This question already has answers here:
Random number generator only generating one random number
(15 answers)
Closed 7 years ago.
This is really weird, and I cannot see why this is happening. In the foreach cycle, I am iterating through a class A collection, and for each class, I call the Count() method, where r1 and r2 numbers are generated from range [-1,1]. The problem is that Random.Next returns the same "random" numbers for each instance. When the results for the first instance are 0 and -1, the same ones will be returned from following instances. Please, could you tell me why this is happening? Also, I cannot get different results in each class A instance. This is the code:
class a
{
Random rnd = new Random();
private void Count()
{
int r1 = rnd.Next(-1, 1);
int r2 = rnd.Next(-1, 1);
}
}
class b
{
List<a> listofA=new list<a>();
foreach (a ACLASS in listofA)
{
ACLASS.Count();
}
}
The problem is that you are creating instances of the Random class too close in time.
When you create a Random object, it's seeded with a value from the system clock. If you create Random instances too close in time, they will all be seeded with the same random sequence.
Create a single Random object and pass its reference to the constructor when you create instances of the "a" class, instead of creating one Random object for each "a" instance.
Use a single, static Random number generator for all instances of the class.
class a
{
private static Random rnd;
static a() {
rnd = new Random();
}
private void Count()
{
int r1 = rnd.Next(-1, 2);
int r2 = rnd.Next(-1, 2);
}
}
Note the change to give you numbers in the range -1,1 rather than -1,0
You're creating new instances of Random very close together (your loop is very tight) so each instance is effectively using the same seed value.
A better approach would be to create one instance and pass that to your Count method.
You probably know this next bit, but I'll include it here for completeness:
The MSDN has the details on this, but basically your problem is the Random.Next method you're using generates:
A 32-bit signed integer greater than or equal to minValue and less than maxValue; that is, the range of return values includes minValue but not maxValue. If minValue equals maxValue, minValue is returned.
because of this your calls will return -1 or 0.
You include a random instance for each A instance. It sounds like they're all getting the same default seed value. You probably want to make a static random for all A instances and use it repeatedly, or alternatively provide a seed value to the Random() instance in the A constructor.
Related
This question already has answers here:
Instantiating Random in or out of the loop
(7 answers)
Closed 2 years ago.
I'm getting some very strange behavior, please help!
I'm iterating through an object array, the participate property is a bool. The method randomly assigns each person a true or false (50/50 right now).
As the code is right now, all five people either get all true or all false.
When the message box is enabled, the code works properly (each person gets a true or false, not all the same)!!!!
for (int i = 0; i < 5; i++)
{
//other code removed, I'll add it if it turns out to be important to the problem
person[i].participate = doesPersonJoin();
}
public bool doesPersonJoin()
{
bool joinBool = true;
Random rnd = new Random();
int r = rnd.Next(1, 100);
//MessageBox.Show(r.ToString());
if (r > 50)
{
joinBool = false;
}
return joinBool;
}
The random number generator works by setting a seed value based by default on the current clock time. Without MessageBox, the code runs so fast all of the random instances have the same seed and therefore produce the same value when .Next() is called.
To fix this, you should create ONE random instance you can use throughout your program, or at least one instance (possibly static) for use in that class.
I'm using the current clock ticks as a seed for random number generation. The random number is used in a pseudo GUID and a check in my database will make sure it doesn't already exist before returning. On average, this method will be called around 10k times in succession during the life of the process.
My concern is that an identical number might be generated back to back resulting in multiple unnecessary recursive calls to my database checking for the same ID. I'd like to avoid this if possible. What is the best way to test this scenario?
If it matters, application is .NET 4 and database is SQL Server 2008.
private static string GenerateUniqueDelId()
{
// Generate a random integer using the current number of clock ticks as seed.
// Then prefix number with "DEL" and date, finally padding random integer with leading zeros for a fixed 25-character total length.
int seed = (int)DateTime.Now.Ticks;
Random number = new Random(seed);
string id = string.Format("DEL{0}{1}", DateTime.Today.ToString("yyyyMMdd"), number.Next().ToString("D14"));
// Lookup record with generated ID in Sesame. If one exists, call method recursively.
string query = "SELECT * FROM Lead WHERE Esm_Id = #Esm_Id";
SqlParameter[] parameters = { new SqlParameter("#Esm_Id", id) };
if (DataManager.GetRow(query, parameters, DelConnection.Sesame) != null) return GenerateUniqueDelId();
// Otherwise, return ID.
return id;
} //// End GenerateUniqueDelId()
You are right in your concern: You should move the creation of your Random instance out of your method body - otherwise you will re-seed with the same value many times which results in the same number sequence.
Also you are kinda re-inventing the wheel: the default constructor of the Random class already uses the current clock time as default seed.
The question is why don't you avoid all of this and just use an auto-generated Guid on the database side?
Quoting Jon Skeet
When you see the word "random" in a question title on Stack Overflow you can almost guarantee it will be the same fundamental problem as countless similar questions. This article takes a look at why randomness causes so many problems, and how to address them.
