about multi threading

Question 1.

You are given a paragraph , which contain n number of words, you are given m threads. What you need to do is , each thread should print one word and give the control to next thread ... this way each thread will keep on printing one word , in case last thread come, it iwill invoke the first thread ... procedure will repeat until all the words are printed in paragraph. Finally all the thread should exit gracefully. What kind of synchronization will use ?

Question 2.
How could you make sure that thread A ,B and C run sequentially without using join method?

These question was asked in interview and found this site is too good for c# so hope to get answer as soon as possible.

thank you in advance.

For question 1, the simplest solution is to use a lock with some additional code to make sure that the 'm' threads print the words in order. For example:

using System;

using System.Threading;

class Program

{

static int m, count, next;

static object padlock = new object();

static string[] words;

static void Main()

{

string paragraph = "The quick brown fox jumps over the lazy dog.";

words = paragraph.Split(' ');

m = 3; // number of threads, say

count = 0; // word counter

next = 0; // thread counter

for(int i = 0; i < m; i++)

{

Thread t = new Thread(Printer);

t.Name = i.ToString();

t.Start();

}

Console.ReadKey();

}

static void Printer()

{

while(count < words.Length) PrintWord(); // keep going whilst there are words left

}

static void PrintWord()

{

// ensure that the threads print each word in order

if (Thread.CurrentThread.Name != next.ToString()) return;

lock(padlock)

{

if (count >= words.Length) return; // double check no more words left

Console.Write("{0} ", words[count]);

count++;

next++;

if (next == m) next = 0; // start at beginning again

}

You can check that the threads are printing the words in order by replacing this line:

Console.Write("{0} ", words[count]);

with this:

Console.Write("{0}{1} ", words[count], Thread.CurrentThread.Name);

The output should then be:

The0 quick1 brown2 fox0 jumps1 over2 the0 lazy1 dog.2

The simplest solution for question 2 is to use a flag (isFinished, say) which each thread can set to true when it's finished running. The main thread can wait until this flag is set before resetting it to false and starting the next thread. Here's an example:

using System;

using System.Threading;

class Program

{

static bool isFinished;

static void Main()

{

isFinished = false;

Thread A = new Thread(MethodA);

A.Start();

while (!isFinished); // wait until thread has finished

Console.WriteLine();

isFinished = false;

Thread B = new Thread(MethodB);

B.Start();

while (!isFinished);

Console.WriteLine();

isFinished = false;

Thread C = new Thread(MethodC);

C.Start();

while (!isFinished);

Console.ReadKey();

}

static void MethodA()

{

for (int i = 1; i < 10; i++) Console.WriteLine("Iteration {0} in thread A", i);

isFinished = true; // signal main thread

}

static void MethodB()

{

for (int i = 1; i < 10; i++) Console.WriteLine("Iteration {0} in thread B", i);

isFinished = true;

}

static void MethodC()

{

for (int i = 1; i < 10; i++) Console.WriteLine("Iteration {0} in thread C", i);

isFinished = true;

}

The output is:

Iteration 1 in thread A

Iteration 2 in thread A

Iteration 3 in thread A

Iteration 4 in thread A

Iteration 5 in thread A

Iteration 6 in thread A

Iteration 7 in thread A

Iteration 8 in thread A

Iteration 9 in thread A

Iteration 1 in thread B

Iteration 2 in thread B

Iteration 3 in thread B

Iteration 4 in thread B

Iteration 5 in thread B

Iteration 6 in thread B

Iteration 7 in thread B

Iteration 8 in thread B

Iteration 9 in thread B

Iteration 1 in thread C

Iteration 2 in thread C

Iteration 3 in thread C

Iteration 4 in thread C

Iteration 5 in thread C

Iteration 6 in thread C

Iteration 7 in thread C

Iteration 8 in thread C

Iteration 9 in thread C

2 Replies

Know the answer? Post it — somebody with the same question will find it here.

Trisha13y ago
thank you vuples, your solution is simple and meaningful.
Vulpes13y ago
For question 1, the simplest solution is to use a lock with some additional code to make sure that the 'm' threads print the words in order. For example:

using System;
using System.Threading;

class Program
{
static int m, count, next;
static object padlock = new object();
static string[] words;

static void Main()
{
string paragraph = "The quick brown fox jumps over the lazy dog.";
words = paragraph.Split(' ');
m = 3; // number of threads, say
count = 0; // word counter
next = 0; // thread counter

for(int i = 0; i < m; i++)
{
Thread t = new Thread(Printer);
t.Name = i.ToString();
t.Start();
}

Console.ReadKey();
}

static void Printer()
{
while(count < words.Length) PrintWord(); // keep going whilst there are words left
}

static void PrintWord()
{
// ensure that the threads print each word in order
if (Thread.CurrentThread.Name != next.ToString()) return;

lock(padlock)
{
if (count >= words.Length) return; // double check no more words left
Console.Write("{0} ", words[count]);
count++;
next++;
if (next == m) next = 0; // start at beginning again
}
}
}

You can check that the threads are printing the words in order by replacing this line:

Console.Write("{0} ", words[count]);

with this:

Console.Write("{0}{1} ", words[count], Thread.CurrentThread.Name);

The output should then be:

The0 quick1 brown2 fox0 jumps1 over2 the0 lazy1 dog.2

The simplest solution for question 2 is to use a flag (isFinished, say) which each thread can set to true when it's finished running. The main thread can wait until this flag is set before resetting it to false and starting the next thread. Here's an example:

using System;
using System.Threading;

class Program
{
static bool isFinished;

static void Main()
{
isFinished = false;
Thread A = new Thread(MethodA);
A.Start();
while (!isFinished); // wait until thread has finished
Console.WriteLine();

isFinished = false;
Thread B = new Thread(MethodB);
B.Start();
while (!isFinished);
Console.WriteLine();

isFinished = false;
Thread C = new Thread(MethodC);
C.Start();
while (!isFinished);

Console.ReadKey();
}

static void MethodA()
{
for (int i = 1; i < 10; i++) Console.WriteLine("Iteration {0} in thread A", i);
isFinished = true; // signal main thread
}

static void MethodB()
{
for (int i = 1; i < 10; i++) Console.WriteLine("Iteration {0} in thread B", i);
isFinished = true;
}

static void MethodC()
{
for (int i = 1; i < 10; i++) Console.WriteLine("Iteration {0} in thread C", i);
isFinished = true;
}
}

The output is:

Iteration 1 in thread A
Iteration 2 in thread A
Iteration 3 in thread A
Iteration 4 in thread A
Iteration 5 in thread A
Iteration 6 in thread A
Iteration 7 in thread A
Iteration 8 in thread A
Iteration 9 in thread A

Iteration 1 in thread B
Iteration 2 in thread B
Iteration 3 in thread B
Iteration 4 in thread B
Iteration 5 in thread B
Iteration 6 in thread B
Iteration 7 in thread B
Iteration 8 in thread B
Iteration 9 in thread B

Iteration 1 in thread C
Iteration 2 in thread C
Iteration 3 in thread C
Iteration 4 in thread C
Iteration 5 in thread C
Iteration 6 in thread C
Iteration 7 in thread C
Iteration 8 in thread C
Iteration 9 in thread C