Sorting Values in RobotC

VEX V5 General Discussion General Forum
#1

I’m trying to implement a median filter on my robot, and in order to do so I need to sort 5 values and retrieve the middle one. Does anyone know how I can do this?

#2

Probably a bit over the top just for this but the library that BNS wrote has this functionality.

For sorting a small number of values you can use a simple bubble sort algorithm.

#3

Thank you!
I was aware of this library, but I like to learn how to do things myself. There would be no library if they didn’t learn how to do the things in it. :slight_smile:

#4

So try and write a simple bubble sort in robotc. If you have problems continue to post in this thread and I will give you some more help.

#5 
bool Solved = false;
typedef struct{
	char Sort[3];
} Sonar;

Sonar old;
Sonar new;

task main()
{
	old.Sort[0] = 1;
	old.Sort[1] = 4;
	old.Sort[2] = 3;

	while(!Solved)
	{
		if (old.Sort[0] > old.Sort[1])
			new.Sort[1] = old.Sort[1];
		  new.Sort[0] = old.Sort[0];
		  old.Sort[0] = new.Sort[1];
		  old.Sort[1] = new.Sort[0];
		if (old.Sort[1] > old.Sort[2])
			new.Sort[2] = old.Sort[2];
		  new.Sort[1] = old.Sort[1];
		  old.Sort[2] = new.Sort[1];
		  old.Sort[1] = new.Sort[2];
		if ((old.Sort[0] < old.Sort[1]) && (old.Sort[1] < old.Sort[2]))
			Solved = true;
	  
			
		writeDebugStream("%i,%i,%i,",old.Sort[0],old.Sort[1],old.Sort[2]);
		writeDebugStreamLine("");
	}

}

This is the code I am using and it is not working. What is wrong?

#6

Well, the problem is that when using a statement like this.


if (old.Sort[0] > old.Sort[1])
    new.Sort[1] = old.Sort[1];
    new.Sort[0] = old.Sort[0];
    old.Sort[0] = new.Sort[1];
    old.Sort[1] = new.Sort[0];

Only the first statement will execute if the if statement is true, I think you probably meant to write this.


if (old.Sort[0] > old.Sort[1])
    {
    new.Sort[1] = old.Sort[1];
    new.Sort[0] = old.Sort[0];
    old.Sort[0] = new.Sort[1];
    old.Sort[1] = new.Sort[0];
    }

It’s a rather complex way of swapping those two values. Here is an example of a bubble sort (completely unoptimized) in ROBOTC.


void
bubbleSort(  int *a, int length )
{
    int i, tmp;
    bool swapped = false;

    do
        {
        swapped = false;
      
        for(i=1;i<length;i++)
            {
            if( a* > a* )
                {
                tmp = a*;
                a* = a*;
                a* = tmp;
          
                swapped = true;
                }
            }
        } while(swapped);
}

task main( )
{
    int  a] = {4, 1, 3, 7, 2, 0};
    int  i, len;
    
    len = (sizeof(a)/sizeof(int));
    
    for(i=0;i<len;i++)
        writeDebugStream("%d ", a*);
    writeDebugStreamLine("" );
    
    bubbleSort( a, len );    

    for(i=0;i<len;i++)
        writeDebugStream("%d ", a*);
    writeDebugStreamLine("" );
}

Notice how I use a temporary variable to swap the two values.********

#7

I agree with jpearman on using the bubble sort algorithm for sorting the small data size. However, only use bubble sort when dealing with small input sizes because the algorithm’s run-time grows exponentially as you get more values – causing your software to be inefficient and slow – in this case use the merge sort algorithm as it has a way faster average run-time of O(n lg n), while bubble has a run-time of average O(n^2).

#8