Creating a list of random elements using recursion

Question

I was asked to define a recursive function that takes in two parameters:

n
valmax

and returns a list of n numbers picked randomly from the interval [0 , valmax]

`

import random

def random_list(n, valmax, lst = []):
    """
    parameters : n of type int;
                 valmax of type int;
    returns    : a list of n numbers picked randomly from the interval 
                 [0, valmax]
    """
    if len(lst) == n:     
        return lst
    return [random.randint(0, valmax)] + random_list(n, valmax)

print(random_list(10,100))`

However, I'm getting an

RecursionError

How can I fix my code so that it returns a list with n random numbers in the interval [0, valmax] ?

you are trying to slice lst in your recursive step, but the list is empty at first. you may want to rethink if you'd want to slice it in the first place. — Paritosh Singh, Nov 11 '18 at 19:17

score 3 · Accepted Answer · answered Nov 11 '18 at 19:19

Your logic is wrong. You need each function call to return n random integers, so you do not need to pass it in a list.

Each function generates a single random number in the range [0, valmax] and concatenates it to the random list of integers which is of length one less (n-1) which it gets from calling itself recursively.

The base case is when n == 1, in which case we return an empty list.

import random
def random_list(n, valmax):
    if n == 0:
        return []
    return [random.randint(0, valmax)] + random_list(n-1, valmax)

and a test:

random_list(10, 20)
#[20, 9, 4, 7, 3, 4, 3, 18, 19, 9]

score 2 · Answer 2 · answered Nov 11 '18 at 19:21

Instead of keeping a default parameter (which can also cause unexpected behavior on consecutive calls), use yield for a cleaner solution. Also, simply use random.randint(0, valmax) to generate a single random integer between 0 and valmax:

import random
def random_list(n, valmax):
  if n:
    yield random.randint(0, valmax)
    yield from random_list(n-1, valmax)

print(list(random_list(10, 10))) #create a list of length ten with random values between 0 and 10, inclusive.

Output:

[4, 6, 9, 1, 10, 2, 2, 8, 2, 10]

score 1 · Answer 3 · answered Nov 11 '18 at 20:14

You could write a generic build_list function -

import random

def identity (x):
  return x

def build_list (size, proc = identity):
  if size == 0:
    return []
  else:
    return build_list (size - 1, proc) + [ proc (size - 1) ]

print (build_list (5))
# [ 0, 1, 2, 3, 4 ]

print (build_list (5, lambda _: random.randint (0, 10)))
# [ 4, 7, 7, 3, 6 ]

random_list could be a specialization of build_list -

def random_list (size, valmax):
  return build_list (size, lambda _: random.randint (0, valmax))

print (random_list (5, 10))
# [ 1, 7, 4, 7, 0 ]

Creating a list of random elements using recursion

3 Answers3