user248237dfsf - 7 months ago 28

Python Question

I'm trying to find an efficient way to pair together rows of data containing integer points, and storing them as Python objects. The data is made up of

`X`

`Y`

`(x_1, y_1), (x_2, y_2), ...`

`get_data`

`def get_data(N=100000, M=10):`

import random

data = []

for n in range(N):

pair = [[str(random.randint(1, 10)) for x in range(M)],

[str(random.randint(1, 10)) for x in range(M)]]

row = [",".join(pair[0]),

",".join(pair[1])]

data.append(row)

return data

The parsing code I have now is:

`class Point:`

def __init__(self, a, b):

self.a = a

self.b = b

def test():

import time

data = get_data()

all_point_sets = []

time_start = time.time()

for row in data:

point_set = []

first_points, second_points = row

# Convert points from strings to integers

first_points = map(int, first_points.split(","))

second_points = map(int, second_points.split(","))

paired_points = zip(first_points, second_points)

curr_points = [Point(p[0], p[1]) \

for p in paired_points]

all_point_sets.append(curr_points)

time_end = time.time()

print "total time: ", (time_end - time_start)

Currently, this takes nearly 7 seconds for 100,000 points, which seems very inefficient. Part of the inefficiency seems to stem from the calculation of

`first_points`

`second_points`

`paired_points`

Another part of the inefficiency seems to be the building up of

`all_point_sets`

`all_point_sets.append(...)`

How can this be sped up? thanks.

Answer

Simply running with pypy makes a big difference

```
$ python pairing_strings.py
total time: 2.09194397926
$ pypy pairing_strings.py
total time: 0.764246940613
```

disable gc didn't help for pypy

```
$ pypy pairing_strings.py
total time: 0.763386964798
```

namedtuple for Point makes it worse

```
$ pypy pairing_strings.py
total time: 0.888827085495
```

using itertools.imap, and itertools.izip

```
$ pypy pairing_strings.py
total time: 0.615751981735
```

Using a memoized version of int and an iterator to avoid the zip

```
$ pypy pairing_strings.py
total time: 0.423738002777
```

Here is the code I finished with.

```
def test():
import time
def m_int(s, memo={}):
if s in memo:
return memo[s]
else:
retval = memo[s] = int(s)
return retval
data = get_data()
all_point_sets = []
time_start = time.time()
for xs, ys in data:
point_set = []
# Convert points from strings to integers
y_iter = iter(ys.split(","))
curr_points = [Point(m_int(i), m_int(next(y_iter))) for i in xs.split(",")]
all_point_sets.append(curr_points)
time_end = time.time()
print "total time: ", (time_end - time_start)
```

Source (Stackoverflow)