Sort ArrayList of objects based on boolean value

Programming Questions

amnon.owed

Sort ArrayList of objects based on boolean value

in Programming Questions • 2 years ago

All right, suppose I have a class Photo with a lot of stuff in it. Including the boolean selected. Now I have a lot of these Photo's in an ArrayList. I would like to sort these photos based on the boolean selected. Where the true ones are sorted to the front of the list and the false ones are sorted to the end. I've looked into both "implements comparable" and creating a "custom comparator", but so far I've not been able to implement either into a workable solution. Below is a basic sketch which is comparable (no pun intended) to the actual sketch. Now I ask of you...

What would be the most efficient way to sort this ArrayList?

ArrayList <Photo> photos = new ArrayList <Photo> ();
void setup() {
size(200,200);
for (int i=0; i<10; i++) {
float flipcoin = random(1);
if (flipcoin>0.3) { photos.add(new Photo(i,false)); }
else { photos.add(new Photo(i,true)); }
}
// sort ArrayList here on the boolean selected
for (Photo p : photos) {
println(p.selected + "," + p.index);
}
// sorted ArrayList with the true ones first and after that the false ones
}
class Photo {
int index;
boolean selected;
int a,b,c,d;
float e,f,g,h;
PImage i,j,k;
Photo(int index, boolean selected) {
this.index = index;
this.selected = selected;
}
}

Replies(4)

PhiLho

Re: Sort ArrayList of objects based on boolean value

2 years ago

For such easy, binary criterion, the easiest is to make a swap sort, working in O(n):

int ff = 0; // First false index
Photo pf = photos.get(0); // photo with false
for (int i = 0; i < photos.size(); i++)
{
Photo p = photos.get(i);
if (p.selected)
{
// Replace it with the current photo with false
photos.set(i, pf);
// Put the selected photo in place of the first false photo (and move this index)
photos.set(ff++, p);
// Next photo is the next to swap (might also advance ff up to the next unselected photo)
pf = photos.get(ff);
}
}
println();
for (Photo p : photos) {
println(p.selected + "," + p.index);
}

amnon.owed

Re: Sort ArrayList of objects based on boolean value

2 years ago

Great, thanks!

CivicImag..

Re: Sort ArrayList of objects based on boolean value

1 year ago

Hi, I know I'm digging up an old topic here, but I have a similar question.

I want to be able to select one image and either move it to the front or to the back. I used phi.lho's code and it worked, sort of.

1.) I don't want to swap the first item with the last, I just want to move the selected item back and bump all other items ahead.

2.) I tried flipping the code around and it removes the highest indexed item from the array list.

Any tips? Array lists will be my new favourite thing if I can get this to work.

Thanks so much.

void keyPressed () {
if (key == 't') {
shapeMove.add(new ShapeMove("tree.gif", 100+btn*50, 100, 150, 150));
btn++;
}
if (key == 'q') {
int ff = 0; // First false index
ShapeMove sf = (ShapeMove)shapeMove.get(0); // photo with false
for (int i = 0; i < shapeMove.size(); i++) {
ShapeMove s = (ShapeMove)shapeMove.get(i);
if (s.selected) {
// Replace it with the current photo with false
shapeMove.set(i, sf);
// Put the selected image in place of the first false photo (and move this index)
shapeMove.set(ff++, s);
// Next photo is the next to swap (might also advance ff up to the next unselected photo)
sf = (ShapeMove)shapeMove.get(ff);
}
}
}
if (key == 'w') {
int ff = shapeMove.size()-1; // First false index
ShapeMove sf = (ShapeMove)shapeMove.get(0); // photo with false
for (int i = shapeMove.size()-1; i >= 0; i--) {
ShapeMove s = (ShapeMove)shapeMove.get(i);
if (s.selected)
{
// Replace it with the current photo with false
shapeMove.set(i, sf);
// Put the selected image in place of the first false photo (and move this index)
shapeMove.set(ff--, s);
// Next photo is the next to swap (might also advance ff up to the next unselected photo)
sf = (ShapeMove)shapeMove.get(ff);
}
}
}
}

Manindra29

Re: Sort ArrayList of objects based on boolean value

1 year ago

Try sorting it using Collections.sort() function. This function can sort the objects of an ArrayList if they implement the Comparable interface by having a method called compareTo(Object obj). You need to override compareTo() in your declaration of ShapeMove class.

Check out this example code:

Node is a class which has a selected data member and the list sorted on its basis.

import java.util.*;
void setup() {
size(200, 200);
ArrayList arrayList = new ArrayList();
System.out.println("Unsorted List:");
// Add 10 random elements to arrayList
for (int i = 0; i < 10; i++) {
Node n = new Node();
System.out.println(n.toString());
arrayList.add(n);
}
Collections.sort(arrayList);
System.out.println("Sorted array:");
for (int i = 0; i < arrayList.size(); i++) {
System.out.println(arrayList.get(i));
}
}
public class Node implements Comparable<Node> {
boolean selected;
int value;
public Node() {
// Assign random values
value = (int) (random(100));
selected = random(10) > 5 ? true : false;
}
public String toString() {
return "Node: " + value + ", " + selected;
}
public int compareTo(Node node) {
if (node.selected)
// give node higher preference
return 1;
else
// give node lower preference
return -1;
}
}

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