terratunaz - 8 months ago 69
Python Question

# Selection Sort Algorithm Python

Working on implementing this algorithm using Python. I thought my logic was okay but apparently not as Python is complaining. The while loop is causing issues. If I remove that it works as expected but obviously doesn't sort the whole list. My thought process -> Use Linear Search to find the smallest number -> Append that new number to a list -> Remove that number from the current list -> Loop through that same list (but with smallest number removed) again -> Repeat process until we've gone through the entire list "x" number of times. "x" being equal to the length of the list. The problem I think I'm running into is that the list never gets updated every time I run through the for loop? I keep getting the error

`Line 21: ValueError: list.index(x): x not in list`
.
Even though "x" is in the list. Any idea as to what I'm doing wrong?

``````"""
Selection sort algorithm.
"""

import random
ls = []
max_number = 10
while len(ls) < max_number:
ls.append(random.randint(1,101))
print ls

def selection_sort(items_to_sort):
smallest_number = items_to_sort[0]
current_number = 0
sorted_items = []
item_len = len(items_to_sort)
while item_len > 0:
for item in items_to_sort[:]:
if item < smallest_number:
smallest_number = item
items_to_sort.pop(items_to_sort.index(smallest_number))
sorted_items.append(smallest_number)
item_len -= 1
return sorted_items
print selection_sort(ls)
``````

Answer

It looks like you're not re-initializing the `smallest_number` variable, so after the first execution of your `while` loop - you look for a value smaller than the previous value which you just `pop`ed from the list.

When you don't find a value smaller than the previously smallest value which is no longer in the list, you try to `pop` the same `smallest_number` as in the previous iteration of the `while` loop. However, that value is no longer in the `items_to_sort` list which is why you get the `ValueError`

Try moving the line `smallest_number = items_to_sort[0]` to be the first line executed in every iteration of your `while` loop.

Source (Stackoverflow)