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eferion
Asked: 2020-06-23 00:36:33 +0800 CST 2020-06-23 00:36:33 +0800 CST

slow read cin

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By default reading with cinis excruciatingly slow compared to its counterpart scanf... when logic tells us it shouldn't:

  • With cina direct call is made to the function that knows what the type of the target variable is.
  • With scanfthe function you have to read the format string (with the dozens of possibilities that exist) to know how you have to interpret the input and then process it.

However, with a simple example it is easy to see that reality is different:

#include <iostream>
#include <chrono>
#include <string>

void cinFunc()
{
  int N;
  for( int i=0; i<100000; i++)
    std::cin >> N;
}

void scanfFunc()
{
  int N;
  for( int i=0; i<100000; i++)
    scanf(" %d",&N);
}


template<typename Func>
void Test(std::string const& title, Func f)
{
  auto start = std::chrono::steady_clock::now();
  f();
  auto end = std::chrono::steady_clock::now();
  auto diff = end - start;
  std::cout << "Test " << title << ":\t" << std::chrono::duration<double, std::milli>(diff).count() << " ms\n";
}

int main()
{
  Test("cin",cinFunc);
  Test("scanf",scanfFunc);
  return EXIT_SUCCESS;
}

The program is executed with a file containing 200,000 1-digit integers separated by a space, and the results speak for themselves:

Test cin:       64.7967 ms
Test scanf:     49.855 ms

What is the reason that C++'s own reading is so slow? How can this situation be corrected?

c++

2 Answers

  1. Best Answer

    By default iostreamit is synchronized with stdioto ensure that the standard input and output functions both from Cand C++share a single buffer, since without synchronization they would both have independent buffers, this would cause problems like:

    #include <iostream>
#include <cstdio>

using namespace std;

int main()
{
    cout << "Hola";
    printf("mundo");
    cout << "adios";
}

    Without the synchronization you will never know if you will get Holaadiosmundoor Holamundoadiosor adiosholamundosince both printfand coutthey have different buffers in consequence the order is undefined. Unfortunately the sync issue introduces a significant performance penalty for iostream, but it is possible to disable it if you don't intend to mix stdioand iostream.

    ios_base::sync_with_stdio(false);

    This would give a significant performance boost even better than stdio.

    • 4

  2. I have looked at your code in cpp.sh and it cinis faster than with the use of scanf, seeing that I removed the optimizations and it is still faster cin, "I don't know" what it should be considering your question and the times it shows , you can take a look at the online ide maybe I'm missing something.

    On the other hand you can use std::ios::sync_with_stdio(false);if you don't need synchronized with the standard c maybe that will help you.

    I have also looked at other online ides like coliru and the same thing, coliru uses GCC 7.1.0, in case it helps you, the coliru test does not have any optimization either and it is about-std=c++11

    • 2