Modifing an array from multiple threads

Question

So i hava a thread that replaces the smallest char in an array of chars with another, random char. The thread runs until the array consists only of 'z'. But when i create multiple threads i get weird outputs and im wondering wether its only problem with printing or if my code is actually incorect.

run() method in my thread:

public void run() {
    char[] expected = { 'z', 'z', 'z', 'z', 'z', 'z', 'z', 'z', 'z', 'z' };
    while (timesModified < 100 && !Arrays.equals(array, expected)) {
        try {
            array[findMinIndex(array)] = rndChar();
            timesModified++;
            Thread.sleep(2000);
            for (char c : array) {
                System.out.print(c + " ");
            }
            System.out.println();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

My main:

public static void main(String[] args) {
        char[] sa_table = new char[10];
        Arrays.fill(sa_table, 'a');
        Thread t = new Thread(new AddToTable(sa_table));
        Thread t1 = new Thread(new AddToTable(sa_table));
        Thread t2 = new Thread(new AddToTable(sa_table));
        t.start();
        t1.start();
        t2.start();
        try {
            t.join();
            t1.join();
            t2.join();
        } catch (Exception e) {
            System.out.println("Interrupted");
        }
    }

Output from console:

n n n a a a a a a a a a a a a a a a a a a a a a 
a a a a a a 

n u U a n u U a a a a a a a 
a a a a a a 
n u N a a a a a a a 
n n u o a a a a a a a 
u o K a a a a a a 
n u o i a a a a a a 
n u o i A a a a a a 
n u o i A a a a a a 
n u o i G a a a a a 
n u o i K a a a a a 
n u o i u a a a a a 
n u o i u s a a a a 
n u o i u s x a a a 
n n n u o i u s u o i u s x l a a 
u o i u s x l D a 
x l h a 
n u o i u s x l h H 
n u o i u s x l h D 
n u o i u s x l h a 
n u o i u s x l h r 
n u o i u s x l R r 
n u o i u s x l v r 
n u o C u s x l v r 
n u o b u s x l v r 
n n u o S u s x l v r 
u o D u s x l v r 
n u o T u s x l v r 
n u o l u s x l v r 
n u o M u s x l v r 
n u o M u s x l v r 
n u o t u s x l v r 
n u o t u s x m v r 
n u o t u s x J v r 
n u o t u s x R v r 
n u o t u s x i v r 
n u o t u s x t v r 
F u o t u s x t v r 
A u o t u s x t v r 
D u o t u s x t v r 
l u o t u s x t v r 
e u o t u s x t v r 
L u o t u s x t v r 
L u o t u s x t v r 
o u o t u s x t v r 
m u o t u s x t v r 
s u o t u s x t v r 
s u C t u s x t v r 
s u D t u s x t v r 
s u q t u s x t v r 
s u W t u s x t v r 
s u Y t u s x t v r 
s u C t u s x t v r 
s u s t u s x t v r 
s u s t u s x t v h 
s u s t u s x t v D 
s u s t u s x t v w 
I u s t u s x t v w 
c u s t u s x t v w 
g u s t u s x t v w 
b u s t u s x t v w 
N u s t u s x t v w 
O u s t u s x t v w 
u u s t u s x t v w 
u u E t u s x t v w 
u u u u C t u s x t v w 
u T u T t u s x t v w 
t u s x t v w 
u u u X t u s x t v w u X t u s x t v w 

u u D t u s x t v w 
u u u T t u s x t v w 
u u X t u s x t v w 
u e t u s x t v w 
u u f t u s x t v w 
u u C t u s x t v w 
u u u t u s x t v w 
u u u t u o x t v w 
u u u t u i x t v w 
u u u t u P x t v w 
u u u t u v x t v w 
u u u b u v x t v w 
u u u u u u K u v x t v w 
u u u x u v x t v w 
x u v x K v w 
u u u x u v x W v w 
u u u x u v x c v w 
u u u u u x u v x e v w 
u u u u x u v x n v w 
u u u x u v x q v w 
x u v x T v w 
u u u x u v x w v w 
z u u x u v x w v w 
z P u x u v x w v w 
z T u x u v x w v w 
z z b u x u v x w v w b u x u v x w v w 

z p u x u v x z p u x u v x w v w 
w v w

As you can see, the output often has different lenght and i'm not sure what causes it

Could be because the printing from different threads is interleaving. Try using Arrays.toString(array) to print and see what happens. — OldCurmudgeon
– OldCurmudgeon, Commented Nov 23, 2017 at 14:38

Turtle · Accepted Answer · 2017-11-23 14:44:47Z

2

Your problem comes from here:

  for (char c : array) {
       System.out.print(c + " ");
  }
  System.out.println();

This loop can be paused while in the middle in one thread and start in another, so you have the printing of two threads that mix before you call the println().

For example, when you have:

n n u o S u s x l v r 
u o D u s x l v r

It means that you had this String: n u o S u s x l v r, and one thread started printing it, it printed the n, then another took over, printed everything -> n n u o S u s x l v r + a \n (println()), then the first one ended what he was doing, and printed the rest: u o D u s x l v r.

answered Nov 23, 2017 at 14:44

Turtle

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Comments

Wollo · Accepted Answer · 2017-11-23 14:49:47Z

Without looking into the code to check if it does the actual task correctly, the answer to the varying length is simple: Three threads print out values concurrently, so you do not know which thread printed out which value.

For example, these lines:

u u u u C t u s x t v w 
u T u T t u s x t v w 
t u s x t v w

were printed by three threads. That's why the total number of values is correct, and so is the number of line breaks (3). Try printing in a way that includes the thread id with each value, and you'll see you did not get one line per thread each time, but a mixture of values and line breaks from each of the participating threads.

To avoid this, you may use a StringBuilder to assemble each thread's values and then print them out in one go.

Try this:

    public void run() {
    char[] expected = { 'z', 'z', 'z', 'z', 'z', 'z', 'z', 'z', 'z', 'z' };
    while (timesModified < 100 && !Arrays.equals(array, expected)) {
        try {
            array[findMinIndex(array)] = rndChar();
            timesModified++;
            Thread.sleep(2000);
            StringBuilder sb = new StringBuilder();
            for (char c : array) {
                sb.append(c + " ");
            }
            System.out.println(sb);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Fabian Schmidt · Accepted Answer · 2017-11-23 14:39:41Z

0

It seems like you are facing the "Mutal exclusion" or "Mutex" problem here:

https://en.wikipedia.org/wiki/Mutual_exclusion

You have to make sure that the array is not edited by multilple threads at the same time.

answered Nov 23, 2017 at 14:39

Fabian Schmidt

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3 Comments

Gordon Gessler Over a year ago

Editing array from different threads at the same time wouldn't affect the size of the array tough. Someone in the comments above already helped me with printing, but i'll definietly take a look at the mutex thing. Thanks anyway

Turtle Over a year ago

It wouldn't explain why he got different length on his arrays. It's probably more of a printing problem: his for ... print is interlapping with other thread's

Turtle Over a year ago

@FabianSchmidt No problem ^^ It may've been the case if he was tempering with his array's size though. Also (just a note), if possible, you can quote the relevant information from the page you quote (in case the site goes off: I agree, it's not so plausible for wikipedia, but we never know :p)

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