Array with a class overwrites old one

Programming Questions

Bohdan

Array with a class overwrites old one

in Programming Questions • 2 years ago

String [] alldays = new String[3];
String [] fulldays = new String[6];
int [][] days = new int [6][7];
PersonDay[] person = new PersonDay[4];
void setup(){
size(200,200);
//set everything to zero
for (int a = 0; a < 6; ++a){
for (int b = 0; b < 6; ++b){
days[b][a] = 0;
}
}
for (int a = 0; a < alldays.length; ++a) {
person[a] = new PersonDay(days);
}
/// rip up the data ///
alldays = loadStrings("time.txt");
for( int a = 0; a < alldays.length; ++a){
fulldays = split(alldays[a], ',');
for( int l = 0; l < 6; ++l){
days[l] = int(split(fulldays[l], '\t'));
}
/// put the data to apporpiate place in the array ///
person[a].days = days;
}
/// see that it failed, placed person[1].days in person[0].days as well
println(person[0].days[1][0]);
}
void draw(){
}

the "time.txt" is this

,1900 2200,,1530 2130,,0830 1130,,

,1400 1700,1100 1400,1100 1300 1600 1800 1900 2100,1100 1600,1600 1700,,

and the class

class PersonDay {
int[][]days;
PersonDay(int[][] days) {
this.days = days;
}
}

So I don't understand why person[0].days is being the same as person[1].days?

Is there a better way to store 3 layers of data(thats all I really want to do)?

so that there is the person lvl, days lvl, time lvl. If it was working I would have this...

Replies(3)

PhiLho

Re: Array with a class overwrites old one

2 years ago

OK, first, let's make some little changes so that I can test it without creating my own text file (I am lazy...).
I cleaned up a bit on my way, with comments to explain the changes:

String[] alldays; // No need to initialize as you overwrite it
String[] fulldays; // Idem
int[][] days = new int[6][]; // Idem for second dim
// Lazy, I won't create a text file to test this
String[] loadStringsData =
{
",1900 2200,,1530 2130,,0830 1130,,",
",1400 1700,1100 1400,1100 1300 1600 1800 1900 2100,1100 1600,1600 1700,,"
};
PersonDay[] persons = new PersonDay[3];
void setup(){
size(200,200);
for (int a = 0; a < persons.length; ++a) {
persons[a] = new PersonDay(days);
}
/// rip up the data ///
// OK, but I use my trick above
// alldays = loadStrings("time.txt");
alldays = loadStringsData;
for( int a = 0; a < alldays.length; ++a){
fulldays = split(alldays[a], ',');
for( int l = 0; l < 6; ++l){
days[l] = int(split(fulldays[l], ' ')); // Changed for my test only
}
/// put the data to appropriate place in the array ///
persons[a].days = days;
}
/// see that it failed, placed person[1].days in person[0].days as well
println(persons[0].days[1][0]);
println(persons[1].days[1][0]);
println(Arrays.toString(persons[0].days));
println(Arrays.toString(persons[1].days));
}
class PersonDay {
int[][] days;
PersonDay(int[][] days) {
this.days = days;
}
}

The last two println shows exactly the same data, which is interesting because the exotic names are actually references to objects: you see that persons[0].days and persons[1].days share the same objects.
It is somehow similar to the problem I answered to in https://forum.processing.org/topic/adding-one-object-to-multiple-arraylists thread.
So, when you do persons[a].days = days; you just assign to the class' field the global array days that you update, so all classes will just have the last version of the data.
This also illustrate what I say in another thread ( https://forum.processing.org/topic/probably-trivial-speed-memory-question): declare the variables in the most inner loop where you need them.

I changed the program to show a better way to do it:

PersonDay[] persons = new PersonDay[3];
void setup(){
size(200,200);
/// rip up the data ///
alldays = loadStrings("time.txt");
for( int a = 0; a < alldays.length; ++a){
String[] fulldays = split(alldays[a], ',');
int[][] days = new int[6][];
for( int l = 0; l < 6; ++l){
days[l] = int(split(fulldays[l], '\t'));
}
/// put the data to appropriate place in the array ///
persons[a] = new PersonDay(days);
}
}

I haven't checked all the results, but at least I get different values now...

Bohdan

Re: Re: Array with a class overwrites old one

2 years ago

Wow, Thank you so much for your quick and well explained response.
I don't quite still understand what the difference in, and why it works but thank you very much.

But as you said in the other form... it's a bug...

PhiLho

Re: Array with a class overwrites old one

2 years ago

Basically, all the classes you created just pointed to the days global array. When you changed the values in this array, all the classes reflected these changes, as they share the same array.

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