how the code produce always different string in different length

Question

3.00/5 (2 votes)

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C#

public static string createRandomString(int Length)
       {
           string _allowedChars = "abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNOPQRSTUVWXYZ";
           Random randNum = new Random();
           int outputLength = (int)(randNum.NextDouble() * Length); //<<<<<<<<< moved out of loop
           char[] chars = new char[outputLength];  //<<<<<<<< use outputLength here
           // not used    int allowedCharCount = _allowedChars.Length;
           for (int i = 0; i < outputLength; i++)
           {
               chars[i] = _allowedChars[(int)((_allowedChars.Length) * randNum.NextDouble())];
           }
           return new string(chars);
       }

hi this code some times generate same string and some times generate different string

how can i rectify this?i want to generate different string always please reply

Posted 22-Jun-11 23:45pm

suryaswathi

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Comments

[no name] 23-Jun-11 5:54am

just a suggestion why dont u use GUID and trim top 6 or top 8 char.

Sergey Alexandrovich Kryukov 23-Jun-11 12:22pm

Please do not re-post.
--SA

7 solutions

Solution 1

The thing you have to understand is that Random isn't truly random. It's actually a pseudo random number generator - which means that the numbers are generated based off a statistical algorithm (in this case, it's a version of Donald Knuth's algorithm). If you start from the same seed number every time, then you generate the same sequence every time. Internally, the seed for the Random class is based on the system clock, but if Random instances are created sufficiently close together the same value will be picked up.

You have two real choices here:

1. Use the same instance of Random (don't recreate it in this method every time).
2. Use the parameterised Random constructor and pass in the tick count - enough ticks will have elapsed between calls (unless you are running in multiple threads where it can't be guaranteed) to generate a new seed.

Posted 22-Jun-11 23:56pm

Pete O'Hanlon

Solution 2

I think you need two randoms here.
Take this as pseudocode:

1. determine length of the string with a random (int stringlength). (you can use Next method with a Min and Max value, that avoids the casting to int)
2. initiate a stringbuilder (sb)
3. start a for loop : for(int i = 0; i < stringlength; i++)
4: in the for loop generate a NEW random number with the Next method with MIN value = 0 and MAX value = _allowedChars.Lenght-1; 
5. append the character with index number from step 4 to the stringbuilder (sb.Append(_allowedChars[randomintbetween0and_allowedcharslength]);)
6. when for loop finishes return sb.toString();

this should do the trick.

Posted 22-Jun-11 23:57pm

V.

Solution 5

What if outputLength == 0, as can arise ?

And you shouldn't you null-terminate the chars string ?

Posted 23-Jun-11 0:05am

YDaoust

Solution 6

try this
System.Guid.NewGuid
this method is used to generate the unique identifiers.

Posted 23-Jun-11 0:21am

ambarishtv

Solution 7

"different string always"

Sorry but that cant be done. It's trivial math: |letter set| ^ numberOfLettersInString may be a quite large number, but it is definitely not infinite. Thus repetitions are bound to occurr if "always" is taken in the sense of "forever".

Cheers!

--MRB

Posted 23-Jun-11 1:21am

Manfred Rudolf Bihy

Solution 8

Hi,

One simple solution.

1 ) Obtain clock tick (which is different at different times.)
2 ) and develop logic based on the clock tick counter.

This should work.

Regards,
Prafulla

Posted 23-Jun-11 1:51am

PrafullaVedante

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OriginalGriff · Accepted Answer · 2011-06-23T00:00:00

There are a few things to do here.
1) Move the definition of randNum to class level - do not generate a new instance each time you want a random number. Since the new instance is initialized partly from the time, this will reduce the chances of getting the same sequence repeated.
2) Don't use NextDouble, then multiply it by the length. Instead, use a random int instead: Random.Next[^]

private Random randNum = new Random();

public static string createRandomString(int Length)
   {
   string _allowedChars = "abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNOPQRSTUVWXYZ";
   int outputLength = randNum.Next(Length);
   char[] chars = new char[outputLength];
   for (int i = 0; i < outputLength; i++)
      {
      chars[i] = _allowedChars[randNum.Next(allowedChars.Length)];
      }
   return new string(chars);
   }

"in this

private Random randNum = new Random(); public static string createRandomNumbers() { int Length=5; string _allowedChars = "0123456789"; int outputLength = randNum.Next(Length); char[] chars = new char[outputLength]; for (int i = 0; i < outputLength; i++) { chars[i] = _allowedChars[randNum.Next(allowedChars.Length)]; } return new string(chars); }

it didnt take my length and produce duplicate code"

"how to rectify this i want to generate unique number in lenght = 5"

Then do it the easy way:

private static Random randNum = new Random();
public static string createRandomString()
   {
   return randNum.Next(100000).ToString("00000");
   }

Which will return a 5 character string with a random number in it.