CodeGym/Help with Java Tasks/requirements are not right

Level 9

10.10.2019
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requirements are not right

Question about the task

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I don't know why it doesn't work. I checked it few times, didn't find any error and results are right, but still requirements are not fulfill

1. Create a list of words and populate it yourself.
2. The fix method should:
2.1. remove all words containing the letter "r" from the list
2.2. duplicate all words containing the letter "l".
2.3. if a word contains both "r" and "l", then leave it unchanged.
2.4. don't do anything to other words.

For example:
rose lyre love

Output:
lyre love love

Requirements:

The program must not read data from the keyboard.
The fix method should remove all words containing the letter "r" from the list. There is an exception: words containing both "r" and "l" should be left alone.
The fix method must duplicate words containing the letter "l" (add another instance of this word to the list). There is an exception: words containing both "l" and "r" should not be duplicated.
The fix method should not do anything with words that contain both "l" and "r".
The fix method should not do anything with words that don't contain either "l" or "r".

package com.codegym.task.task07.task0716; import java.util.ArrayList; /* R or L */ public class Solution { public static void main(String[] args) throws Exception { ArrayList<String> list = new ArrayList<String>(); list.add("rose"); // 0 list.add("myle"); // 1 list.add("lye"); // 2 list.add("myre"); list.add("syle"); list.add("karl"); list.add("max"); list = fix(list); for (String s : list) { System.out.println(s); } } public static ArrayList<String> fix(ArrayList<String> list) { // write your code here for(int i = 0; i < list.size(); i++){ if(list.get(i).contains("r") && list.get(i).contains("l")) { return list; } else if(list.get(i).contains("r")) { list.remove(i); i--; } else if(list.get(i).contains("l")) { list.add(list.get(i)); } else{ return list; } } return list; } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

10 October 2019, 14:06

When the requirements say " if a word contains both "r" and "l", then leave it unchanged.", they mean that you should leave the specific word unchanged, not the list. Both lines 35 and 50 are the main reason you are failing. After that fix you will still have an infinite loop because you are adding elements that is increasing the size of the list but not adjusting the count of the index. You will need to figure out how to compensate properly for this. Ask any questions if you are still stuck.

Dominik

Level 9 , Poland

10 October 2019, 22:34

Ok, thanks. I changed it how you wrote. Changed lines 35 and 50, and make loop not infinite by added i++, now code looks like this

for(int i = 0; i < list.size(); i++){
           if(list.get(i).contains("r") && list.get(i).contains("l"))
           {
               continue;
           }

            else if(list.get(i).contains("r"))
            {
                list.remove(i);
                i--;
            }
            else if(list.get(i).contains("l"))
           {
               list.add(i, list.get(i));
               i++;
           }
        }
        return list;

Goff

Level 8 , Berlin, Germany

10 October 2019, 13:25

else if(list.get(i).contains("r"))
            {
                list.remove(i);
                i--;

Not the best idea. If condition is true at the very first loop, then counter i goes out of range 0 - list.size. IMHO it will be better to increment counter not in for statement, but manually within this loop.

for(int i = 0; i < list.size();  ){

And if you add an item you have to increment i by 2, not by one.

Guadalupe Gagnon

Level 37 , Tampa, United States

10 October 2019, 14:13

the i-- is perfectly acceptable and will not make the range go to negative. Here is an example: An arraylist with string values (index added for clarity) [0:"tank", 1:"ream", 2:"real"] ream would need to be removed and real would need to be duplicated ream would be removed when i = 1, the remaining values with indices would be: [0:"tank", 1: "real"]. Without the i-- after an item is removed, the next loop cycle would be i = 2 and real would never be check. This would fail the task. The main problem he is going to have to solve is the infinite loop created by adding values.

Goff

Level 8 , Berlin, Germany

10 October 2019, 23:12

I mean if you have [0:"ream", 1:"tank", 2:"real"]. ream would be removed when i = 0, and i would be set to -1 and -1 is out of range.

