Coś jest nie tak

Pytania dotyczące zadania

Rozwiązane

Nie wiem dlaczego program usuwa tylko trzy pierwsze wyrazy z listy, dodaje je na koniec i usuwa teraz ostatni wyraz z listy. A przynajmniej tak to dla mnie wygląda . Albo dodaje pierwszy pusty ciąg. Ale dlaczego ?

package pl.codegym.task.task07.task0720; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.*; /* Przestawiono w samą porę */ public class Solution { public static void main(String[] args) throws IOException { Scanner sc = new Scanner(System.in); ArrayList<String> list = new ArrayList<>(); int m = sc.nextInt(); int n = sc.nextInt(); //tutaj wpisz swój kod for(int i = 0; i<m ; i++){ list.add(sc.nextLine()); } for(int k = 0; k<n; k++){ list.add(list.get(0)); list.remove(list.get(0)); } for(String x : list){ System.out.println(x); } } }

Komentarze (9)

Popularne
Najnowsze
Najstarsze

Musisz się zalogować, aby dodać komentarz

Gellert Varga

Poziom 23 , Szekesfehervar, Hungary

11 listopada 2021, 17:54

I can say the same as Lupe. I just added a couple Sys.o.pr() commands to your code to see what happens to your list inside the loop. Try it yourself in the following way:

for(int k = 0; k<n; k++) {
   System.out.println("k=" + k + ", " + list);
   list.add(list.get(0));
   list.remove(list.get(0));
   System.out.println("k=" + k + ", " + list);
}

I tested it with this input: 7 3 01a 02a 03a 04a 05a 06a 07a Output: k=0, [, 01a, 02a, 03a, 04a, 05a, 06a] k=0, [01a, 02a, 03a, 04a, 05a, 06a, ] k=1, [01a, 02a, 03a, 04a, 05a, 06a, ] k=1, [02a, 03a, 04a, 05a, 06a, , 01a] k=2, [02a, 03a, 04a, 05a, 06a, , 01a] k=2, [03a, 04a, 05a, 06a, , 01a, 02a] You can see that first of all the first element in the list is an empty string, which is then added to the end of the list and after it moves backwards in the indexes step by step. So I think the ordering operation of the list is fine, only there is an unnecessary element of empty String, which comes from using the nextInt() and nextLine() methods in sequence, as Lupe explained.

Guadalupe Gagnon

Poziom 37 , Tampa, United States

11 listopada 2021, 15:11

It is because nextInt() does not consume the new line character (when you hit enter). It will consume all spaces and newLines up until the next int, then consume the full int stopping at (and not consuming) a space, new line, or carriage return (and maybe a few more characters). The nextLine() method will grab everything up to and including the new line. Because nextInt() leaves the last new line on the console, then nextLine() consumes everything up to including the next new line, the first time you call newLine() after nextInt() it will consume the new line that is left from the nextInt() method. You have to remember this if you intend to use a Scanner with its methods. The nextDouble(), nextBoolean(), nextShort(), etc methods all behave the same way. To avoid this mistake you can just use the nextLine() method and then parse to int, however you can simply just call .nextLine() in between the last call of nextInt() and the loop that uses .nextLine() (somewhere on or around line 20).

Guadalupe Gagnon

Poziom 37 , Tampa, United States

11 listopada 2021, 15:37

Quick example in code:

public static void main(String[] args) {
   String x = "\n  17\n 15\n 21  s15\n 22\n end";
   System.out.println("Printing String 'x':");
   System.out.println(x);
   System.out.println("End Printing String");

   System.out.println();
   System.out.println("Creating Scanner to print ints and nextLine from 'x'");
   Scanner s = new Scanner(new ByteArrayInputStream(x.getBytes(StandardCharsets.UTF_8)));
   System.out.println("\tPrinting: " + s.nextInt()); // 17
   System.out.println("\tPrinting: " + s.nextInt()); // 15
   System.out.println("\tPrinting: " + s.nextInt()); // 21
   System.out.println("\tPrinting: " + s.nextLine()); // "  s15"
   System.out.println("\tPrinting: " + s.nextInt()); // 22
   System.out.println("\tPrinting: " + s.nextLine()); // BLANK
}

This just declares a String and outputs it to the screen. Lines 3 and 5 output the start and end of the String so you can see that everything between them, when printed to the screen, is the String. Line 9 creates a Scanner to read that same String, 'x'. The Scanner acts exactly the same as if you were reading from the console. You can see line 10 prints "17", ignoring the leading new line character and spaces. The next line prints 15, also ignoring leading "white space", Same thing with line 12. Now at line 13 it switches to the nextLine() method and outputs " s15". This is because the previous line only consumed up until "21" which means the nextLine() consumes everything up to and including the next new line char. Line 14 outputs the next int. Finally line 15 calls the nextLine() method but outputs nothing instead of "end". Once again this is because the previous line called nextInt() which does not consume any trailing white space, leaving the new line character for the nextLine() method to consume.

Gellert Varga

Poziom 23 , Szekesfehervar, Hungary

11 listopada 2021, 18:19

I'd like to add a little something to make sure everyone can test this code, that the use of StandardCharsets requires one more 'import' at the beginning of the code:

import java.nio.charset.*;

Also, to expand the illustrating, I would add one more print command on line 16:

System.out.println("\tPrinting: " + s.nextLine()); // " end"

Guadalupe Gagnon

Poziom 37 , Tampa, United States

11 listopada 2021, 18:26

Good points. IntelliJ just automatically adds the proper import statements without having to think about it. I should have said that it needed to be added in case someone not using intelliJ (or another IDE) tries out this small example.

Gellert Varga

Poziom 23 , Szekesfehervar, Hungary

11 listopada 2021, 18:59

przydatny

Well, yes, I still don't use IntelliJ and I always have to add 'import' statements manually... I can practice which class is in which package:) In this context, I would like to ask if there is any significance, too much memory usage, or any drawbacks to using the second of the following two versions (because of laziness): 1.: import java.io.BufferedReader; import java.io.IOException; 2.: import java.io.*;

Guadalupe Gagnon

Poziom 37 , Tampa, United States

11 listopada 2021, 20:11

It is my understanding that it matters not. The program, when compiled, is going to only include the classes that you use and not every single class specified by the import statements. If that were the case then no one would use the dot star.

Gellert Varga

Poziom 23 , Szekesfehervar, Hungary

11 listopada 2021, 20:21

That's good news:) because I've always been lazy to write out all those import classes, and I always just use the dot star.

Paweł Knapik

Poziom 18 , Lipinki, Polska

12 listopada 2021, 21:24

wow. I use scanner from always and probably i never used it in this configuration so i was no idea abou this. Thanks for explenations.