Fast method to generate random string sequence [duplicate]

Question

I have a function which returns a string of size N containing a random sequence of characters form a small set {A,B,C,D}. I generate this line as:

def gen_line(N):
  tline = ""
  for i in range(N):
    xrand = random.random()
    if( xrand < 0.25 ):
      ch = "A" 
    elif( xrand < 0.50 ):
      ch = "B" 
    elif( xrand < 0.75 ):
      ch = "C" 
    elif( xrand < 1.00 ):
      ch = "D" 
    else:
      print "ERROR: xrand = %f"%( xrand )
    tline = tline+ch
  return tline

but this is, no doubt, a very inefficient way to do things. Is there a better, more pythonic, way to accomplish this?

So your problem is not speed but how pythonic your solution is? — syntonym
– syntonym, Commented May 6, 2015 at 18:50

Shashank · Accepted Answer · 2015-05-06 18:56:00Z

2

Try using random.choice with str.join.

>>> x = 'abcd'
>>> ''.join(random.choice(x) for _ in range(10))
'aabbdbadbc'

edited May 6, 2015 at 18:56

answered May 6, 2015 at 18:54

Shashank

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3 Comments

Shashank Over a year ago

@ndpu The problem with random.sample is that it samples without replacement. You will not be able to generate strings like 'aa'.

drjrm3 Over a year ago

This is actually much slower. Why would that be? I am looking for a line of length 60, if it matters. It takes maybe twice as long.

Shashank Over a year ago

@Laurbert515 There are several reasons why it could be slower, so I'm not sure. Benchmarks are often difficult to explain without rigorous testing and source code analysis. Instead of using random.choice, you could try n = len(x) - 1; ''.join(x[randint(0, n)] for _ in xrange(N)) Who knows, it could be faster or slower than other methods. Or just use np.random.choice from the other answer.

EdChum · Accepted Answer · 2015-05-06 18:55:32Z

0

You can use np.random.choice:

In [13]:

import random
a = np.array(list('abcd'))
%timeit ''.join(np.random.choice(a, 10000))

def gen_line(N):
  tline = ""
  for i in range(N):
    xrand = random.random()
    if( xrand < 0.25 ):
      ch = "A" 
    elif( xrand < 0.50 ):
      ch = "B" 
    elif( xrand < 0.75 ):
      ch = "C" 
    elif( xrand < 1.00 ):
      ch = "D" 
    else:
      print("ERROR: xrand = %f"%( xrand ))
    tline = tline+ch
  return tline

%timeit gen_line(10000)
100 loops, best of 3: 6.39 ms per loop
100 loops, best of 3: 11.7 ms per loop

answered May 6, 2015 at 18:55

EdChum

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