python - 如何使用 numpy 或 pandas 仅在 python 中的两行之间读取数据？

Question

我有一个这样的数据文件:

# column 1 is the angle of incidence (degrees)
# column 2 is the wavelength (microns)
# column 3 is the transmission probability
# column 4 is the reflection probability
      14.2000     0.300000  0.01     0.999920
      14.2000     0.301000  0.02     0.999960
      14.2000     0.302000  0.03     0.999980
      14.2000     0.303000  0.04     0.999980
      14.2000     0.304000  0.06     0.999980
      14.2000     0.305000  0.08     0.999970
      14.2000     0.306000  0.2     0.999950
      14.2000     0.307000  0.4     0.999910
      14.2000     0.308000  0.8     0.999860
      14.2000     0.309000  0.9     0.999960
      14.2000     0.310000  0.8     0.999990
      14.2000     0.311000  0.4     0.999980
      14.2000     0.312000  0.2     0.999960
      14.2000     0.313000  0.06     0.999940
      14.2000     0.314000  0.03     0.999930
      14.2000     0.315000  0.02     1.00000
      14.2000     0.316000  0.01     1.00000

所需的输出文件output.csv是这样的:

# column 1 is the angle of incidence (degrees)
# column 2 is the wavelength (microns)
# column 3 is the transmission probability
# column 4 is the reflection probability
      14.2000     0.304000  0.06     0.999980
      14.2000     0.305000  0.08     0.999970
      14.2000     0.306000  0.2     0.999950
      14.2000     0.307000  0.4     0.999910
      14.2000     0.308000  0.8     0.999860
      14.2000     0.309000  0.9     0.999960
      14.2000     0.310000  0.8     0.999990
      14.2000     0.311000  0.4     0.999980
      14.2000     0.312000  0.2     0.999960
      14.2000     0.313000  0.06     0.999940
      14.2000     0.314000  0.03     0.999930


      # conditions are: 
      # output first element of column3 >= 0.05   i.e. 0.06
      # output last  element of column3  < 0.05   i.e. 0.03

      # for the second may be we need to get the index of second 0.06 and 
      #     get the value of next index.

我们如何在 python pandas 或 numpy 中这样做？

我最初的尝试是这样的:

#!/usr/bin/env python
# -*- coding: utf-8 -*- 
# Author    : Bhishan Poudel 
# Date      : June 16, 2016 


# Imports
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

#==============================================================================
# read in a file
infile = 'filter_2.txt'
colnames = ['angle', 'wave','trans', 'refl']
print('{} {} {} {}'.format('\nreading file : ', infile, '','' ))
df = pd.read_csv(infile,sep='\s+', header = None,skiprows = 0,
                 comment='#',names=colnames,usecols=(0,1,2,3))

print(df)

# find value of wavelength just above 0.05
print("\n")
df         = df[(df['trans'] >=  0.05) ]
print(df)

一些类似的链接如下:
How to read between 2 specific lines in python

Answer 1

我会完全跳过 pandas 或 numpy

fo = open('filter_3.txt', 'w')
with open('filter_2.txt', 'r') as fi:
    line = fi.readline()
    while line:
        split = line.split()
        if (split[0] == '#') or (float(split[2]) >= 0.027):
            print line,
            fo.write(line)

        line = fi.readline()

fo.close()

# column 1 is the angle of incidence (degrees)
# column 2 is the wavelength (microns)
# column 3 is the transmission probability
# column 4 is the reflection probability
      14.2000     0.302000  0.028     0.999980
      14.2000     0.303000  0.030     0.999980
      14.2000     0.304000  0.032     0.999980
      14.2000     0.305000  0.030     0.999970
      14.2000     0.306000  0.028     0.999950

---

新增一行代码

fo = open('filter_3.txt', 'w')
with open('filter_2.txt', 'r') as fi:
    new_line = fi.readline()
    old_line = None
    while new_line:
        split_new = new_line.split()
        if old_line is not None:
            split_old = old_line.split()

        cond0 = False if old_line is None else (split_old[0] == '#')
        cond1 = split_new[0] == '#'
        cond2 = float(split_new[2]) >= 0.05
        cond3 = False if old_line is None else (float(split_old[2]) >= 0.05)

        if (cond1 or cond2) or (cond3 and not cond0):
            print new_line,
            fo.write(new_line)
            printed_old = True

        old_line = new_line
        new_line = fi.readline()

fo.close()

# column 1 is the angle of incidence (degrees)
# column 2 is the wavelength (microns)
# column 3 is the transmission probability
# column 4 is the reflection probability
      14.2000     0.304000  0.06     0.999980
      14.2000     0.305000  0.08     0.999970
      14.2000     0.306000  0.2     0.999950
      14.2000     0.307000  0.4     0.999910
      14.2000     0.308000  0.8     0.999860
      14.2000     0.309000  0.9     0.999960
      14.2000     0.310000  0.8     0.999990
      14.2000     0.311000  0.4     0.999980
      14.2000     0.312000  0.2     0.999960
      14.2000     0.313000  0.06     0.999940
      14.2000     0.314000  0.03     0.999930