Random String generator python

Question

I made a simple program in Python to generate a random string with 5 numbers in it:

import random

numcount = 5
fstring = ""

for num in range(19): #strings are 19 characters long
    if random.randint(0, 1) == 1:
        x = random.randint(1, 26)
        x += 96
        fstring += (chr(x).upper())
    elif not numcount == 0:
        x = random.randint(0, 9)
        fstring += str(x)
        numcount -= 1

print(fstring)

Not too hard, right? Except for one incredibly strange thing: the strings it returns are of a random length. I have run the code several times, and here are some of my results:

>>> ================================ RESTART ================================
>>> 
VQZ99HA5DER0CES4
>>> ================================ RESTART ================================
>>> 
05PS0T86LOZS
>>> ================================ RESTART ================================
>>> 
E2QX8296XK
>>> ================================ RESTART ================================
>>> 
M5X9K457QDNBPX

I can't figure out what's going on... Can anyone point me in the right direction?

Without doing any deep-level analysis, you can see that sometimes both ifs will fail (leading to a shorter fstring) and sometimes, randomly, they won't. — Michael Lorton
– Michael Lorton, Commented Jun 19, 2014 at 22:02
Sometimes, very rarely, they won't. On average you'd get 9.5 iterations in the numeric branch, and only 5 will do anything. Pretty unlikely you'd only get 5 or fewer iterations. — Tom Zych
– Tom Zych, Commented Jun 19, 2014 at 22:05

Martijn Pieters · Accepted Answer · 2014-06-19 22:01:33Z

You flip a coin 19 times; 50 percent of the time you pick a letter, the other 50 percent you pick a digit, but only up to 5 times. If you hit the number option more often, you do not add anything.

So you build a string up to 19 characters, but it can be shorter. On average it'll be 9.5 letters, and 5 digits.

Only pick numbers if you still have numbers to pick:

import string
import random

numcount = 5
chars = []

for num in range(19): #strings are 19 characters long
    if numcount and random.random() < 0.5:
        chars.append(str(random.randint(0, 9)))
        numcount -= 1
    else:
        chars.append(random.choice(string.ascii_uppercase))

fchars = ''.join(chars)

Demo:

>>> import string
>>> import random
>>> string.ascii_uppercase
'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
>>> numcount = 5
>>> chars = []
>>> for num in range(19): #strings are 19 characters long
...     if numcount and random.random() < 0.5:
...         chars.append(str(random.randint(0, 9)))
...         numcount -= 1
...     else:
...         chars.append(random.choice(string.ascii_uppercase))
... 
>>> ''.join(chars)
'3M6G97OEHP6TGYRONPV'
>>> len(chars)
19

Alexandr Abramov · Accepted Answer · 2014-06-19 22:04:26Z

0

import random

numcount = 5
fstring = ""


while numcount > 0:
    if random.randint(0, 1) == 1:
        x = random.randint(1, 26)
        x += 96
        fstring += (chr(x).upper())
    elif not numcount == 0:
        x = random.randint(0, 9)
        fstring += str(x)
    numcount -= 1

print(fstring)

answered Jun 19, 2014 at 22:04

Alexandr Abramov

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2 Comments

Tim Over a year ago

This can generate strings as short as 5 characters, i dont think he wants that

Martijn Pieters Over a year ago

This produces only 5 characters.

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