Styjun

Im learning python but i have some problems with my scripts yet.

I have a file similar to:

1 5
2 5
3 5
4 2
5 1
6 7
7 7
8 8

I want to print the pairs of numbers 2-1 in consecutive lines, just taking the column 2 to find them, and then, print the column 1 and 2 with the results. The result will be similar to this:

4 2 
5 1 

I'm trying to do it with python, because my file has 4,000,000 data. So, this is my script:

import linecache

final_lines = []
with open("file.dat") as f:
for i, line in enumerate(f, 1):
 if "1" in line:
 if "2" in linecache.getline("file.dat", i-1):
 linestart = i - 1 
 final_lines.append(linecache.getline("file.dat", linestart))
print(final_lines)

and the result is:

['2n', '2n', '2n']

What I must to change in my script to fit the result that I want?, Can you guide me please? Thanks a lot.

would work i think

import re
with open("info.dat") as f:
 for match in re.findall("d+ 2[sn]*d+ 1",f.read()):
 print match

see also : https://repl.it/repls/TatteredViciousResources

another alternative is

lines = f.readlines()
for line,nextline in zip(lines,lines[1:]):
 if line.strip().endswith("2") and nextline.strip().endswith("1"):
 print(line+nextline)

Use a for loop with enumerate with a if statement to condition the lines, and then if the condition is true, append the two lines into the list final_lines:

final_lines = []
with open('file.dat') as f:
 lines = f.readlines()
 for i,line in enumerate(lines):
 if line.split()[1] == '2' and lines[i+1].split()[1] == '1':
 final_lines.extend([line,lines[i+1]])

And now:

print(final_lines)

Would return your desired list.

You are a beginner at Python which is great, so I am going to take a more elementary approach. It's a huge file so you are better reading a line at at time and keeping only that line, but you actually need two lines to identify the pattern so keep two. consider the following:

 fp = open('file.dat')
 last_line = fp.readline()
 next_line = fp.readline()
 while next_line:
 # logic to split the lines into a pair 
 # of numbers and check to see if the 
 # 2 and 1 end last_line and next_line
 # and outputting
 last_line = next_line
 next_line = fp.readline()

This follows good, readable software patterns, and requires a minimum of resources.