将一个列表中的项目附加到另一个列表答案

【问题标题】：Appending an item from one list to another将一个列表中的项目附加到另一个列表
【发布时间】：2021-03-17 09:26:57
【问题描述】：

我这里有从文件读取的代码，应该创建和打印一个地区列表，以及有多少地区。

def main():

    #open the file
    myFile = open("Data.txt")

    #read the first line
    firstLine = myFile.readline()

    #initialize a counter
    count = 0

    #for each line in the file
    for dataLine in myFile:

        #strip the end of the line
        dataLine = dataLine.rstrip("\n")

        #split the line into a list
        dataList = dataLine.split(",")

        #create a new list
        districts = []

        #if dataList[3] is not in districts already
        if dataList[3] in districts == False:
            #append dataList[3] to the districts list
            districts.append(dataList[3])
            count = count + 1

    #print the districts list as well as how many were found
    print("Here is a list of all districts:")
    print(" ")
    print(districts)
    print("There are",count,"districts.")
            
    #close the file
    myFile.close

main()

但是，我遇到了一个问题，似乎没有从 dataList 将任何内容添加到地区列表中。我确信这与我的代码措辞方式有关，但我不清楚它可能是什么。任何帮助将不胜感激。

【问题讨论】：

注意rstrip("\n")的使用可以替换为strip()，因为它默认会删除换行符

标签： python list for-loop append

【解决方案1】：

要在数据列表中查找唯一值，您可以使用set

所以，这里因为第 4 列的数据是区，你想找到这个列表 dataList（这是所有区的列表）的唯一值

unique_districts = list(set(districts)

这里set() 将地区转换为唯一值，list() 将其转换回列表

def main()
    # get contents of the file into a single list
    with open("Data.txt","r") as fid:
        contents = fid.readlines()

    # assuming the district data is in 4th column
    districts=list(set([line.split(",")[3].rstrip() for line in contents]))
    # length of the districts list is the count

    print("Here are all the districts")
    print(districts)
    print("There are", len(districts), "districts.")

【讨论】：

【解决方案2】：

命令

    #if dataList[3] is not in districts already
    if dataList[3] in districts == False:

应该是

    #if dataList[3] is not in districts already
    if dataList[3] not in districts:

此外，你有命令

    #create a new list
    districts = []

在您的 for 循环中，因此您一次又一次地将其设为空，使其没有机会获得多个元素。将其移出循环。

顺便说一句，为什么不使用内置模块csv 而不是手动解析每一行来创建其部件列表？这是您的代码的修改版本：

import csv

def main():

    #open the file
    with open("Data.txt") as my_file:
        reader = csv.reader(my_file, delimiter=",")
        
        #ignore the first line
        next(reader)

        #initialize a counter
        count = 0

        #create a new list
        districts = []
        
        #for each line in the file, AUTOMATICALLY CHANGED BY csv.reader() TO A LIST
        for data_list in reader:

            #if data_list[3] is not in districts already
            if data_list[3] not in districts:
                #append data_list[3] to the districts list
                districts.append(data_list[3])
                count += 1

    #print the districts list as well as how many were found
    print("Here is a list of all districts:")
    print()
    print(districts)
    print("There are", count, "districts.")

注意：

我使用名称 data_list 代替名称 dataList 与 PEP 8 一致

【讨论】：

【解决方案3】：

if dataList[3] in districts 总是返回 false。

考虑改成

if dataList[3] not in districts:

此外，如 cmets 中所述，您可能还希望将行更改为：

dataLine = dataLine.rstrip()

【讨论】：