在Python中将数据文件列拆分为单独的数组

Question

I'm new to python and have been trying to figure this out all day. I have a data file laid out as below,

time    I(R_stkb)

Step Information: Temp=0  (Run: 1/11)

0.000000000000000e+000  0.000000e+000

9.999999960041972e-012  8.924141e-012

1.999999992008394e-011  9.623148e-012

3.999999984016789e-011  6.154220e-012

(Note: No empty line between the each data line.)

I want to plot the data using matplotlib functions, so I'll need the two separate columns in arrays.

I currently have

def plotdata():

Xvals=[], Yvals=[]
i = open(file,'r')

for line in i:
    Xvals,Yvals = line.split(' ', 1)

print Xvals,Yvals

But obviously its completely wrong. Can anyone give me a simple answer to this, and with an explanation of what exactly the lines mean would be helpful. Cheers.

Edit: The first two lines repeat throughout the file.

Answer 1

This is a job for the * operator on the zip method.

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


>>> zip(*asdf)
[(1, 3, 5), (2, 4, 6)]

So in the context of your data it might be something like:

handle = open(file,'r')
lines = [line.split() for line in handle if line[:4] not in ('time', 'Step')]
Xvals, Yvals = zip(*lines)

or if your really need to be able to mutate the data afterwards you could just call the list constructor on each tuple:

Xvals, Yvals = [list(block) for block in zip(*lines)]

Answer 2

One way to do it is:

Xvals=[]; Yvals=[]
i = open(file,'r')

for line in i:
    x, y = line.split(' ', 1)
    Xvals.append(float(x))
    Yvals.append(float(y))

print Xvals,Yvals

Note the call to the float function, which will change the string you get from the file into a number.

Answer 3

This is what numpy.loadtxt is designed for. Try:

import numpy as np
import matplotlib.pyplot as plt

data = np.loadtxt(file, skiprows = 2) # assuming you have time and step information on 2 separate lines 
                                      # and you do not want to read them
plt.plot(data[:,0], data[:,1])
plt.show()

EDIT: if you have time and step information scattered throughout the file and you want to plot data on every step, there is a possibility of reading all the file to memory (suppose it's small enough), and then split it on time strings:

l = open(fname, 'rb').read()
for chunk in l.split('time'):
    data = np.array([s.split() for s in chunk.split('\n')[2:]][:-1], dtype = np.float)
    plt.plot(data[:,0], data[:,1])
    plt.show()

Or else you could add the # comment sign to the comment lines and use np.loadxt .

Answer 4

If you want to plot this file with matplotlib, you might want to check out it's plotfile function. See the official documentation here .