Can Python generate a random number that excludes a set of numbers, without using recursion?

Question

I looked over Python Docs (I may have misunderstood), but I didn't see that there was a way to do this (look below) without calling a recursive function.
What I'd like to do is generate a random value which excludes values in the middle.

In other words,
Let's imagine I wanted X to be a random number that's not in
range(a - b, a + b)
Can I do this on the first pass,
or
1. Do I have to constantly generate a number,
2. Check if in range(),
3. Wash rinse ?

As for why I don't wish to write a recursive function,
1. it 'feels like' I should not have to
2. the set of numbers I'm doing this for could actually end up being quite large, and
... I hear stack overflows are bad, and I might just be being overly cautious in doing this.

I'm sure that there's a nice, Pythonic, non-recursive way to do it.

Solution 1

Use random.choice(). In this example, a is your lower bound, the range between b and c is skipped and d is your upper bound.

import random
numbers = range(a,b) + range(c,d)
r = random.choice(numbers)

Solution 2

Generate one random number and map it onto your desired ranges of numbers.

If you wanted to generate an integer between 1-4 or 7-10, excluding 5 and 6, you might:

1. Generate a random integer in the range 1-8
2. If the random number is greater than 4, add 2 to the result.

The mapping becomes:

Random number:    1  2  3  4  5  6  7  8
Result:           1  2  3  4  7  8  9 10

Doing it this way, you never need to "re-roll". The above example is for integers, but it can also be applied to floats.

Solution 3

A possible solution would be to just shift the random numbers out of that range. E.g.

def NormalWORange(a, b, sigma):
    r = random.normalvariate(a,sigma)
    if r < a:
        return r-b
    else:
        return r+b

That would generate a normal distribution with a hole in the range (a-b,a+b).

Edit: If you want integers then you will need a little bit more work. If you want integers that are in the range [c,a-b] or [a+b,d] then the following should do the trick.

def RangeWORange(a, b, c, d):
    r = random.randrange(c,d-2*b) # 2*b because two intervals of length b to exclude
    if r >= a-b:
        return r+2*b
    else:
        return r

Solution 4

I may have misunderstood your problem, but you can implement this without recursion

def rand(exclude):
    r = None
    while r in exclude or r is None:
         r = random.randrange(1,10)
    return r

rand([1,3,9])

though, you're still looping over results until you find new ones.

Solution 5

The fastest solution would be this (with a and b defining the exclusion zone and c and d the set of good answers including the exclusion zone):

offset = b - a
maximum = d - offset
result = random.randrange(c, maximum)
if result >= a:
    result += offset

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