Python sorted list insertion

Question

I need help for an exercise in python where I have to fill a preexistent code.

The function take an integer and a sorted list, it inserts the value a and then returns the new list, I have to replace the dots by the correct answer

It looks like this :

def insert(a, tab):
    l = list(tab)
    l.append(a)
    i = ...
    while a < ... :
        l[i+1] = ...
        l[i] = a
        i = ...
    return l

>>> insert(3,[1,2,4,5])
[1, 2, 3, 4, 5]

Thanks for the help.

It seems like you are supposed to do something similar to bubble sort - swap positions of elements at indices `i` and `i+1`, and keep doing that from right to left until the inserted element is in the correct position. Try solving it yourself with this idea in mind. — Luka Mesaric, Jan 28 '21 at 16:07
Feel the code structure and variable name is try to confuse people...(or written poorly!) — Daniel Hao, Jan 28 '21 at 17:16

rhorwitz · Accepted Answer · 2021-01-28T16:47:16.103

1

def insert(a, tab):
    l = list(tab)
    l.append(a)
    i = len(l) - 2
    while a < l[i] and i >= 0:
        l[i+1] = l[i]
        l[i] = a
        i -= 1
    return l

edited Jan 28 '21 at 16:47

answered Jan 28 '21 at 16:14

rhorwitz

104
2

1

As far as I can tell, this will crash if the element has to be inserted at the beginning of the list. However, checking `i` in the loop condition to avoid that problem does not seem possible with the original code. – Luka Mesaric Jan 28 '21 at 16:22
Good catch. Updated to fix – rhorwitz Jan 28 '21 at 16:47
Nice fix. For anyone wondering, by having `i>=0` after `a < l[i]`, `l[-1]` will be accessed in the last iteration. In Python that is not a problem, but in other languages it might cause an error. `i>=0` should be checked first, but the provided code template does not allow doing that. That is probably an oversight. – Luka Mesaric Jan 29 '21 at 17:15

score 0 · Answer 2 · answered Jan 28 '21 at 17:11

Disclaimer - Nothing new here. Just for a reference, make the variable more descriptive and simplify the code. (for future reading, or myself)

def insert(x, lst):
    ''' lst is a sorted list'''
    ''' bubble sort - '''
    
    lst.append(x)   # to the end
    
    i = len(lst) - 2
    
    while i >=0 and lst[i] > x:
        lst[i+1], lst[i] = lst[i], x
        i -= 1
    return lst
  

insert(3,[1,2,4,5])
[1, 2, 3, 4, 5]

score 0 · Answer 3 · answered Jan 28 '21 at 18:49

Easy, we can even do it by replacing only one of the ...:

def insert(a, tab):
    l = list(tab)
    l.append(a)
    i = ...
    while a < (l.sort() or a) :
        l[i+1] = ...
        l[i] = a
        i = ...
    return l

Testing it:

>>> insert(3,[1,2,4,5])
[1, 2, 3, 4, 5]

score -2 · Answer 4 · answered Jan 28 '21 at 16:05

-2

Taking inspiration from the answer above, I would say something that also duplicates the list instead of overwriting on it, like the following

def insert_and_sort(a, tab):
    newtab = list(tab)
    newtab.append(a)
    sort(newtab)
    return newtab

answered Jan 28 '21 at 16:05

Francesco Rossetto

1
1

Python sorted list insertion

4 Answers4