UT Martin Competitive Programming Guidebook

User Tools

Site Tools

Trace:
python3:input_output

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revision Previous revision
Next revision 		Previous revision
python3:input_output [2018/08/14 16:32]
jguerin Renamed "advanced input", rewrote intro to section to carry more useful information. 		python3:input_output [2018/08/14 19:04] (current)
jguerin
Line 3: Line 3:
  
 ===== Input Basics ===== ===== Input Basics =====
-Input in Python3 is handled via the ''%%input()%%'' function, and reads a newline-terminated string from standard input.((Unlike C++ and Java, input is line-based rather than token-based.)) Additional processing is done to the resulting string.+Input in Python3 is handled via the ''%%input()%%'' function, and reads a newline-terminated string from standard input.((Unlike C++ and Java, input is line-based rather than token-based.)) Any additional processing is done to the resulting string.
  
 <code python> <code python>
Line 58: Line 58:
  
 ===== Advanced Input ===== ===== Advanced Input =====
-''input()'' will scale well for many easy and mid-level contest problems. For mid to upper problems with significant bounds on reads and writes ''input()'' will carry an increased risk of "time limit exceeded" judgements.((There is no guaranteed cutoff, but for many problems ''input()'' is //increasingly likely// to fail around 10<sup>3</sup>≤//n//≤10<sup>4</sup> lines.))+''input()'' will scale well for many easy and mid-level contest problems. For mid to upper problems with significant bounds on reads and writes ''input()'' will carry an increased risk of //time limit exceeded// judgements.((There is no guaranteed cutoff, but for many problems ''input()'' is //increasingly likely// to fail around 10<sup>3</sup>≤//n//≤10<sup>4</sup> lines.)) 
 + 
 +=== stdin.readline() === 
 +''readline()'' is identical to the interface provided by ''input()'' except it //retains// the ''\n'' that is used to terminate the read.((This may require special handling in certain circumstance and can be safely ignored in others. E.g., ''.split()'' will //not// behave differently vs. ''input()''.))
  
 <code python> <code python>
Line 66: Line 69:
 </code> </code>
  
-<code python> 
->>> from sys import * 
->>> x = stdin.readline().split() 
-Hello World! # x=["Hello", "World!"] 
-</code> 
  
-''readlines()'' is stopped by an End Of File (EOF) character.  You can send this from the terminal with <ctrl-D>+=== stdin.readlines() === 
 +''readlines()'' processes an entire file, terminated by an //end of file// character.((This behavior can be simulated on the terminal with ''<ctrl>+d''.))
  
 <code python> <code python>
Line 80: Line 79:
 To Everyone! # x=["Hello\n", "To Everyone!\n"] To Everyone! # x=["Hello\n", "To Everyone!\n"]
 </code> </code>
 +
 +''<ctrl>+d'' would be used to terminate this example after the ''!''.
 +
 +''readlines()'' loads //and// stores an entire file into memory (as a list) its performance will exceed other options in Python3 for all but the most enormous files.((See the table below for an example where ''readlines()'' required storing ∼1GB of data.))
 +
 +
 ==== Benchmarks ==== ==== Benchmarks ====
 +The following benchmarks demonstrate the increased likelihood of failure of ''input()'' as input sizes increase. All files used for testing can be found [[files:python3:io_tests|here]].
 +
 10 characters per line (//n//= number of lines): 10 characters per line (//n//= number of lines):
 | //n//          | input() | sys.stdin.readline() | sys.stdin.readlines() | | //n//          | input() | sys.stdin.readline() | sys.stdin.readlines() |
-| 10<sup>4</sup> | .034    | .016                 | .018                  | +| 10<sup>4</sup> | .034s    | .016s                 | .018s                  | 
-| 10<sup>5</sup> | .146    | .052                 | .030                  | +| 10<sup>5</sup> | .146s    | .052s                 | .030s                  | 
-| 10<sup>6</sup> | 1.301   | .301                 | .130                  |+| 10<sup>6</sup> | 1.301s   | .301s                 | .130s                  |
  
  
 1000 characters per line (//n//= number of lines): 1000 characters per line (//n//= number of lines):
 | //n//          | input() | sys.stdin.readline() | sys.stdin.readlines() | | //n//          | input() | sys.stdin.readline() | sys.stdin.readlines() |
-| 10<sup>4</sup> | .046    | .037                 | .033                  | +| 10<sup>4</sup> | .046s    | .037s                 | .033s                  | 
-| 10<sup>5</sup> | .282    | .183                 | .143                  | +| 10<sup>5</sup> | .282s    | .183s                 | .143s                  | 
-| 10<sup>6</sup> | 2.728   | 1.430                | 1.723                 | +| 10<sup>6</sup> | 2.728s   | 1.430s                | 1.723s((The ''readlines()'' version is actually slower than ''readline()'' on the largest dataset.  It is attempting to store about 1GB in memory here, causing a slowdown, but still faster than ''input()''. 
- +))                 |
- +
-The ''readlines()'' version is actually slower than ''readline()'' on the largest dataset.  It is attempting to store about 1GB in memory here, causing a slowdown, but still faster than ''input()''. +
- +
-<file python input.py> +
-num_lines = int(input()) +
- +
-for i in range(num_lines): +
-    line = input() +
-</file> +
- +
-<file python readline.py> +
-from sys import stdin +
- +
-num_lines = int(input()) +
- +
-for i in range(num_lines): +
-    line = stdin.readline() +
-</file> +
- +
-<file python readlines.py> +
-from sys import stdin +
- +
-num_lines = int(input()) +
- +
-lines = stdin.readlines() +
-</file>+
  
-<file python read_test.py> +The above tests were designed to showcase minimal reading functionality other than temporary storage.((We deliberately avoided additional processing such as typecasts, ''map()'', and ''split()'', as these are non-IO considerations in Python3.))
-from sys import argv +
-from random import * +
-from string import *+
  
-for i in range(int(argv[1])): 
-    print(''.join(choice(ascii_lowercase + ' ') for _ in range(int(argv[2])))) 
-</file> 
  
-((''python3 read_test.py 1000 10 > out.txt'')) 
-((''python3 read_test.py 1000 1000 > out.txt'')) 
  
python3/input_output.1534282364.txt.gz · Last modified: 2018/08/14 16:32 by jguerin

Page Tools

Except where otherwise noted, content on this wiki is licensed under the following license: CC Attribution-Noncommercial-Share Alike 4.0 International
CC Attribution-Noncommercial-Share Alike 4.0 International Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki