如何在Python- .txt文件中保存列表并访问列表中的值

Question

I want to create a module that checks whether a number is prime or not, and to list all of the primes before it.

The functions in the module would take a bit of time if it had to generate the list of primes every single time it was created.

How do I open a .txt file that is in the same folder as the program is in, then have it access the data from it and create a list based off of that? The .txt file will be saved a list afterwards when the function is completed. I also need a bit of guidance on how to actually save the file in a given location.

Any suggestions or tips would be great. Thanks!

Answer 1

I would pickle a set object, you could do this:

import cPickle as pickle

primes_set = set([2,3,5])

pickle.dump(primes_set, open('primes_set.pkl', 'wb')) 

my_primes = pickle.load(open('primes_set.pkl', 'rb')) # to load the object

After you created a pickle file, you could use it like this:

def get_primes(my_nums):
    '''
    Return only prime numbers from inputted list.
    '''
    my_primes = pickle.load(open('primes_set.pkl', 'rb'))
    return [num for num in my_nums if num in my_primes]

Answer 2

Have a look at io in python http://docs.python.org/2/tutorial/inputoutput.html

as you seem to be doing number crunching i would use pytables (hdf5 binary file) to save load and update arrays

http://www.pytables.org/moin

Answer 3

You have several options. If you want to save the list as a list, the Pickle is probably the best way to go.

If you don't mind processing your txt file then you could just store it as a csv , which python will turn back into a list readily.

Answer 4

In order to open a file, just use

filereader = open("example.txt")

then the variable filereader will contain the file's data.

allprimes = []
for line in filereader:
    allprimes.append(int(line)) #assuming one number is stored per line

allprimes will now contain all numbers stored in the file.