How do I delete empty lists from my list?

Question

I have the following txt file:

1 2 3 4 5 6 7 8 9

2 3 4 5 6 7 6 8 9
3 4 5 6 7 7 8 8 9

1 2 3 4 5 6 7 8 9

4 5 6 7 8 9 10 11

5 6 7 6  7 8 8 9 15

when opening it and generating a list of lists many positions become empty, to try to solve this problem I used the following code:

with open('C:/Anaconda/loto.txt', 'r') as f: 
   results = [[int(entry) for entry in line.split() != []] for line in f.readlines()] 
   print(results)

and is making that mistake:

----> 2 Results = [[int(entry) for entry in line.split() != []] for line in f.readlines()]

Would anyone know how to solve this problem?

Answer 1

This line here has a problem:

   results = [[int(entry) for entry in line.split() != []] for line in f.readlines()] 
#                                                  ^-- falta um IF

When you use the for conditionally, must use the format

for name in list if condition

That in your case, it should be something like:

   results = [[int(entry) for entry in line.split() if entry != []] for line ... etc

So when entry is empty, will not be added to the internal list of results

Taking advantage, you can put a condition in the outside list, to filter empty lines if you want:

   results = [...  igual linha de cima ...] for line in f.readlines() if line.split()] 

Here, the line.split() will be considered true if they are not blank lines.

If you’re having a little trouble understanding Py’s quirks, here’s a less cryptic alternative to the code, which can be easily adapted to lists:

How to read hyphenated numbers in a text file?

Answer 2

Bacco’s answer explains well what the problem was, but I would like to add some comments.

The function readlines will read the entire contents of the file at once, placing each line of the file in a list item. For small files - and for applications that waste memory - this will not be a problem, but if you need to process very large files, it will be a problem. One way around this is by using generators. It is possible to work with each line of the file separately, reading one by one and doing the necessary processing. To set a generator, just use the yield:

def list_of_integers(filename):
    with open(filename) as stream:
        for line in stream:
            line = line.strip()
            if line:
                yield [int(number) for number in line.split()]

So you can go through all the lines as follows:

for numbers in list_of_integers('C:/Anaconda/loto.txt'):
    print(numbers)

The result would be:

[1, 2, 3, 4, 5, 6, 7, 8, 9]
[2, 3, 4, 5, 6, 7, 6, 8, 9]
[3, 4, 5, 6, 7, 7, 8, 8, 9]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
[4, 5, 6, 7, 8, 9, 10, 11]
[5, 6, 7, 6, 7, 8, 8, 9, 15]

^{See working on Repl.it}

Other comments:

• When opening the read-only file, you can omit the mode argument, 'r';
• Avoid variables with name f, because they will not be readable in the course of the code;
• If the file lines are too large too, it is convenient to return a generator generators replacing the brackets in yield by parentheses. This way, the line will remain in memory only once and not twice - even if for a short period of time;
• The function strip is therefore necessary, in that case, the character \n is maintained in line;