Generate Unique Random String with Letters and Numbers in Lower Case

How to generate random string of numbers and letters ,In form (2 letters + 4 numbers + 2 letters) Example AD1256Cv

Use this-

private string RandomString()
        {
            var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
            var numbers = "0123456789";
            var stringChars = new char[8];
            var random = new Random();

            for (int i = 0; i < 2; i++)
            {
                stringChars[i] = chars[random.Next(chars.Length)];
            }
            for (int i = 2; i < 6; i++)
            {
                stringChars[i] = numbers[random.Next(numbers.Length)];
            }
            for (int i = 6; i < 8; i++)
            {
                stringChars[i] = chars[random.Next(chars.Length)];
            }

            var finalString = new String(stringChars);
            return finalString;
        }

Random string generation with upper case letters and digits

Answer in one line:

''.join(random.choice(string.ascii_uppercase + string.digits) for _ in range(N))

or even shorter starting with Python 3.6 using random.choices():

''.join(random.choices(string.ascii_uppercase + string.digits, k=N))

A cryptographically more secure version: see this post

''.join(random.SystemRandom().choice(string.ascii_uppercase + string.digits) for _ in range(N))

In details, with a clean function for further reuse:

>>> import string
>>> import random
>>> def id_generator(size=6, chars=string.ascii_uppercase + string.digits):
...    return ''.join(random.choice(chars) for _ in range(size))
...
>>> id_generator()
'G5G74W'
>>> id_generator(3, "6793YUIO")
'Y3U'

How does it work ?

We import string, a module that contains sequences of common ASCII characters, and random, a module that deals with random generation.

string.ascii_uppercase + string.digits just concatenates the list of characters representing uppercase ASCII chars and digits:

>>> string.ascii_uppercase
'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
>>> string.digits
'0123456789'
>>> string.ascii_uppercase + string.digits
'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'

Then we use a list comprehension to create a list of 'n' elements:

>>> range(4) # range create a list of 'n' numbers
[0, 1, 2, 3]
>>> ['elem' for _ in range(4)] # we use range to create 4 times 'elem'
['elem', 'elem', 'elem', 'elem']

In the example above, we use [ to create the list, but we don't in the id_generator function so Python doesn't create the list in memory, but generates the elements on the fly, one by one (more about this here).

Instead of asking to create 'n' times the string elem, we will ask Python to create 'n' times a random character, picked from a sequence of characters:

>>> random.choice("abcde")
'a'
>>> random.choice("abcde")
'd'
>>> random.choice("abcde")
'b'

Therefore random.choice(chars) for _ in range(size) really is creating a sequence of size characters. Characters that are randomly picked from chars:

>>> [random.choice('abcde') for _ in range(3)]
['a', 'b', 'b']
>>> [random.choice('abcde') for _ in range(3)]
['e', 'b', 'e']
>>> [random.choice('abcde') for _ in range(3)]
['d', 'a', 'c']

Then we just join them with an empty string so the sequence becomes a string:

>>> ''.join(['a', 'b', 'b'])
'abb'
>>> [random.choice('abcde') for _ in range(3)]
['d', 'c', 'b']
>>> ''.join(random.choice('abcde') for _ in range(3))
'dac'

Generate Unique Random String With Letters And Numbers In Lower Case

If you are using ruby 1.9.2 you can use SecureRandom:

irb(main):001:0> require 'securerandom'
=> true
irb(main):002:0> SecureRandom.hex(13)
=> "5bbf194bcf8740ae8c9ce49e97"
irb(main):003:0> SecureRandom.hex(15)
=> "d2413503a9618bacfdb1745eafdb0f"
irb(main):004:0> SecureRandom.hex(32)
=> "432e0a359bbf3669e6da610d57ea5d0cd9e2fceb93e7f7989305d89e31073690"

Generate random string/characters in JavaScript

I think this will work for you:

function makeid(length) {
    var result           = '';
    var characters       = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    var charactersLength = characters.length;
    for ( var i = 0; i < length; i++ ) {
        result += characters.charAt(Math.floor(Math.random() * charactersLength));
    }
    return result;
}

console.log(makeid(5));

php Random string generation with letters and numbers

Use the below code.

For digits and characters

$length = 10;    
echo substr(str_shuffle('0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'),1,$length);

For only characters

$length = 10;    
echo substr(str_shuffle('ABCDEFGHIJKLMNOPQRSTUVWXYZ'),1,$length);

For only digits

$length = 10;    
echo substr(str_shuffle('0123456789'),1,$length);

How can I generate random alphanumeric strings?

I heard LINQ is the new black, so here's my attempt using LINQ:

private static Random random = new Random();

public static string RandomString(int length)
{
    const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    return new string(Enumerable.Repeat(chars, length)
        .Select(s => s[random.Next(s.Length)]).ToArray());
}

(Note: The use of the Random class makes this unsuitable for anything security related, such as creating passwords or tokens. Use the RNGCryptoServiceProvider class if you need a strong random number generator.)

