General Input/Output Python discussion

zingoba 2010-12-18 19:15:03 UTC #1

Input and type conversion seem to be taking a lot of time in python. I'm new to python and the following is my first code in python

Spoiler removed :)

I wrote it for the problem http://www.spoj.pl/problems/M00PAIR/
And it goes without saying that I'm getting TLE but I've seen the list of accepted python solutions to the problem and they all seem to complete within a second. Is it because of a better algorithm or have they used some other input method(say, implicit conversion)? I am absolutely new to the language and any pointers will be appreciated.
(Expecting hendrik to turn up )

hendrik 2010-12-18 19:27:43 UTC #2

Try to find a way to read the input without the try-except clause. That costs time which you dont have.

zingoba 2010-12-18 20:12:42 UTC #3

Tried

while 1:
         a=raw_input()
         if len(a)==0:
                        break
         b=int(a)
         print fun(b)

Result : TLE

hendrik 2010-12-18 20:29:43 UTC #4

Still not optimal.Look at the Python module sys. There are other functions which you can use.
Once you have improved the IO you should also take a look at your algo. Your recursive approach is too slow. Think about something different.

numerix 2010-12-18 20:30:04 UTC #5

No, that's not the correct way. Search in the forum and you'll find some other hints on how to read until EOF in Python.
Then you'll get AC in < 1 s.

numerix 2010-12-18 20:31:23 UTC #6

No, it's fast enough, as any value is only calculated once.

hendrik 2010-12-18 20:39:04 UTC #7

numerix: Thanks for correcting me. Perhaps I didnt understand the algo completely.

zingoba 2010-12-18 20:55:34 UTC #8

while 1:
        a=sys.stdin.readline()
        if not a:
                 break
        # continue with the rest of it

Result : TLE

Similar result for:

for a in sys.stdin:
                   b=int(a)
                   printf fun(b)

I think the type conversion takes considerable time too. Can't that be optimized? Isn't there any function which doesn't read in strings and reads integers instead?

numerix 2010-12-18 21:04:22 UTC #9

No loop for the input ...

zingoba 2010-12-18 21:13:24 UTC #10

Does that mean reading the whole input in one go and then parsing it in a loop to print back to console?

numerix 2010-12-18 21:28:22 UTC #11

Yes, but It's less than parsing, it's just reading it. "sys.stdin" is a good choice, but readline() is not the best method ...

You're right, type conversion is expensive. You can read integers instead of strings using the input()-function, but that's not really what you want (in fact input() is nothing more than eval(raw_input()) and is veeeery slow). You can speedup the conversion a bit, if you read the whole input and convert it to integer in one pass using the map()-function.

zingoba 2010-12-19 20:56:35 UTC #12

map() requires the second argument to be a list( or a similar iterator data structure), how do I read the whole input in the form of a list of strings? If I get the input as a list of strings(lets call it 'inlist'), I can simply do this:

# read input in 'inlist'
L=map(int,inlist);
for b in L:
            print fun(b)

numerix 2010-12-19 21:46:44 UTC #13

stdin behaves like a file-object: http://docs.python.org/release/2.6.6/library/stdtypes.html#file-objects

Good approach.

zingoba 2010-12-19 22:21:40 UTC #14

This is what I did :

a=sys.stdin.read()
l=map(f,a)
for b in l:
	print fun(b)

Result: NZEC
pretty sure the error because 'a' isnt an iterator type and is a string object instead. I don't know how to read input!

hendrik 2010-12-19 23:33:04 UTC #15

Are you aware what sys.stdin.read returns. If you use that function there is one more step needed.
You can use map on strings but then the function that you pass to map must be applicable for all characters. Otherwise exception. Example:

map(int,"123")

map(int,"123\n")

NOK or in terms of SPOJ NZEC

numerix 2010-12-20 11:07:11 UTC #16

I posted a link where you find all you need ...

zingoba 2010-12-21 20:58:33 UTC #17

Thank you.

zingoba 2010-12-31 07:27:01 UTC #18

Used split() to get through. I'm wondering what the best way would be to read input in python after having seen so many python solution finish in under 0.05s for the same problem for which my (memoized)solution took over 0.95s. What's psyco and how do we use that? Could you also let me know how you'd read input( all at once) to the end of file which is of the form shown below:

Several lines having two integers each. We could use split() but then how'd we split the returned string to get the corresponding integers for each test case? I'd do something like this:

L=sys.stdin.read().splitlines()
for a in L:
          a=a.split(' ')
          a=map(int,a)
          ##Use a[0] and a[1] as required or simply use int(a[0]) and int(a[1]) and skip step 2

Is there a better way?

thechamp 2011-01-22 15:12:55 UTC #19

How do i input in python when number of integers/characters depend on a number present in line itself and they all are separated by space.
input:

3 100 200 300
1 934
5 78 6 100 98 90

Here first number in line is the number of integers following it. and i want to take input as:
first number stored in a variable n
and following 'n' numbers in an array a
so for first line, n=3, a[0]=100, a[1]=200, a[3]=300 .. that way

EDIT: the way in C++ i can scan two integers separated by any number of space easily by cin>>x>>y; how to do that in python?

numerix 2011-01-22 15:21:07 UTC #20

Read a line, split it into a list, convert all elements into integers. Then take first element from list (-> n) and take the rest of the list as your array. If you really need the array zero-based, remove first element from list.
If that description doesn't help: read the Python tutorial: http://docs.python.org/release/2.6.6/library/index.html