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Reputation: 149

Using std::ios::sync_with_stdio(false) can be faster than scanf and printf?

I just know using cin and cout will be slower than scanf and printf. However, the top answer says using std::ios::sync_with_stdio(false) can be faster than scanf&printf. However, I did this experiment and found it wasn't right. Is it my fault? Why?

What is a right code to use cin&cout faster than scanf&printf including reading files. Thanks.

Here is my code:

Generating data

#include<cstdio>
#include<iostream>
#include<ctime>
#include<fstream>
#include<iomanip>
FILE *data = freopen("test.in","w",stdin);
//FILE *fi = freopen("test.in","r",stdin);
//FILE *fo = freopen("test.out","w",stdout);
int main()
{
    for(int i=1;i<10000000;i++)
        printf("%d\n",i);
    return 0;
}

Using scanf&printf

#include<cstdio>
#include<iostream>
#include<ctime>
#include<fstream>
#include<iomanip>
//FILE *data = freopen("test.in","w",stdin);
FILE *fi = freopen("test.in","r",stdin);
FILE *fo = freopen("test.out","w",stdout);
int main()
{
    int ans = 1;
    while(~scanf("%d",&ans))
    {
        printf("%d\n",ans);
    }
    return 0;
}

Using cin & cout ( std::ios::sync_with_stdio(true) )

#include<cstdio>
#include<iostream>
#include<ctime>
#include<fstream>
#include<iomanip>
//FILE *data = freopen("test.in","w",stdin);
FILE *fi = freopen("test.in","r",stdin);
FILE *fo = freopen("test.out","w",stdout);
int main()
{
    std::ios::sync_with_stdio(true);
    int ans = 1;
    while(std::cin>>ans)
    {
        std::cout<<ans<<std::endl;
    }
    return 0;
}

Using std::ios::sync_with_stdio(false)

#include<cstdio>
#include<iostream>
#include<ctime>
#include<fstream>
#include<iomanip>
//FILE *data = freopen("test.in","w",stdin);
FILE *fi = freopen("test.in","r",stdin);
FILE *fo = freopen("test.out","w",stdout);
int main()
{
    std::ios::sync_with_stdio(false);
    int ans = 1;
    while(std::cin>>ans)
    {
        std::cout<<ans<<std::endl;
    }
    return 0;
}

Result:

scanf version:                           4.918 seconds
iostream version:                        51.266 seconds
iostream with sync_with_stdio(false):    31.815 seconds

Upvotes: 1

Views: 2509

Answers (1)

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385102

You're flushing std::cout every single time, with std::endl.

Don't do that. Instead, just output a '\n' if you want to match the behaviour of the printf version:

std::cout << ans << '\n';

Upvotes: 7

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