# [Tutor] List comprehensions to search a list--amazing!

boB Stepp robertvstepp at gmail.com
Thu Mar 19 05:20:17 CET 2015

```I hope extolling the beauty and power of Python on this list is
allowed, because I have had a large "WOW!!!" moment tonight. I had a
problem I was working on at work this afternoon. I have a list of ~
10,000 floating point numbers, which run from largest to smallest.
There are duplicates scattered throughout, so I might have something
like: [5942.6789, 5942.6789, 5941.000003, 5941.01, 5941.01, ... ],
etc. I wanted to search the list for a test value, which, depending on
the particular list (I can have many such lists, each different from
the other.), could conceivably be anywhere within the given list. I
needed to return the index where the list values change from being
just greater than the test value to just less than the test value at
the very next index position. I spent a good chunk of my afternoon
writing a binary search function and wondering what theoretically the
optimum search algorithm would be, got interrupted (as usual on this
project), and decided to look at my books at home to see if a better
solution would be staring at me from some book (Like there usually
is!).

I haven't studied list comprehensions formally yet, but a snippet of
code in a book caught my eye where the author was discussing filtering
data in a list. This led me to try:

The generalized problem:

L = [V0, V1, ..., Vn], where V0 >= V1 >= V2 >= ... >= Vn .
Find index i, such that V[i] >= Vt >= V[i + 1], where Vt is the test
value being searched for. I need to know the indices i and i + 1,
which I need to interpolate based on where Vt falls.

The solution (As a sublist, S)  I worked out tonight after
experimenting with comprehension syntax is:
S = [i for i, V in enumerate(L) if L[i] >= Vt >= L[i + 1]]

And, of course, the index i I need is:
i = S[0]

I tested this out with concrete examples in the interpreter, such as
with a list, L:

L = [item for item in range(10000, 0, -1)]

and trying different test values. It was blazingly fast, too!

All I can say is: WOW!!!

--
boB
```