integer, Character> myMap;

myTreeMap.putIfAbsent(myArray.get(i), (char) myArray.get(i));

during filling map,tried to convert Integer to char, but it fails. ( my file was filled by letters) So take different approuch to replace second argument 'character' to 'Integer' and using it like index from ArrayList. Code was short, and that task passes. 👍 Whole code you can see there -> task1805

Thomas

Poziom 41 , Bayreuth, Germany

12 sierpnia, 05:48

przydatny

Yep, that's what you can do. But why use that intermediate list? And a Map you not necessarily need. If you want a sorted container you also could use a TreeSet. As you should not count occurrences and just look if a byte exists you only hava one value to memorize, the byte itself. Means no key is necessary.

Set<Integer> bytes = new TreeSet<>();

try (BufferedReader console = new BufferedReader(new InputStreamReader(System.in));
     InputStream in = new FileInputStream(console.readLine())) {

    while (in.available() > 0)
        bytes.add(in.read());
}

pufIfAbsent() isn't really neccessary in this case. It just checks if a value exists before adding it. put() would just replace an entry if it exists. In any case, the key would stay the same. But good to see that you're getting used to collections. It's a long way to learn coding. Important is to never give up and to keep learning without taking long breaks.

Robert

Poziom 20 , Debica, Poland

13 sierpnia, 21:30

Yeah, for me my last years was like brutal life, during learning the coding always show up something who punch 🥊 so hard who take ma out, out my routine for months 😵. Basically, In many levels of live. But you know life is life, not always we can handle it like we want. My approach is like that, to treat task like mind breakers, not like school punish home task. I think, that thinking change approach in good way. I hope there will be some fruits in the future. And will, can use that knowledge. I am grateful for your help, as it is very encouraging. 👍 Even when sometime is something hard to understand, ex. simply nesting the code ;) Yeah. Returns in coding are difficult at the learning level. “The cycle of crashing into the wall.”😅

Thomas

Poziom 41 , Bayreuth, Germany

14 sierpnia, 09:54

You have to take it as it comes. It took me several attempts before I got over a critical point and finally understood some of the concepts of programming languages. However, it's easier if you can stick to learning. Then you don't forget so much between attempts ;). Either way, it's always good to know something about programming. If you have questions, don't hesitate to ask. It's always a good repetition for me.

Robert

Poziom 20 , Debica, Poland

10 sierpnia, 11:52

I do little change line 48, 49.

System.out.println("internal loop: " + memory + "?" + byteTable.get(j) + " is repeat " + "countVal=" + counter); // test print
} else System.out.println("internal loop: " + memory + "?" + byteTable.get(j) + " is not repeat " +"countVal=" + counter);

Now looks like this:

default: [49, 49, 51, 49, 49, 52, 49, 49]

external InMemory: 49(index 0)

internal loop: 49?49 is repeat countVal=1
internal loop: 49?49 is repeat countVal=2
internal loop: 49?51 is not repeat countVal=2
internal loop: 49?49 is repeat countVal=3
internal loop: 49?49 is repeat countVal=4
internal loop: 49?52 is not repeat countVal=4
internal loop: 49?49 is repeat countVal=5
internal loop: 49?49 is repeat countVal=6

external InMemory: 49(index 1)

internal loop: 49?49 is repeat countVal=1
internal loop: 49?49 is repeat countVal=2
internal loop: 49?51 is not repeat countVal=2
internal loop: 49?49 is repeat countVal=3
internal loop: 49?49 is repeat countVal=4
internal loop: 49?52 is not repeat countVal=4
internal loop: 49?49 is repeat countVal=5
internal loop: 49?49 is repeat countVal=6

external InMemory: 51(index 2)

internal loop: 51?49 is not repeat countVal=0
internal loop: 51?49 is not repeat countVal=0
internal loop: 51?51 is repeat countVal=1
internal loop: 51?49 is not repeat countVal=1
internal loop: 51?49 is not repeat countVal=1
internal loop: 51?52 is not repeat countVal=1
internal loop: 51?49 is not repeat countVal=1
internal loop: 51?49 is not repeat countVal=1
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 8, Size: 8
	at java.util.ArrayList.rangeCheck(ArrayList.java:657)
	at java.util.ArrayList.get(ArrayList.java:433)
	at pl.codegym.task.task18.task1804.Solution.main(Solution.java:53)

Process finished with exit code 1

Robert

Poziom 20 , Debica, Poland

10 sierpnia, 11:58

it should go for third element in external loop, but it's throwing exception :/