Check his article about random number generators
http://csharpindepth.com/Articles/Chapter12/Random.aspx
basically his solution looks like:
using System;
using System.Threading;
public static class RandomProvider
{
private static int seed = Environment.TickCount;
private static ThreadLocal<Random> randomWrapper = new ThreadLocal<Random>(() =>
new Random(Interlocked.Increment(ref seed))
);
public static Random GetThreadRandom()
{
return randomWrapper.Value;
}
}
This question already has answers here:
Random number generator only generating one random number
(15 answers)
Closed 6 years ago.
I have a class:
public class MyClass
{
public int MyMethod()
{
Random rand = new Random();
return rand.Next() % 10 + 1;
}
}
And 2 objects of it:
MyClass obj1 = new MyClass();
MyClass obj2 = new MyClass();
The problem is that obj1.MyMethod() == obj2.MyMethod() always. Why does it happen? What's the best way to avoid it?
Create your random object static
public class MyClass
{
public static Random rand = new Random();
public int MyMethod()
{
return rand.Next() % 10 + 1;
}
}
Random works on System.DatTime.Now.Ticks.
If we do like this
Random rand = new Random();
internally it happens as
Random rand = new Random(System.DateTime.Now.Ticks);
Just think for a moment the only thing which is not constant in system is System Time.
When ever using Random class make its object once and use its method Next() where ever you want. You will find this situation in loops when random object is created inside loops.
In your code they are created one after another, they get created by same Ticks seed value.
Create your random object static and then they won't be same.
you are creating a new Random every time. When you create a new instance of Random without explicitly specifying the seed value it uses System.DatTime.Now.Ticks as the seed. Due to the speed of calls they are happening at the same 'Tick' so the same seed value is used. As all Random instances generate the exact same sequence of 'random' number for the same seed value the same 'random' value is generated by both instances.
This has been covered many times before on the site and you should search for one of those answers.
But basically you need to create your Random object once and reuse it. This could be done statically , or at least as a class variable.
You should read this question and its answers to find a better approach.
To clarify, multiple students objects and all of them are getting the same value.
I know this question has been asked before, but I have had no luck with the other posts about his topic. I have a random number generator ranging from 1-3. I then us %2 to make a bool value true or false. Every time I run the program I either get all true or all false. Here is my code. I know that random is not really random. What can I do to get more random numbers.
Random random = new Random();
public Student()
{
int randomLevel=random.Next(1,3);
level = (randomLevel % 2 == 0);
}
public bool readingLevel()//this always returns one value for the entire program.
{
return level;
}
You are only assigning a random value to 'level' once during the constructor, so it will always have the initial value.
Try:
public bool readingLevel()
{
return (random.Next(1,3) % 2 == 0);
}
Edit:
Static Random random = new Random();
...
It looks like you're trying to get a random number!
Clippy
Well you can try something like this:
static Random random = new Random();
public Student()
{
lock (random)
{
int randomLevel=random.Next(1,3);
level = (randomLevel % 2 == 0);
}
}
public bool readingLevel()//this always returns one value for the entire program.
{
return level;
}
The problem with your snippet seems to be that you are instantiating a new Random class with each of your class instances.
This is not the way Random is supposed to be used, but instead a single Random class instance should be used for acquiring multiple random numbers.
The Rationale for this is that Random in .Net uses a pseudo random algorithm based on state (a seed) that changes every time you ask for a new random number. By instantiating multiple random classes in a relatively short time span, there is a high chance that all of them will be initiated with the same seed (Based on system time) and all will give the same random number.
It looks like your random generator is an instance variable of the Student. Since the generator uses the current time as the seed, if you create a bunch of students within a short time, they will all have each have a generator with the same seed and the same results. You could make the random generator a static variable or, better yet, use constructor injection and pass the level into the Student's constructor.
class Student
{
private static Random random = new Random();
public Student()
{
level = random.NextDouble() < 0.5;
}
public bool readingLevel()
{
return level;
}
}
or use constructor injection so your student class is deterministic.
class Student
{
private boolean level;
public Student(boolean readingLevel)
{
this.level = readingLevel;
}
public boolean readingLevel()
{
return level;
}
}
Create only one instance of Random and reuse it. Creating multiple instances of random in quick succession seeds to the same value and thus leads to the same sequence.
If your code in single threaded you can simply use a static property to hold the instance of Random.
The default seed value is derived from the system clock and has finite resolution. As a result, different Random objects that are created in close succession by a call to the default constructor will have identical default seed values and, therefore, will produce identical sets of random numbers. This problem can be avoided by using a single Random object to generate all random numbers. You can also work around it by modifying the seed value returned by the system clock and then explicitly providing this new seed value to the Random(Int32) constructor. For more information, see the Random(Int32) constructor.
http://msdn.microsoft.com/en-us/library/h343ddh9.aspx
public Student()
{
int randomLevel=random.Next(1,3);
level = (randomLevel % 2 == 0);
}
seems very much like a constructor for the Student class. In this constructor you are basically calculating a random number and storing it inside the level field. So if you use the same instance of Student throughout your entire program and call the readingLevel() method multiple times on this instance it will obviously return the same value -> the one that was done during the construction of this instance and that you stored in the level field.
