Question: i have a file with information in the pattern of:

```
.343423 1
.434322 1
.453434 1
.534342 1

```

Equal size each line and row in sorted order..I have a variable "a" with a value and need to get the row number which it closest to "a" comparing with the values in the 1st column..

Till now i was coping the 1st column element into list and then using bisect method i am getting the row\_num...but as i need to perform this many times..this has become painfully slow as i need to copy some 4000 element to list everytime..

so now i am thinking of doing it with dict instead of datastructure as i would be faster...but i dont know whether we can use dict in bisect if possible how can we use for this case please suggest...
if not possible are they any method for loading data into list faster then normal???
thanking you...

Answer: 
Here is a way to use `bisect` without reading the entire file. The OS will end up reading much more of the file than you need regardless, so you won't see a performance gain until `data.txt` big enough

```
from os import SEEK_END
from bisect import bisect

class ListProxy(object):
    def __init__(self, f):
        self.f = f
        self.line_len = len(f.readline())
        self.f.seek(0, SEEK_END)
        self.num_lines = self.f.tell()//self.line_len

    def __len__(self):
        return self.num_lines

    def __getitem__(self, idx):
        self.f.seek(idx*self.line_len)
        return float(self.f.read(7))

with open("data.txt") as f:
    lp = ListProxy(f)    
    num = .44
    idx = bisect(lp, num)
    if idx != 0 and num - lp[idx-1] < lp[idx] - num:
        idx -=1
    print num, idx

```
Dicts are unordered, so using bisect on them is meaningless.

I can think of a couple of options:

1) Keep the data in a sorted list of (key, value) tuples. This will allow you to use bisect to find the closest element. This is fine if this is the only thing you ever want to do with the list, and if the list does not change much over time (since it will need to be resorted each time, which will have a cost).

2) use a balanced binary tree datastructure - there are several [Python implementations available on PyPi](http://pypi.python.org/pypi?%3Aaction=search&term=binary%20tree). This will give you dictionary-like semantics while being able to find the closest element like bisect does. The first item in the PyPi search is [bintrees](http://pypi.python.org/pypi/bintrees/0.3.0), which looks like it will do everything you want. It acts like a dictionary but has additional methods to get the items before and after a given value. This will let you efficiently find the closest number.