Why is reading lines from stdin much slower in C than Python

2025-01-26 (Last Modified: 2025-01-26)

Speechmaking information from modular enter (stdin) is a cardinal cognition successful programming, but the show tin change dramatically betwixt languages. Galore builders are amazed to detect that C++, famed for its velocity, tin beryllium importantly slower than Python once speechmaking traces from stdin. This seemingly counterintuitive behaviour stems from the underlying mechanisms all communication employs for enter/output operations and however they work together with the working scheme. Knowing these variations is important for optimizing show successful I/O-sure functions.

The Function of Buffering

A cardinal cause contributing to the show disparity lies successful the buffering methods utilized by C++ and Python. C++’s modular enter watercourse (std::cin) is usually tied to buffered enter. This means information isn’t publication straight from the console oregon enter origin all clip a publication cognition is carried out. Alternatively, information is gathered successful a buffer, and the programme reads from this buffer. Piece buffering tin better ratio for ample inputs, it introduces overhead once speechmaking idiosyncratic strains. Python’s enter mechanics, connected the another manus, frequently employs greater-flat buffering and performs formation-primarily based speechmaking by default. This formation-oriented attack simplifies speechmaking from stdin and tin pb to improved show once dealing with formation-by-formation enter.

Moreover, the measurement of the buffer and the manner it’s managed drama important roles. Smaller buffers whitethorn necessitate much predominant interactions with the working scheme, expanding overhead. Conversely, bigger buffers mightiness trim action frequence however may present latency if the programme wants to delay for the buffer to enough. This commercial-disconnected is indispensable to see once optimizing I/O show.

Communication-Circumstantial I/O Implementations

C++ provides good-grained power complete I/O operations, offering antithetic ranges of abstraction. Piece this flexibility permits for optimization successful circumstantial eventualities, it tin besides beryllium a origin of complexity. Python, by opposition, usually makes use of increased-flat I/O capabilities that frequently grip buffering and another debased-flat particulars robotically. This advanced-flat attack tin simplify codification and better show successful communal situations similar speechmaking traces from stdin.

See this illustration: speechmaking a ample record formation by formation. Successful C++, utilizing std::getline with std::cin tin beryllium slower owed to the underlying buffered enter and related overhead. Python’s enter() oregon sys.stdin.readline() capabilities are frequently quicker successful this script arsenic they’re optimized for formation-based mostly enter. This quality is magnified once processing monolithic datasets wherever I/O turns into a bottleneck.

Optimizing C++ for Stdin Enter

Piece C++’s default stdin dealing with whitethorn beryllium slower successful any circumstances, location are methods to optimize show. Disabling synchronization betwixt C++ streams and C-kind I/O tin importantly trim overhead. This tin beryllium achieved utilizing std::ios::sync_with_stdio(mendacious);. Moreover, untie std::cin from std::cout utilizing std::cin.necktie(nullptr); tin forestall pointless flushing of output buffers.

Utilizing less-flat I/O capabilities similar fread oregon publication tin supply much power complete buffering and better show for ample inputs. This attack requires manually managing buffers and dealing with newline characters, however tin supply important velocity positive factors once accomplished accurately. Nevertheless, the added complexity mightiness not beryllium justified for smaller datasets oregon if simplicity is paramount.

Applicable Implications and Issues

The noticed show quality successful speechmaking from stdin betwixt C++ and Python frequently turns into noticeable once dealing with ample datasets oregon interactive purposes wherever enter velocity is important. Successful competitory programming, for case, utilizing Python’s enter() for speechmaking strains is mostly most well-liked owed to its velocity vantage. Successful technological computing oregon information processing involving monolithic records-data, optimizing C++’s stdin dealing with tin beryllium important for attaining optimum show. Knowing these nuances tin importantly contact the ratio of your purposes.

Selecting the correct implement for the occupation is critical. If show is paramount once speechmaking from stdin, Python mightiness beryllium a amazing however effectual prime. Nevertheless, if analyzable information processing oregon scheme-flat action is required, optimized C++ codification tin inactive outperform Python. Finally, the champion attack relies upon connected the circumstantial wants of the exertion.

C++’s std::cin tin beryllium optimized by disabling synchronization and untying from std::cout.
Python’s enter() oregon sys.stdin.readline() are mostly sooner for formation-by-formation enter.

Place the origin and measurement of the enter information.
Take the due communication and I/O strategies.
Benchmark and optimize show wherever essential.

Research another enter/output associated articles present.

Featured Snippet: Piece C++ is famed for its velocity, Python tin beryllium amazingly sooner once speechmaking strains from stdin owed to variations successful buffering and I/O implementation. Optimizing C++ codification tin mitigate this, however Python stays a beardown contender for I/O-certain duties involving formation-by-formation enter.

FAQs

Q: Is Python ever sooner than C++ for speechmaking from stdin?

A: Nary, it relies upon connected the circumstantial implementation and information dimension. Optimized C++ tin outperform Python, particularly with ample binary information.

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The show quality betwixt C++ and Python once speechmaking strains from stdin boils behind to however all communication handles buffering and I/O operations. Piece C++ provides larger power, Python frequently offers amended retired-of-the-container show for formation-by-formation enter. Knowing these nuances and using due optimization strategies are important for gathering businesslike functions, particularly once dealing with ample datasets oregon interactive packages. Dive deeper into I/O show and research precocious strategies to maximize your exertion’s ratio. See exploring additional sources connected scheme-flat programming and businesslike record dealing with. C++ getline Documentation and Python sys.stdin Documentation supply invaluable accusation. Besides, Wikipedia’s leaf connected Modular Streams provides a bully overview of the subject.

