如何读取文件并仅用特定位数打印行

Question

input_file = input("Open what file:")

try:
    input_file = open(input_file)
    for line_str in input_file:
         if input_file == 4 and line_str.isdigit():
    print(line_str)

except IOError:
    print("The input file doesn't exist.")
    sys.exit(1)

    input_file.close

In my text file I had these numbers:

174862
2000
2400
9996
12
55

How do I make so that only the numbers with at least 4 digits are printed out?

Answer 1

You need to strip the new line char which is appended to line_str.

Try something like this:

input_file = "file.txt"

try:
    input_file = open(input_file)
    for line_str in input_file:
        line_str = line_str.strip()

        if len(line_str) == 4 and line_str.isdigit():
            print(line_str)

except IOError:
    print("The input file doesn't exist.")
    sys.exit(1)

    input_file.close

You can also use a regex to see if you have a set of 4 digits. I can add how that can be accomplished if you want me to.

EDIT: To match using a regex, you can use the below

import re
line_str = line_str.strip()

    if re.match(r'^[0-9]{4}$', line_str):
            print "RE " + line_str

re is pythons regular expression module. re.match will return a None object if there is no match. Hence you can use it directly in the if condition. r'^[0-9]{4}$' - is basically your regular expression. '^' denotes beginning, '$' denotes end. [0-9] is the number range, and {} denotes count.

Hope it clears things up.

Answer 2

Instead of comparing input_file == 4 (which will never be true, since input_file is a file object, not an integer), you want to compare the len of your line_str with 4. Try:

for line_str in input_file:
    if len(line_str) >= 4 and line_str.isdigit():
        print(line_str)

This will print only lines that are at least four characters long and consist only of digits.

You probably should also be using a with statement to handle the closing of the file, but that's a minor issue (it will probably work just find without it).

Answer 3

import re
try:
    input_file = open("input.txt",'r')
    x=input_file.read()
    print re.findall(r"(\d{4,})",x)


except IOError:
    print("The input file doesn't exist.")
    sys.exit(1)

    input_file.close

Try this.This will give all numbers of greater than length 4 .

This doesnt loop through file as well.

Answer 4

You can use the len() function to check the length of the number you are reading.

I have modified your program.

import sys
input_file = input("Open what file:")

try:
    with open(input_file) as in_file:
        for line_str in in_file:
         if len(line_str.strip()) >= 4 and line_str.strip().isdigit():

            print(line_str.strip())

except IOError:
    print("The input file doesn't exist.")
    sys.exit(1)

Also you don't require file close when you use with .