从列表中读取Python中读取文本文件的第n行

Question

I have a function gen_rand_index that generates a random group of numbers in list format, such as [3,1] or [3,2,1]

I also have a textfile that that reads something like this:

red $1

green $5

blue $6

How do I write a function so that once python generates this list of numbers, it automatically reads that # line in the text file? So if it generated [2,1], instead of printing [2,1] I would get "green $5, red $1" aka the second line in the text file and the first line in the text file?

I know that you can do print(line[2]) and commands like that, but this won't work in my case because each time I am getting a different random number of a line that I want to read, it is not a set line I want to read each time.

row = str(result[gen_rand_index]) #result[gen_rand_index] gives me the random list of numbers
file = open("Foodinventory.txt", 'r')
for line in file:
    print(line[row])
file.close()

I have this so far, but I am getting this

error: invalid literal for int() with base 10: '[4, 1]'

I also have gotten

TypeError: string indices must be integers

butI have tried replacing str with int and many things like that but I'm thinking the way I'm just approaching this is wrong. Can anyone help me? (I have only been coding for a couple days now so I apologize in advance if this question is really basic)

Answer 1

Okay, let us first get some stuff out of the way

Whenever you access something from a list the thing you put inside the box brackets [] should be an integer, eg: [5] . This tells Python that you want the 5th element. It cannot ["5"] because 5 in this case would be treated as a string

Therefore the line row = str(result[gen_rand_index]) should actually just be row = ... without the call to str . This is why you got the TypeError about list indices

Secondly, as per your description gen_rand_index would return a list of numbers.

So going by that, why don;t you try this

indices_to_pull = gen_rand_index()
file_handle = open("Foodinventory.txt", 'r')
file_contents = file_handle.readlines() # If the file is small and simle this would work fine
answer = []

for index in indices_to_pull:
    answer.append(file_contents[index-1])

Explanation

1. We get the indices of the file lines from gen_rand_index
2. we read the entire file into memory using readlines()
3. Then we get the lines we want, Rememebr to subtract 1 as the list is indexed from 0

Answer 2

The error you are getting is because you're trying to index a string variable ( line ) with a string index ( row ). Presumably row will contain something like '[2,3,1]' .

However, even if row was a numerical index, you're not indexing what you think you're indexing. The variable line is a string, and it contains (on any given iteration) one line of the file. Indexing this variable will give you a single character. For example, if line contains green $5 , then line[2] will yield 'e' .

It looks like your intent is to index into a list of strings, which represent all the lines of the file.

If your file is not overly large, you can read the entire file into a list of lines, and then just index that array:

with open('file.txt') as fp:
    lines = fp.readlines()

print(lines[2]).

In this case, lines[2] will yield the string 'blue $6\\n' .

To discard the trailing newline, use lines[2].strip() instead.

Answer 3

I'll go line by line and raise some issues.

row = str(result[gen_rand_index]) #result[gen_rand_index] gives me the random list of numbers

Are you sure it is gen_rand_index and not gen_rand_index() ? If gen_rand_index is a function, you should call the function. In the code you have, you are not calling the function, instead you are using the function directly as an index.

file = open("Foodinventory.txt", 'r')
for line in file:
    print(line[row])
file.close()

The correct python idiom for opening a file and reading line by line is

with open("Foodinventory.txt.", "r") as f:
    for line in f:
        ...

This way you do not have to close the file; the with clause does this for you automatically.

Now, what you want to do is to print the lines of the file that correspond to the elements in your variable row . So what you need is an if statement that checks if the line number you just read from the file corresponds to the line number in your array row .

with open("Foodinventory.txt", "r") as f:
    for i, line in enumerate(f):
        if i == row[i]:
            print(line)

But this is wrong: it would work only if your list's elements are ordered. That is not the case in your question. So let's think a little bit. You could iterate over your file multiple times, and each time you iterate over it, print out one line. But this will be inefficient: it will take time O(nm) where n==len(row) and m == number of lines in your file.

A better solution is to read all the lines of the file and save them to an array, then print the corresponding indices from this array:

arr = []
with open("Foodinventory.txt", "r") as f:
    arr = list(f) 
for i in row:
    print(arr[i - 1]) # arrays are zero-indiced