CODEWITHSUNDEEP
pythonlist
rikovvv
rikovvv

Reputation: 71

How to insert elements into the list at arbitrary positions?

I have this

>>> a = [1, 4, 7, 11, 17]

Is there any way to add 4 characters '-' randomly between the other elements to achieve, for example

['-', 1, '-', 4, 7, '-', '-', 11, 17]

Upvotes: 4

Views: 7712

Answers (4)

boardrider
boardrider

Reputation: 6185

Since performance is not an issue, the following is another solution to your issue (amended per @AChampion comment):

from __future__ import print_function

import random

_max = 4
in_list = [1, 4, 7, 11, 17]
out_list = list()

for d in in_list:
    if _max:
        if random.choice([True, False]):
            out_list.append(d)
        else:
            out_list.extend(["-", d])
            _max -= 1
    else:
        out_list.append(d)

# If not all 4 (-max) "-" have not been added, add the missing "-"s at random.
for m in range(_max):
    place = random.randrange(len(out_list)+1)
    out_list.insert(place, "-")

print(out_list)

Which gives:

$ for i in {1..15}; do python /tmp/tmp.py; done
[1, '-', 4, '-', '-', 7, 11, '-', 17]
['-', 1, 4, '-', '-', 7, 11, '-', 17]
['-', 1, 4, '-', 7, '-', 11, 17, '-']
[1, '-', 4, '-', '-', 7, '-', 11, 17]
[1, '-', 4, '-', '-', 7, 11, '-', 17]
['-', 1, 4, '-', 7, 11, '-', 17, '-']
['-', '-', 1, '-', 4, '-', 7, 11, 17]
[1, 4, '-', 7, '-', '-', '-', 11, 17]
['-', 1, 4, 7, '-', 11, '-', '-', 17]
[1, 4, '-', '-', '-', 7, '-', 11, 17]
['-', '-', 1, 4, 7, 11, '-', 17, '-']
['-', '-', 1, '-', 4, '-', 7, 11, 17]
['-', 1, '-', 4, '-', 7, 11, '-', 17]
[1, '-', 4, '-', 7, '-', 11, '-', 17]
[1, '-', '-', 4, '-', 7, 11, '-', 17]

Upvotes: 0

AChampion
AChampion

Reputation: 30258

You could randomly interleave the '-'s using iterators and random.sample():

In [1]:
a = [1, 4, 7, 11, 17]
pop = [iter(a)]*len(a) + [iter('-'*4)]*4
[next(p) for p in random.sample(pop, k=len(pop))]

Out[1]:
['-', '-', 1, '-', 4, 7, 11, '-', 17]

Upvotes: 2

user2390182
user2390182

Reputation: 73460

You could simply do:

import random
for _ in range(4):
    a.insert(random.randint(0, len(a)), '-')

The loop body inserts a '-' at a random index between 0 and len(a)(inclusive). However, since inserting into a list is O(N), you might be better off performance-wise constructing a new list depending on the number of inserts and the length of the list:

it = iter(a)
indeces = list(range(len(a) + 4))
dash_indeces = set(random.sample(indeces, 4))  # four random indeces from the available slots
a = ['-' if i in dash_indeces else next(it) for i in indeces]

Upvotes: 8

tigrank
tigrank

Reputation: 31

Python has insert(index, value) list method that will do the trick. What you want is:

import random

l = [1, 2, 3, 4]
for x in range(0, 4): # this line will ensure 4 element insertion
   l.insert(random.randrange(0, len(l)-1), '-')

randrange() will generate random integers from within index range of your list. Thats it.

Upvotes: 2

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