Unique random string with alphanumberic required in Ruby

down   = ('a'..'z').to_a
up     = ('A'..'Z').to_a
digits = ('0'..'9').to_a
all    = down + up + digits
[down.sample, up.sample, digits.sample].
  concat(7.times.map { all.sample }).
  shuffle.
  join
  #=> "TioS8TYw0F"

[Edit: The above reflects a misunderstanding of the question. I'll leave it, however. To have no characters appear more than once:

def rnd_str
  down   = ('a'..'z').to_a
  up     = ('A'..'Z').to_a
  digits = ('0'..'9').to_a
  [extract1(down), extract1(up), extract1(digits)].
    concat(((down+up+digits).sample(7))).shuffle.join
end

def extract1(arr)
  i = arr.size.times.to_a.sample
  c = arr[i]
  arr.delete_at(i)
  c
end

rnd_str #=> "YTLe0WGoa1" 
rnd_str #=> "NrBmAnE9bT"

down.sample.shift (etc.) would have been more compact than extract1, but the inefficiency was just too much to bear.

If you do not want to repeat random strings, simply keep a list of the ones you generate. If you generate another that is in the list, discard it and generate another. It's pretty unlikely you'll have to generate any extra ones, however. If, for example, you generate 100 random strings (satisfying the requirement of at least one lowercase letter, uppercase letter and digit), the chances that there will be one or more duplicate strings is about one in 700,000:

t = 107_518_933_731
n = t+1
t = t.to_f
(1.0 - 100.times.reduce(1.0) { |prod,_| prod * (n -= 1)/t }).round(10)
  #=> 1.39e-07

where t = C(62,10) and C(62,10) is defined below.

An alternative

There is a really simple way to do this that turns out to be pretty efficient: just sample without replacement until a sample is found that meets the requirement of at least lowercase letter, one uppercase letter and one digit. We can do that as follows:

DOWN   = ('a'..'z').to_a
UP     = ('A'..'Z').to_a
DIGITS = ('0'..'9').to_a
ALL    = DOWN + UP + DIGITS

def rnd_str
  loop do
    arr = ALL.sample(10)
    break arr.shuffle.join unless (DOWN&&arr).empty? || (UP&&arr).empty? || 
    (DIGITS&&arr).empty?
  end
end

rnd_str #=> "3jRkHcP7Ge" 
rnd_str #=> "B0s81x4Jto

How many samples must we reject, on average, before finding a "good" one? It turns out (see below if you are really, really interested) that the probability of getting a "bad" string (i.e, selecting 10 characters at random from the 62 elements of all, without replacement, that has no lowercase letters, no uppercase letters or no digits, is only about 0.15. (15%). That means that 85% of the time no bad samples will be rejected before a good one is found.

It turns out that the expected number of bad strings that will be sampled, before a good string is sampled, is:

0.15/0.85 =~ 0.17

The following shows how the above probability was derived, should anyone be interested.

Let n_down be the number of ways a sample of 10 can be drawn that has no lowercase letters:

n_down = C(36,10) = 36!/(10!*(36-10)!)

where (the binomial coefficient) C(36,10) equals the number of combinations of 36 "things" that can be "taken" 10 at a time, and equals:

C(36,10) = 36!/(10!*(36-10)!) #=> 254_186_856

Similarly,

n_up = n_down #=> 254_186_856

and

n_digits = C(52,10) #=> 15_820_024_220

We can add these three numbers together to obtain:

n_down + n_up + n_digits #=> 16_328_397_932

This is almost, but not quite, the number of ways to draw 10 characters, without replacement, that contains no lowercase letters characters, uppercase letters or digits. "Not quite" because there is a bit of double-counting going on. The necessary adjustment is as follows:

n_down + n_up + n_digits - 2*C(26,10) - 3
  #=> 16_317_774_459

To obtain the probability of drawing a sample of 10 from a population of 62, without replacement, that has no lowercase letter, no uppercase letter or no digit, we divide this number by the total number of ways 10 characters can be drawn from 62 without replacement:

(16_317_774_459.0/c(62,10)).round(2)
  #=> 0.15

Generate shortest possible random unique letters-only string in a table

Turn the integer primary key into a base 26 number... aka letters.

0  becomes a
1  becomes b
25 becomes z
26 becomes aa
27 becomes ab

There's no need to store this representation in the database, store it as an integer and convert it to letters at the display level.

Because the primary key increments this isn't particularly secure, it can be guessed.

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If you wish to have the code expire, but not the associated data, create a second table which stores the code, when it was created, and a foreign key back to the data. Use an INSERT trigger to add a code.

CREATE TABLE user_codes (
    code     INTEGER PRIMARY KEY AUTO_INCREMENT,
    uid      INTEGER FOREIGN KEY users(id),
    created  TIMESTAMP 
);

As above, turn that integer code into base 26 where the digits are all letters.

This also protects the user's primary key, someone might be able to do something malicious with that maybe, better to not leak it.

This also isn't particularly secure and can be guessed. You can improve on it by using the technique in this answer to pick unique random numbers.

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If it must be random and unique, consider using a UUID_SHORT() call to generate a unique 64 bit number, which is basically the time plus a server ID. Then convert that to base 26 and there to letters. Unfortunately this will require something like 14 letters.

...but because it's based on time this also isn't particularly secure and can be guessed.

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Finally, ask yourself if the code must be globally unique, or if it's enough for the person to know the code for a particular account. For example, a verification code.

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