Buffering methods
I/O implementations
stdin
C++ I/O
Python I/O
Show optimization
Enter/Output

Question & Answer :
I wished to comparison speechmaking strains of drawstring enter from stdin utilizing Python and C++ and was shocked to seat my C++ codification tally an command of magnitude slower than the equal Python codification. Since my C++ is rusty and I’m not but an adept Pythonista, delight archer maine if I’m doing thing incorrect oregon if I’m misunderstanding thing.

(TLDR reply: see the message: cin.sync_with_stdio(mendacious) oregon conscionable usage fgets alternatively.

TLDR outcomes: scroll each the manner behind to the bottommost of my motion and expression astatine the array.)

C++ codification:

#see <iostream> #see <clip.h> utilizing namespace std; int chief() { drawstring input_line; agelong line_count = zero; time_t commencement = clip(NULL); int sec; int lps; piece (cin) { getline(cin, input_line); if (!cin.eof()) line_count++; }; sec = (int) clip(NULL) - commencement; cerr << "Publication " << line_count << " strains successful " << sec << " seconds."; if (sec > zero) { lps = line_count / sec; cerr << " LPS: " << lps << endl; } other cerr << endl; instrument zero; } // Compiled with: // g++ -O3 -o readline_test_cpp foo.cpp

Python Equal:

#!/usr/bin/env python import clip import sys number = zero commencement = clip.clip() for formation successful sys.stdin: number += 1 delta_sec = int(clip.clip() - start_time) if delta_sec >= zero: lines_per_sec = int(circular(number/delta_sec)) mark("Publication {zero} traces successful {1} seconds. LPS: {2}".format(number, delta_sec, lines_per_sec))

Present are my outcomes:

$ feline test_lines | ./readline_test_cpp Publication 5570000 strains successful 9 seconds. LPS: 618889 $ feline test_lines | ./readline_test.py Publication 5570000 strains successful 1 seconds. LPS: 5570000

I ought to line that I tried this some nether Mac OS X v10.6.eight (Snowfall Leopard) and Linux 2.6.32 (Reddish Chapeau Linux 6.2). The erstwhile is a MacBook Professional, and the second is a precise beefy server, not that this is excessively pertinent.

$ for i successful {1..5}; bash echo "Trial tally $i astatine `day`"; echo -n "CPP:"; feline test_lines | ./readline_test_cpp ; echo -n "Python:"; feline test_lines | ./readline_test.py ; executed

Trial tally 1 astatine Mon Feb 20 21:29:28 EST 2012 CPP: Publication 5570001 traces successful 9 seconds. LPS: 618889 Python:Publication 5570000 traces successful 1 seconds. LPS: 5570000 Trial tally 2 astatine Mon Feb 20 21:29:39 EST 2012 CPP: Publication 5570001 traces successful 9 seconds. LPS: 618889 Python:Publication 5570000 traces successful 1 seconds. LPS: 5570000 Trial tally three astatine Mon Feb 20 21:29:50 EST 2012 CPP: Publication 5570001 traces successful 9 seconds. LPS: 618889 Python:Publication 5570000 traces successful 1 seconds. LPS: 5570000 Trial tally four astatine Mon Feb 20 21:30:01 EST 2012 CPP: Publication 5570001 strains successful 9 seconds. LPS: 618889 Python:Publication 5570000 traces successful 1 seconds. LPS: 5570000 Trial tally 5 astatine Mon Feb 20 21:30:eleven EST 2012 CPP: Publication 5570001 strains successful 10 seconds. LPS: 557000 Python:Publication 5570000 traces successful 1 seconds. LPS: 5570000

Small benchmark addendum and recap

For completeness, I idea I’d replace the publication velocity for the aforesaid record connected the aforesaid container with the first (synced) C++ codification. Once more, this is for a 100M formation record connected a accelerated disk. Present’s the examination, with respective options/approaches:

By default, cin is synchronized with stdio, which causes it to debar immoderate enter buffering. If you adhd this to the apical of your chief, you ought to seat overmuch amended show:

std::ios_base::sync_with_stdio(mendacious);

Usually, once an enter watercourse is buffered, alternatively of speechmaking 1 quality astatine a clip, the watercourse volition beryllium publication successful bigger chunks. This reduces the figure of scheme calls, which are usually comparatively costly. Nevertheless, since the Record* primarily based stdio and iostreams frequently person abstracted implementations and so abstracted buffers, this might pb to a job if some have been utilized unneurotic. For illustration:

int myvalue1; cin >> myvalue1; int myvalue2; scanf("%d",&myvalue2);

If much enter was publication by cin than it really wanted, past the 2nd integer worth wouldn’t beryllium disposable for the scanf relation, which has its ain autarkic buffer. This would pb to surprising outcomes.

To debar this, by default, streams are synchronized with stdio. 1 communal manner to accomplish this is to person cin publication all quality 1 astatine a clip arsenic wanted utilizing stdio features. Unluckily, this introduces a batch of overhead. For tiny quantities of enter, this isn’t a large job, however once you are speechmaking thousands and thousands of traces, the show punishment is important.

Thankfully, the room designers determined that you ought to besides beryllium capable to disable this characteristic to acquire improved show if you knew what you have been doing, truthful they supplied the sync_with_stdio methodology. From this nexus (accent added):

If the synchronization is turned disconnected, the C++ modular streams are allowed to buffer their I/O independently, which whitethorn beryllium significantly sooner successful any instances.