import an array in python

Question

How can I import an array to python (numpy.arry) from a file and that way the file must be written if it doesn't already exist.

For example, save out a matrix to a file then load it back.

Answer 1

Checkout the entry on the numpy example list. Here is the entry on .loadtxt()

>>> from numpy import *
>>>
>>> data = loadtxt("myfile.txt")                       # myfile.txt contains 4 columns of numbers
>>> t,z = data[:,0], data[:,3]                         # data is 2D numpy array
>>>
>>> t,x,y,z = loadtxt("myfile.txt", unpack=True)                  # to unpack all columns
>>> t,z = loadtxt("myfile.txt", usecols = (0,3), unpack=True)     # to select just a few columns
>>> data = loadtxt("myfile.txt", skiprows = 7)                    # to skip 7 rows from top of file
>>> data = loadtxt("myfile.txt", comments = '!')                  # use '!' as comment char instead of '#'
>>> data = loadtxt("myfile.txt", delimiter=';')                   # use ';' as column separator instead of whitespace
>>> data = loadtxt("myfile.txt", dtype = int)                     # file contains integers instead of floats

Answer 2

Another option is numpy.genfromtxt, e.g:

import numpy as np
data = np.genfromtxt("myfile.dat",delimiter=",")

This will make data a numpy array with as many rows and columns as are in your file