So you might consider moving the random number generation logic into the readingLevel() method instead of simply returning the same value over and over again:
public class Student
{
private Random random = new Random();
public bool readingLevel()
{
int randomLevel = random.Next(1,3);
return (randomLevel % 2 == 0);
}
}
Now everytime you call this method on the same instance you should get a new calculation of a random number.
Some others have said this, but I think the point deserves underscoring with an example.
public class Student
{
Random random = new Random();
public Student()
{
int randomLevel=random.Next(1,3);
level = (randomLevel % 2 == 0);
}
public bool readingLevel()//this always returns one value for the entire program.
{
return level;
}
}
public class Program
{
public static void Main()
{
var students = new List<Student>();
for (int i = 0; i < 10; i++)
students.Add(new Student());
//Now you have 10 Students; each Student has its own random number generator
//The generators were created within microseconds of each other, so they most likely have THE SAME SEED
//Because they have the same seed, they will generate identical sequences of numbers
//Each student's reading level is calculated from the first call to .Next(1, 3) on its own RNG.
//The RNGs have the same seed, so each will return the same value for the first call to .Next(1, 3)
//Therefore, all students will have the same reading level!
}
}
You are using random to simulate a true/false situation, so you are trying to limit the result to either 1 or 2. Given that you are doing an odd/even test on the result you might be better off doing:
int randomLevel = random.Next();
level = (randomLevel % 2 == 0);
Also If you create all your students in quick succession there's a good chance that your current code will return the same value for subsequent calls.
Well.
Consider what is happening here. When your Student is constructued, you get some random number, and then set it to the member variable level.
Then, at some other point, you call a function, readingLevel, which returns this previously set value.
Then, you may ponder to yourself: Exactly when would this function give a different value? Well, it will only do so when level gets a different value. And when does that happen? Well, it only happens in the constructor, so, that means, it never happens again for the life of the object ....
Try the following. Move the selection of the random level to the readingLevel function.
Random random = new Random();
public Student()
{
}
public bool readingLevel()//this always returns one value for the entire program.
{
int randomLevel=random.Next(1,3);
level = (randomLevel % 2 == 0);
return level;
}
That is because you are using the random.Next() and the level evaluation within the constructor of your class, remember that the constructor is only executed when you create a new instance of your object, for having it executed several times create a different method where you call the random and the level evaluation, that way you'll get different values every time or use Something like this:
public bool Level()
{
int randomLevel=random.Next(1,3);
level = (randomLevel % 2 == 0);
return level;
}
Random random = new Random(DateTime.Now.Ticks);
This question already has answers here:
Random number generator only generating one random number
(15 answers)
Closed 7 years ago.
This is really weird, and I cannot see why this is happening. In the foreach cycle, I am iterating through a class A collection, and for each class, I call the Count() method, where r1 and r2 numbers are generated from range [-1,1]. The problem is that Random.Next returns the same "random" numbers for each instance. When the results for the first instance are 0 and -1, the same ones will be returned from following instances. Please, could you tell me why this is happening? Also, I cannot get different results in each class A instance. This is the code:
class a
{
Random rnd = new Random();
private void Count()
{
int r1 = rnd.Next(-1, 1);
int r2 = rnd.Next(-1, 1);
}
}
class b
{
List<a> listofA=new list<a>();
foreach (a ACLASS in listofA)
{
ACLASS.Count();
}
}
The problem is that you are creating instances of the Random class too close in time.
When you create a Random object, it's seeded with a value from the system clock. If you create Random instances too close in time, they will all be seeded with the same random sequence.
Create a single Random object and pass its reference to the constructor when you create instances of the "a" class, instead of creating one Random object for each "a" instance.
Use a single, static Random number generator for all instances of the class.
class a
{
private static Random rnd;
static a() {
rnd = new Random();
}
private void Count()
{
int r1 = rnd.Next(-1, 2);
int r2 = rnd.Next(-1, 2);
}
}
Note the change to give you numbers in the range -1,1 rather than -1,0
You're creating new instances of Random very close together (your loop is very tight) so each instance is effectively using the same seed value.
A better approach would be to create one instance and pass that to your Count method.
You probably know this next bit, but I'll include it here for completeness:
The MSDN has the details on this, but basically your problem is the Random.Next method you're using generates:
A 32-bit signed integer greater than or equal to minValue and less than maxValue; that is, the range of return values includes minValue but not maxValue. If minValue equals maxValue, minValue is returned.
because of this your calls will return -1 or 0.
You include a random instance for each A instance. It sounds like they're all getting the same default seed value. You probably want to make a static random for all A instances and use it repeatedly, or alternatively provide a seed value to the Random() instance in the A constructor.