一行中的Python 2d列表生成器

Question

I have a matrix of numbers in a file (distances between cities for solving TSP). I load it up into a list. After that each row in my list looks like this:

' 9999    3    5   48   48    8    8    5    5    3    3    0    3    5    8    8    5\n'

The number of whitespaces between numbers may be different on each line. Now I just want to convert it to a list of lists of ints/floats. I got that with a loop:

matrix = []
for row in lines[begin:end]:
    matrix.append([float(x) for x in row.split()])

It works just fine. I am just curious if I can use a generator here (like I use it in loop), but not in loop, but in one line. I tried something like [float(x) for x in [y.split() for y in lines[begin:end]]] , but it says:

float() argument must be a string or a number, not 'list'

So can it be solved like in one line, or should I leave it in loop?

Answer 1

You could use the following list comprehension

matrix = [[float(i) for i in row.split()] for row in lines]

But for this particular case, it would be much faster to use numpy.genfromtxt

import numpy as np
matrix = np.genfromtxt('your_file', delimeter=' ')

Answer 2

Of course you can.

my_list = [' 9999    3    5   48   48    8    8    5    5    3    3    0    3    5    8    8    5\n', 
           ' 3    12    5   48   48    8    8    5    5    3    3    0    3    5    8    8    5\n']

gen = (list(map(float, f.split())) for f in my_list)

You can now iterate over gen .

For example:

for i in gen:
  print(i)

• First iteration returns: [9999.0, 3.0, 5.0, 48.0, 48.0, 8.0, ...]
• Second one: [3.0, 12.0, 5.0, 48.0, 48.0, 8.0, ...]

---

I am assuming that by generator you actually mean generator by the way and not list-comprehension . If that is not the case, the answer by @Cody is the way to go.