Question

I would like to read a 2d array of integers from stdin (or from a file) in Python.

Non-working code:

from StringIO import StringIO
from array import array

# fake stdin
stdin = StringIO("""1 2
3 4
5 6""")

a = array('i')
a.fromstring(stdin.read())

This gives me an error: a.fromstring(stdin.read())
ValueError: string length not a multiple of item size

Answer 1

Several approaches to accomplish this are available. Below are a few of the possibilities.

Using an array

From a list

Replace the last line of code in the question with the following.

a.fromlist([int(val) for val in stdin.read().split()])

Now:

>>> a
array('i', [1, 2, 3, 4, 5, 6])

Con: does not preserve 2d structure (see comments).

From a generator

Note: this option is incorporated from comments by eryksun.

A more efficient way to do this is to use a generator instead of the list. Replace the last two lines of the code in the question with:

a = array('i', (int(val) for row in stdin for val in row.split()))

This produces the same result as the option above, but avoids creating the intermediate list.

Using a NumPy array

If you want the preserve the 2d structure, you could use a NumPy array. Here’s the whole example:

from StringIO import StringIO
import numpy as np

# fake stdin
stdin = StringIO("""1 2
3 4
5 6""")

a = np.loadtxt(stdin, dtype=np.int)

Now:

>>> a
array([[1, 2],
       [3, 4],
       [5, 6]])

Using standard lists

It is not clear from the question if a Python list is acceptable. If it is, one way to accomplish the goal is replace the last two lines of the code in the question with the following.

a = [map(int, row.split()) for row in stdin]

After running this, we have:

>>> a
[[1, 2], [3, 4], [5, 6]]