Selection Sort (low to high) python

Question

I am trying to write a selection sort algorithm for sorting lists of numbers from lowest to highest.

def sortlh(numList):
    if type(numList) != list:
        print("Input must be a list of numbers.")
    else:
        inf = float("inf")
        sortList = [0]*len(numList)
        count = 0
        while count < len(numList):          
            index = 0
            indexLowest = 0
            lowest = numList[index]
            while index < (len(numList) - 1):
                if numList[index + 1] < numList[index]:
                    lowest = numList[index + 1]
                    indexLowest = index + 1
                    index = index + 1
                else:
                    index = index + 1
            sortList[count] = lowest
            numList[indexLowest] = inf
            count = count + 1
    return sortList

When I run this code on:

sortlh([9,8,7,6,5,4,3,2,1])

I get (as expected):

[1, 2, 3, 4, 5, 6, 7, 8, 9]

However, when I try another example, I get:

sortlh([1,3,2,4,5,7,6,9,8])

[8, 6, 9, 2, 4, 5, 7, 1, 3]

Does anyone see what is going on here?

Answer 1

Here is how I would suggest rewriting your program.

def sortlh(lst_input):
    lst = list(lst_input) # make a copy of lst_input
    i = 0
    while i < len(lst):
        j = i + 1
        i_lowest = i
        lowest = lst[i_lowest]
        while j < len(lst):
            if lst[j] < lowest:
                i_lowest = j
                lowest = lst[i_lowest]
            j += 1
        lst[i], lst[i_lowest] = lst[i_lowest], lst[i]  # swap
        i += 1
    return lst

test = [9,8,7,6,5,4,3,2,1]
assert sortlh(test) == sorted(test)
test = [1,3,2,4,5,7,6,9,8]
assert sortlh(test) == sorted(test)

• We don't test the type of the input. Anything that acts like a list will work, and even an iterator will work.

• We don't "mutate" the original input list. We only work on a copy of the data.

• When we find the lowest number, we swap it with the first number, and then only look at the remaining numbers. Thus we have less work to do on each loop as we have fewer and fewer unsorted numbers.

EDIT:

If you are a beginner, this part might seem too tricky. If it confuses you or you don't like it, just ignore it for now.

This is a more-advanced way to solve this problem in Python. The inner loop simply finds the lowest number and the index of the lowest number. We can use the Python built-in function min() to do this!

We build a "generator expression" that loops over the list, yielding up tuples. Each tuple is the number and its position. Since we want lower numbers to sort lower, we put the number first in the tuple so that min() can properly compare the tuples. Then min() will find the lowest tuple and we get the value and index.

Also, the outer loop is now a for loop with enumerate rather than a while loop using indexing.

def sortlh(lst_input):
    lst = list(lst_input) # make a copy of lst_input
    for i, x in enumerate(lst):
        lowest, i_lowest = min((n, j) for j, n in enumerate(lst) if j >= i)
        lst[i], lst[i_lowest] = lst[i_lowest], lst[i]  # swap
    return lst

test = [9,8,7,6,5,4,3,2,1]
assert sortlh(test) == sorted(test)
test = [1,3,2,4,5,7,6,9,8]
assert sortlh(test) == sorted(test)