Difference in runtime between stdio.h e iostream

Question

Difference in runtime between stdio.h e iostream

I made two codes for an online judge to correct. They are essentially the same. But the with stdio.h is accepted and O with iostream is not, as it exceeds the timeout. Why does this occur?

#include <stdio.h>
int tipo[1000000];
int main(){
    int qtdrot, qtdtipos, i, x, min;
    scanf("%d%d", &qtdrot, &qtdtipos);
    for(i=0; i<qtdrot; ++i){
        scanf("%d", &x);
        tipo[x]++;
    }
    min=1000000;
    for(i=1; i<=qtdtipos; i++){
        if(tipo[i]<min)min=tipo[i];
    }
    printf("%d\n", min);
    return 0;
}

O with iostream:

#include <iostream>
using namespace std;
int tipo[1000000];
int main(){
    int qtdrot, qtdtipos, i, min, t;
    cin>>qtdrot>>qtdtipos;
    for(i=0; i<qtdrot; ++i){
        cin>>t;
        tipo[t]++;
    }
    min=1000000;
    for(i=1; i<=qtdtipos; i++){
        if(tipo[i]<min)min=tipo[i];
    }
    cout<<min<<endl;
    return 0;
}

6

c++runtime

Author: Guilherme Lima, 2016-03-02

Source

1 answers

score 6 · Accepted Answer

cin and cout "spend" a good amount of time synchronizing with the buffers of the stdio. It is not difficult to see programs with operations 3 to 4 times slower compared to scanf and printf (See this example in Soen).

You can reduce runtime by disabling this feature with the command:

std::ios_base::sync_with_stdio(false);

Additionally endl Forces a flush (see this answer in Quora ). In loops \n can become more efficient (in your case, as you only use cout Once the difference is not significant):

cout << min << '\n';

Made the substitutions the example with iostream was on average 13% faster than the example with stdio on my machine / compiler / OS (always be careful generalizing microbenchmarks, but in this case the difference was noticeable).