Python nested loop - get next N lines

Question

I'm new to Python and trying to do a nested loop. I have a very large file (1.1 million rows), and I'd like to use it to create a file that has each line along with the next N lines, for example with the next 3 lines: ``` 1 2 1 3 1 4 2 3 2 4 2 5 ``` Right now I'm just trying to get the loops working with rownumbers instead of the strings since it's easier to visualize. I came up with this code, but it's not behaving how I want it to: ``` with open('C:/working_file.txt', mode='r', encoding = 'utf8') as f: for i, line in enumerate(f): line_a = i lower_bound = i + 1 upper_bound = i + 4 with open('C:/working_file.txt', mode='r', encoding = 'utf8') as g: for j, line in enumerate(g): while j >= lower_bound and j <= upper_bound: line_b = j j = j+1 print(line_a, line_b) ``` Instead of the output I want like above, it's giving me this: ``` 990 991 990 992 990 993 990 994 990 992 990 993 990 994 990 993 990 994 990 994 ``` As you can see the inner loop is iterating multiple times for each line…

Accepted Answer

Based on window example from old docs you can use something like:

```
from collections import deque
from itertools import cycle

try:
    xrange 
except NameError: # python3
    xrange = range

def pack(d):
    tup = tuple(d)
    return zip(cycle(tup[0:1]), tup[1:])

def window(seq, n=2):
    it = iter(seq)
    d = deque((next(it, None) for _ in xrange(n)), maxlen=n)
    yield pack(d)
    for e in it:
        d.append(e)
        yield pack(d)
```

Demo:

```
>>> for l in window([1,2,3,4,5], 4):
...     for l1, l2 in l:
...         print l1, l2
...
1 2
1 3
1 4
2 3
2 4
2 5
```

So, basically you can pass your file to window to get desired result:

```
window(open('C:/working_file.txt', mode='r', encoding='utf8'), 4)
```

Python nested loop - get next N lines

Solution

Problem