无法将条件结果附加到新列表

Question

I'm very new to Python or any programming language in general.

For reference:

1. Assignment
2. My Code
3. Question

1.) The assignment for my class is this: read from text file 'word.txt' and write into a new text file the words from the first text file that fit the criteria. For example, if the function is getListEnd('s', 'word.txt', 'word1.txt') and the criteria is that the words in the ofile must all end with the character 's' then my ifile word.txt=[apples, carrots, cheese, mustard, problems] would be sorted into an ofile word1.txt=[apples, carrots, problems]

2.) Here is my code:

mf1 = open("/Users/XXXX/word.txt", "r")
mf2 = open("/Users/XXXX/word2.txt", "w+")
mylist1=[]                            #list for storing word.txt strings
mylist2=[]                            #list for storing sorted strings

def sort(c):                          #function for sorting mylist1 for 
    for i in mylist1:                        criteria into my list2       
        if c==i[-1]:
            mylist2.append(i)

def getListEnd(c, ifile, ofile):
    for s in mf1:                      #loop for appending word.txt to list 1
        mylist1.append(s)                   
    print(len(mylist1))                #check if mylist1 contains word.txt
    sort(c)                            #append sorted words to list2
    with open('word2.txt', 'w+') as mf2:   #write into word1.txt list2
        for listitem in mylist2:
            mf2.write('%s\n' % listitem)

getListEnd('s', 'word.txt', 'word2.txt')

3.) So the question is: why is it that whenever I try to append into mylist2 the sorted words nothing happens? That is, when I check the length of mylist2 it says 0 even though there are many words in the word.txt file that end in s? I know that there is an error in checking myList1 for characters that end with the last letter 's' but I don't know what and I can't get it to append to mylist2.

Answer 1

There are two things wrong with your program:

1. You were writing to the output file before you had sorted the input
2. You were including newlines in your string so when you compared the last character you were actually comparing newlines.

Here's your program with comments about the changes:

# mf1 = open("word.txt", "r")      # Don't do this here. You read it in in your with block anyway.
# mf2 = open("word2.txt", "w+").   # Don't do this here. You write it in your with block anyway
mylist1=[]
mylist2=[]

def sort(c):
    for i in mylist1:
        if c==i[-1]:
            mylist2.append(i)

def getListEnd(c, ifile):
    with open(ifile, "r") as mf1:
        for s in mf1:  # the strings will still contain the newlines if read this way
                       # so when you check the end of the string you'll actually be comparing
                       # a newline character.
            if "\n" in s:
                mylist1.append(s[:-1]) # if there's a newline character, 
                                       # don't include it when appending to the list
            else:
                mylist1.append(s)
        print("mylist length: ", len(mylist1))
        print("mylist1: ", mylist1)
        sort(c)

getListEnd('s', 'word.txt') # you don't need to pass it any info about the output file
print("mylist2: ", mylist2)

## Your writing block has to come after you've sorted it! Not before
with open('word2.txt', 'w+') as mf2:
    for listitem in mylist2:
        mf2.write('%s\n' % listitem)

More generally:

To create a list with only items that end in the letter "s" you can simply do:

input_list = ['apples', 'carrot', 'johns', 'list']
output = [i for i in input_list if "s" == i[-1]]

print(output)
>> ['apples', 'johns']

The line output = [i for i in input_list if "s" == i[-1] is iterating through your input_list list with i and checking each item to see if the last character of each string i[-1] is the character 's' . It then returns a list with all the items that match the condition. A more expressive way to code it would be:

for word in input_list:
    if word[-1] == "s":
        output.append(word)

With the reading and writing of files it's safer in python to do them within with operators as it ensures that you don't have un-closed files hanging around/is safer scope wise.

A simpler version:

Here's a toy program that I think does what you want:

with open("word.txt", "r") as f:
    mylist1 = f.read().split("\n") # splits the input file around newlines

mylist2 = [i for i in mylist1 if "s" == i[-1]] 

with open("word2.txt", "w") as f:
    for word in mylist2:
        f.write(f"{word}\n") # writes word into the file

if word.txt was:

john
apples
carrots
lewis

Then word2.txt becomes:

apples
carrots
lewis

Answer 2

Reading lines from a file with a for x in file loop gives you a newline character at the end of each line.

So, i[-1] is the newline character, not the last letter of the word.

Answer 3

Here is an example:

with open('words1.txt', 'w') as f_object:
       f_object.write("Apples" + "\n")
       f_object.write("Rubles" + "\n")
       f_object.write("Nope" + "\n")
       f_object.write("Nah" + "\n")
       f_object.write("Georges" + "\n")


f_object.close()
print("Done")

with open('words1.txt', 'r') as f:
lines = f.readlines()


with open('words2.txt', 'w') as f_ob:
     for line in lines:
        line = line.rstrip()
        if line[-1] == 's':
             f_ob.write(line + '\n')

print("Done")

We can simply make a condition that if the word ends in 's' we will add it to the new .txt file. We can grab the last index in the string for example as we iterate through each line. "if" the last index of the line is equal to s, then we can obviously write the line to the file. If not it will skip. The output in your words2.txt file will be:

Apples
Rubles
Georges