UT Martin Competitive Programming Guidebook

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====== Input and Output ======
There is no magic bullet that leads to the fastest input and output in a programming contest. Having said this, there are typically multiple ways of performing and processing input and output in Python3. By understanding the options that are available and how to optimize them (e.g., conducting tests) you can see a real world performance increase, in particular when faced with high volumes of reads and writes.

===== 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.)) Any additional processing is done to the resulting string.

<code python>
>>> x = input()
Hello World! # x="Hello World!"
</code>


<code python>
>>> x = int(input())
42 # x=42
</code>

<code python>
>>> x = input().split()
Hello World! # x=["Hello", "World!"]
</code>

<code python>
>>> (x, y) = map(int, input().split())
1 2 # x=1, y=2
</code>

''[[python3:map_reduce_filter|map()]]'' applies a function across a sequence or iterable structure. The comma '','' [[python3:unpacking | unpacks]] the map object into ''x'' and ''y''.

<code python>
>>> x = list(map(float, input().split()))
1.2 2.3 3.4 4.5 # x=[1.2, 2.3, 3.4, 4.5]
</code>

===== Output Basics =====
Output is handled with the ''%%print()%%'' function.

<code python>
>>> print() # '\n'
</code>

<code python>
>>> print("Hello World!") # "Hello World!\n"
</code>

<code python>
>>> print("Hello", "World!") # "Hello World!\n"
</code>

<code python>
>>> print(1, 2, 3, sep='') # "123\n"
</code>

<code python>
>>> print(1, 2, 3, end='-')
>>> print("a b c") # "1 2 3-a b c\n"
</code>

===== 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.))

=== 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>
>>> from sys import *
>>> x = stdin.readline()
Hello World! # x="Hello World!\n"
</code>


=== 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>
>>> from sys import *
>>> x = stdin.readlines()
Hello
To Everyone! # x=["Hello\n", "To Everyone!\n"]
</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 ====
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):
| //n//          | input() | sys.stdin.readline() | sys.stdin.readlines() |
| 10<sup>4</sup> | .034s    | .016s                 | .018s                  |
| 10<sup>5</sup> | .146s    | .052s                 | .030s                  |
| 10<sup>6</sup> | 1.301s   | .301s                 | .130s                  |


1000 characters per line (//n//= number of lines):
| //n//          | input() | sys.stdin.readline() | sys.stdin.readlines() |
| 10<sup>4</sup> | .046s    | .037s                 | .033s                  |
| 10<sup>5</sup> | .282s    | .183s                 | .143s                  |
| 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 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.))
UT Martin Competitive Programming Guidebook

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