Guadalupe Gagnon

Level 37 , Tampa, United States

10 October 2019, 23:50

Right, but it wont try to access anything at index -1, the next cycle it will increase by one, be 0, then the next value would be tank

Guadalupe Gagnon

Level 37 , Tampa, United States

11 October 2019, 00:17

Here is some example code:

public class Solution {
    public static void main(String[] args) {
        String st = "The main problem he is going to have to solve is the infinite loop created by adding values";
        ArrayList<String> temp = new myArrayList<>();
        for(String x : st.split(" ")) temp.add(x);
        System.out.println("Original values of temp:");
        System.out.println(temp + "\n");


        int x = 0;
        for(int i = 0; i < temp.size(); i++){
            System.out.println("Loop cycle " + x + " starting");
            System.out.println("\ti = " + i);
            String pulled = temp.get(i);
            System.out.println("\tPulling value at index i, value = " + pulled);
            System.out.print("\tChecking if value contains 'e' : ");
            if(pulled.contains("e")){
                System.out.println("TRUE");
                temp.remove(i--);
                System.out.println("\tRemoving value and decreasing i. i now equals: " + i);
            }else{
                System.out.println("FALSE");
            }

            System.out.println("\tEnding current loop cycle, values in temp:");
            System.out.println(temp);
            System.out.println("===============================================================");
            x++;
        }

    }

    static class myArrayList<T> extends ArrayList<T>{

        public String toString() {
            StringBuilder sb = new StringBuilder();
            sb.append("[" + (size() > 0 ? (0 + ":\"" + get(0)) : ("\"")));
            for (int i = 1; i < size(); i++) {
                sb.append(", " + i + ":\"" + get(i)+"\"");

            }
            sb.append("]");
            return sb.toString();
        }
    }
}

Guadalupe Gagnon

Level 37 , Tampa, United States

11 October 2019, 00:21

Some of this code is a bit higher in lever, but focus in the main method. This is creating an array list and removing values inside of a for loop. Every time a value is removed I set i back by one (line 19). This is also how I write my code, with lots of sys.out statements. Once the code is working the way I expect it to then I will shut them off (another topic for later). Copy and paste this code in, run it, and follow the output.

Goff

Level 8 , Berlin, Germany

11 October 2019, 16:33

int x = 0;
        for(int i = 0; i < temp.size();  /*i++*/ ){
            System.out.println("Loop cycle " + x + " starting");
            System.out.println("\ti = " + i);
            String pulled = temp.get(i);
            System.out.println("\tPulling value at index i, value = " + pulled);
            System.out.print("\tChecking if value contains 'e' : ");
            if(pulled.contains("e")){
                System.out.println("TRUE");
                temp.remove(i /*--*/);
                System.out.println("\tRemoving value and decreasing i. i now equals: " + i);
            }else{
                System.out.println("FALSE");
                i++; // here it comes
            }

            System.out.println("\tEnding current loop cycle, values in temp:");
            System.out.println(temp);
            System.out.println("===============================================================");
            x++;
        }

This code is a bit safer (i never goes negative) and is a very little bit faster (i is iterated only when it has to be iterated). Isn't it? It is good to know - in for statement you may omit last parameter or you may use whatever you want and not only i++. For example:

for (int i=0; i<temp.size(); System.out.println(whateverfunction(i++)) anotherfunction;

Guadalupe Gagnon

Level 37 , Tampa, United States

11 October 2019, 17:09

If you are going to do things that way then a while loop would be faster than using a for loop, they have less overhead:

int i = 0;
while(i < temp.size()){
   // all the code from above
}

And yes you can omit the i++. None of the 3 statements inside the parentheses of a for loop are required. For example this will create an infinite loop:

for( ; ; ){
}

Here is a thread on Stackoverflow about for and while loops. Most people say that they are the same but someone actually wrote code that tests them (you can find this in the thread) and the for loop definitely runs slower. https://stackoverflow.com/questions/1165457/java-for-loop-vs-while-loop-performance-difference

Goff

Level 8 , Berlin, Germany

11 October 2019, 22:01

Thanks))

Goff

Level 8 , Berlin, Germany

15 October 2019, 04:09

I measured. for and while run pretty same time.

public static void main(String[] args) {
        int loopL = 1000000000;
        Date startTw = new Date();
        int i=0;
        while (i<loopL){
            i++;
            if ((i&1)==0) i++; // just to do smth
        }
        Date stopTw = new Date();
        System.out.println("while takes "+(stopTw.getTime()-startTw.getTime()));

        Date startTf = new Date();
        for (int j=0; j<loopL; ){
            j++;
            if ((j&1)==0) j++; // just to do smth
        }
        Date stopTf = new Date();
        System.out.println("for takes "+(stopTf.getTime()-startTf.getTime()));
    }

Both loops runs more or less in same time. You may change loop's bodies. But what is interesting: (i&1)==0 runs ~15-20% faster than i%2==0