Error in Selection Sort Algorithm | Together Java | Page 1

tawny steppe Aug 19, 2024, 1:10 AM

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Hello. As the title states, in my personal project, I have designed a selection sort algorithm to organize times from earliest to latest. In order to store times, and name of a task, I made a multi-dimensional String ArrayList as follows:

private List<List<String>> tasks = new ArrayList<List<String>>();

After doing research, I ended up with my first attempt at a selection sort algorithm as follows:

private void sort() {
    int i = 0; int j = 0;
    for(List<String> task : tasks) {
      for(List<String> task2 : tasks) {
        if(Double.parseDouble(task2.get(1)) > Double.parseDouble(task.get(1))) {
          tasks.set(i, task);
          tasks.set(j, task2);
        }
        j++;
      }
      j = 0;
      i++;
    }
  }

With output:

Hello James. Here are your tasks for today -
Name of task: Dinner Time
Start and End times: 17.0-18.0
Name of task: Food Ordering
Start and End times: 8.0-10.0
Name of task: Boardroom Meeting
Start and End times: 11.0-15.0

I have fixed this issue using a different implementation, but I am still curious as to why this did not work with the for-each loop. Please let me know if you need more code for context. Thank you.

dreamy jungleBOT Aug 19, 2024, 1:10 AM

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<@&987246399047479336> please have a look, thanks.

tawny steppe Aug 19, 2024, 1:22 AM

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Yeah, debugging mode isn't making it clearer to me why this output occurs

tired epoch Aug 19, 2024, 1:10 PM

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Why are you storing times using strings

tawny steppe Aug 19, 2024, 3:56 PM

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Because I want to associate a name and a time. Due to my limited knowledge of data structures, I opted for an Array list using strings, since I can convert between string and doubles easy.

lilac remnant Aug 19, 2024, 4:01 PM

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Instead of using double try with HHMM format
eg 8.0 = 0800, 17.0 = 1700

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You can use direct comparison

tawny steppe Aug 19, 2024, 4:41 PM

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lilac remnant Instead of using double try with HHMM format eg 8.0 = 0800, 17.0 = 1700

I see, I'll try and update the current form of my program for this

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but I am still curious as to why this version isn't working

rough spade Aug 19, 2024, 5:00 PM

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can you give sample data?

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i.e. ```java
private List<List<String>> tasks = new ArrayList<List<String>>();

...
tasks.add(List.of(...));

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then we can at least run your code

lilac remnant Aug 19, 2024, 5:15 PM

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the logic in your if condition is not doing anything

tawny steppe Aug 19, 2024, 5:16 PM

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lilac remnant the logic in your if condition is not doing anything

what do you mean?

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okay, this is my sample input, which I used

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"Dinner Time", 17, 18
"Food Ordering", 8, 10
"Boardroom Meeting", 11, 15

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the goal of the "if" statement is to compare whether the task occurs sooner, compared to another

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if it evaluates as true, then it will swap the ArrayList of Strings with each other

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when debugging my program, it is showing that the if statement is evaluating as true

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but even when 'task' and 'task2' meet the appropriate conditions for swap, it doesn't for some reason

tawny steppe Aug 19, 2024, 5:22 PM

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rough spade i.e. ```java private List<List<String>> tasks = new ArrayList<List<String>>(); ...

oh, I think this is what you want?

private List<List<String>> tasks = new ArrayList<List<String>>();
public void setTask(String taskName, String role, double start, double end) {
  ArrayList<String> task = new ArrayList<String>();
  task.add(taskName);
  task.add(String.valueOf(start));
  task.add(String.valueOf(end));
  tasks.add(task);
  sort();
}

dreamy jungleBOT Aug 19, 2024, 5:22 PM

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tawny steppe oh, I think this is what you want? ``` java private List<List<String>> tasks = n...

Detected code, here are some useful tools:

Formatted code

javaprivate List<List<String>> tasks = new ArrayList<List<String>>();
public void setTask(String taskName, String role, double start, double end) {
  ArrayList<String> task = new ArrayList<String>();
  task.add(taskName);
  task.add(String.valueOf(start));
  task.add(String.valueOf(end));
  tasks.add(task);
  sort();
}

tawny steppe Aug 19, 2024, 5:52 PM

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okay, I fixed it. It seems there was an issue with my use of 'i' and 'j'

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I replaced the tasks.set(i, task); and the tasks.set(j, task2); with:

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tasks.set(tasks.indexOf(task), task2);
tasks.set(tasks.indexOf(task2), task);

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and the output is correct

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still dunno how the iterations of i and j makes the program go hawire though

rough spade Aug 19, 2024, 6:12 PM

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tawny steppe oh, I think this is what you want? ``` java private List<List<String>> tasks = n...

yeah thanks i'll look soon

tired epoch Aug 19, 2024, 6:29 PM

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I would try to find a better way to store your 'tasks' better

rough spade Aug 19, 2024, 6:36 PM

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    private void sort() {
        int i = 0, j = 0;
        for(List<String> task : tasks) {
            for(List<String> task2 : tasks) {
                if(Double.parseDouble(task2.get(1)) > Double.parseDouble(task.get(1))) {
                    List<String> temp = tasks.get(i);
                    tasks.set(i, tasks.get(j));
                    tasks.set(j, temp);
                }
                j++;
            }
            j = 0;
            i++;
        }
    }

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deleted my previous messages becuase I think it would just confuse you, so the TLDR is that your swap logic was wrong

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i.e. ```java
List<String> temp = tasks.get(i);
tasks.set(i, tasks.get(j));
tasks.set(j, temp);

tawny steppe Aug 19, 2024, 9:24 PM

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mmm, so it's better not to use the for-each variable

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or, better put, it's more clear not to

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okay, thank you guys

#Error in Selection Sort Algorithm