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Why is printf too slow in C++17(64bit)?
By Hyado, history, 6 years ago, In English

I got stuck in 102253L - Limited Permutation because of slow printf (´・_・`)

Does anyone know why printf is much slower than cout in C++17(64bit)?

I measured execution time in Custom Test

printf

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, a, n) for(ll i = a; i < n; ++i)
int main() {
  ll n = 10000000;
  rep(i, 0, n) printf("%d ", n);
}
  • 3587 ms(GNU C++17 9.2.0 (64bit,2msys))
  • 1996 ms(GNU C++17 7.3.0)

cout

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, a, n) for(ll i = a; i < n; ++i)

int main() {
  ll n = 10000000;
  rep(i, 0, n) cout << 1000000 << " ";
}
  • 1138 ms(GNU C++17 9.2.0 (64bit,2msys))
  • 1544 ms(GNU C++17 7.3.0)
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6 years ago, hide # |
 
Vote: I like it +8 Vote: I do not like it

How cout and printf are written are pretty much implementation independent. If you are particularly bent on performance, you can write custom print functions with putchar or fwrite, which will definitely be faster.

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6 years ago, hide # |
 
Vote: I like it 0 Vote: I do not like it

printf:
0.52user 0.00system 0:00.53elapsed 99%CPU (0avgtext+0avgdata 3500maxresident)k 0inputs+0outputs (0major+124minor)pagefaults 0swaps

cout:
0.56user 0.00system 0:00.56elapsed 99%CPU (0avgtext+0avgdata 3340maxresident)k 0inputs+0outputs (0major+121minor)pagefaults 0swaps

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6 years ago, hide # |
 
Vote: I like it +39 Vote: I do not like it

You can include #include <stdio.h> before #include <bits/stdc++.h> and you will get the same speed as with the 32bit version.

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