How do I read from a file and put it into a list as an integer? Python

Question

This is what I have

file = open("numberlist.txt")
list = []
for line in file:
      line = line.strip()
      list.append(line)

I would line to convert each line into an integer in the for line in file: How do I do this?

Optional: And If I have non numerical character in numberlist.txt ? how can I test that and have to program tell me the file is not OK?

Answer 1

If you have one digit in each line you can directly use a file-object and map function :

with open("numberlist.txt") as f:
     num_list=map(int,f)

Notes : never use of python built-in names as your variable names and also open your file with with statement.

The with statement is used to wrap the execution of a block with methods defined by a context manager.

Read more about the with statement and its usage advantage. https://docs.python.org/2/reference/compound_stmts.html

But if you may have non numerical characters in your file based on whats your file look like you can use some recipes like exception-handling that it can be done with try-except statement.

So if we suppose you have one word (contains digit or character) in each line you can do

num_list=[]
with open("numberlist.txt") as f:

  for line in f:
     try :
       num_list.append(int(line))
     except:
       continue #or other stuffs

Note that this can have a lot of scenarios and based on your code you can use various approaches!

Answer 2

you can use the int constructor or type-caster if you will, to convert string to integer. it throws a ValueError if the conversion is not possible.

int_value = int(strvalue)

a good way to do the same operation on all elements in a list in python is to use the map function. do a help(map) in the interpreter to see how it works

# assuming that the file exists
file = open("numberlist.txt")
my_list = []
for line in file:
        line = line.strip()
        list.append(line)
try:
    list_of_ints = map(int, my_list)
except ValueError:
    print("invalid file")

to check if the list is proper, put the operation in a try-except block.

you can do all of this in a couple of lines by using the with statement

with open("numbers.txt") as file_object:
    try:
        list_of_ints = map(int, (l.strip() for l in file_object))
    except ValueError:
        print("invalid file")

here i am using file_object as an iterable (it gives all the lines in the file). and using the generator syntax to create another iterable on which to apply the map function

Answer 3

file = open("numberlist.txt",'r')
l = []

for line in file:
    line = line.split()
    try:
        l.append(int(line[0]))
    except:
        continue