How to implement Selection Sort within a list?

问题:

So I've got a .txt file as follows:

131,263.07
47,170.14
170,190.01
180,412.69
53,401.53

And I had to read the file so as to output the list like:

131 kms, $263.07
47 kms, $170.14
170 kms, $190.01
180 kms, $412.69
53 kms, $401.53

The code I used was:

def PrintList(table):
    for line in table:
       print(str(line[0]) + " kms, $" + str(line[1]))

file = open(input("Enter file name: ")) 

table = []
for line in file:
    line = line.rstrip().split(",")
    line[0] = int(line[0])
    line[1] = float(line[1])
    table.append(line)

PrintList(table)

file.close()

And now I'd like to sort the list in increasing order of price to obtain:

47 kms, $170.14
170 kms, $190.01
131 kms, $263.07
53 kms, $401.53
180 kms, $412.69

How would I implement this in Python? I've tried doing this using Selection Sort but it just doesn't seem to be working.

Update: Thanks for the input so far. However, I have tried the sort function but I'd like to figure out how to implement this using Selection Sort.

Update: I am unable to post the Selection Sort code that I have used as I have overwritten it, nonetheless given below is an example of the code (which I have used to sort a random list of distances) which I had to modify to sort the aforementioned list in increasing order of price. Hope it's sufficient.

def selectionSort(distance):
    n = len(distance)
    for i in range(n):
        minPlace = searchMin(distance)
        swap(distance, i, minPlace+i)

def searchMin(distance):
    minPlace = 0
    n = len(distance)
    for i in range(1, n):
        if distance[i] < distance[minPlace]:
            minPlace = i
        return minPlace

def swap(distance, i, j):
    temp = distance[i]
    distance[i] = distance[j]
    distance[j] = temp

If there's an easier way to implement this, please let me know. Thanks in advance. Cheers.

回答1:

Python lists come with a sort method already. You can simply call it, specifying a key argument to determine how to sort.

def print_list(table):
    for line in table:
       print(str(line[0]) + " kms, $" + str(line[1]))

with open(input("Enter file name: ")) as f:
    table = []
    for line in f:
        line = line.rstrip().split(",")
        line[0] = int(line[0])
        line[1] = float(line[1])
        table.append(line)

    table.sort(key=lambda line: line[1])
    print_list(table)

Note that I made a couple of additional changes to your program, namely renaming PrintList in accordance with PEP8, and using the with statement so that the file gets closed automatically.

If insist on using Selection Sort (it will be worse than Python's default sorting), implement it in a helper function that fulfills the interface of sorted.

回答2:

Is it absolutely necessary to implement selection sort? I would go with sorted:

PrintList(sorted(table, key = lambda x: x[1]))

where

key = lambda x: x[1]

instructs sorted to use the value of the element with index 1 (price) to compare your objects.

回答3:

after your for-loop:

sorted_table = sorted(table, key=lambda row: row[1], reverse=True)

Sorted_table now contains your tabular data, sorted by the 2nd column. You can then pass sorted_table to your PrintList function. An alternative that avoids the lambda:

from operator import itemgetter
sorted_table = sorted(table, key=itemgetter(1), reverse=True)

For more on python sorting, see: https://wiki.python.org/moin/HowTo/Sorting