直观比较读入的效率

数据生成

数据

随机生成一个长度为 \(5\times 10^7\) ，值域在 \([0,500)\) 的数列。

数据生成器

int main() {
  srand(time(NULL));
  freopen("1.in", "w", stdout);
  for (int i = 1; i <= 50000000; i++) {
    printf("%d ", rand() % 500);
  }
}

对比

以下所有测试均在 -std=c++11,-O2,-O3 下进行。

cin

const int N = 50000005;
int a[N], n = 50000000;
int main() {
  freopen("1.in", "r", stdin);
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
}

cin 关同步流

const int N = 50000005;
int a[N], n = 50000000;
int main() {
  ios::sync_with_stdio(false), cin.tie(0);
  freopen("1.in", "r", stdin);
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
}

scanf

const int N = 50000005;
int a[N], n = 50000000;
int main() {
  freopen("1.in", "r", stdin);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &a[i]);
  }
}

快读

const int N = 50000005;
int a[N], n = 50000000;
int read() {
  int x = 0;
  char ch = getchar();
  while (ch < '0' || ch > '9') ch = getchar();
  while ('0' <= ch && ch <= '9') x = (x << 1) + (x << 3) + ch - '0', ch = getchar(); 
  return x;
}
int main() {
  freopen("1.in", "r", stdin);
  for (int i = 1; i <= n; i++) {
    a[i] = read();
  }
}

快读 + isdigit

const int N = 50000005;
int a[N], n = 50000000;
int read() {
  int x = 0;
  char ch = getchar();
  while (!isdigit(ch)) ch = getchar();
  while (isdigit(ch)) x = (x << 1) + (x << 3) + ch - '0', ch = getchar(); 
  return x;
}
int main() {
  freopen("1.in", "r", stdin);
  for (int i = 1; i <= n; i++) {
    a[i] = read();
  }
}

inline + 快读 + isdigit

const int N = 50000005;
int a[N], n = 50000000;
inline int read() {
  int x = 0;
  char ch = getchar();
  while (!isdigit(ch)) ch = getchar();
  while (isdigit(ch)) x = (x << 1) + (x << 3) + ch - '0', ch = getchar(); 
  return x;
}
int main() {
  freopen("1.in", "r", stdin);
  for (int i = 1; i <= n; i++) {
    a[i] = read();
  }
}

fread

const int N = 50000005;
int a[N], n = 50000000;
char buf[1 << 21], *p1 = buf, *p2 = buf;
int getc() {
  return p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 1 << 21, stdin), p1 == p2) ? EOF: *p1++;
}
int read() {
  int x = 0;
  char ch = getc();
  while (!isdigit(ch)) ch = getc();
  while (isdigit(ch)) x = (x << 1) + (x << 3) + ch - '0', ch = getc();
  return x;
}
int main() {
  freopen("1.in", "r", stdin);
  for (int i = 1; i <= n; i++) {
    a[i] = read();
  }
}

sgetn

const int N = 50000005;
int a[N], n = 50000000;

const int B = 1 << 21;
char Obuf[B], Ibuf[B];
char *optr = Obuf, *ist = 0, *ied = 0;
streambuf *out, *in;
void print(char c) {
  optr == Obuf + B ? (out->sputn(Obuf, B), optr = Obuf), *optr++ = c : *optr++ = c;
}
void flush() {
  out->sputn(Obuf, optr - Obuf);
}
char getc() {
  return ist == ied ? ied = Ibuf + in->sgetn(ist = Ibuf, B), (ist == ied ? -1 : *ist++) : *ist++;
}
int read() {
  int x = 0;
  char ch = getc();
  while (!isdigit(ch)) ch = getc();
  while (isdigit(ch)) x = (x << 1) + (x << 3) + ch - '0', ch = getc();
  return x;
}
int main() {
  ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  in = cin.rdbuf(), out = cout.rdbuf();
  freopen("1.in", "r", stdin);
  for (int i = 1; i <= n; i++) {
    a[i] = read();
  }
}

mmap

#include <bits/stdc++.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
using namespace std;

const int N = 50000005;
int a[N], n = 50000000;

char *pc;
int read() {
  int x = 0;
  char ch = *pc++;
  while (!isdigit(ch)) ch = *pc++;
  while (isdigit(ch)) x = (x << 3) + (x << 1) + ch - '0', ch = *pc++;
  return x;
}

int main() {
  pc = (char *) mmap(NULL, lseek(0, 0, SEEK_END), PROT_READ, MAP_PRIVATE, 0, 0);
  freopen("1.in", "r", stdin);
  for (int i = 1; i <= n; i++) {
    a[i] = read();
  }
}

对比表

读入方式	实验1/s	实验2/s	实验3/s	平均用时/s	效率排名
\(\texttt{cin}\)	\(23.890s\)	\(23.990s\)	\(23.820s\)	\(23.900s\)	\(9\)
\(\texttt{scanf}\)	\(20.490s\)	\(20.310s\)	\(20.350s\)	\(20.383s\)	\(8\)
\(\texttt{cin}\) 关同步流	\(5.936s\)	\(5.970s\)	\(5.862s\)	\(5.923s\)	\(7\)
快读	\(4.601s\)	\(4.670s\)	\(4.666s\)	\(4.646s\)	\(6\)
快读 + \(\texttt{isdigit}\) + \(\texttt{inline}\)	\(4.626s\)	\(4.616s\)	\(4.589s\)	\(4.610s\)	\(5\)
快读 + \(\texttt{isdigit}\)	\(4.627s\)	\(4.569s\)	\(4.611s\)	\(4.602s\)	\(4\)
\(\texttt{fread}\)	\(0.546s\)	\(0.558s\)	\(0.547s\)	\(0.550s\)	\(3\)
\(\texttt{sgetn}\)	\(0.535s\)	\(0.542s\)	\(0.541s\)	\(0.539s\)	\(2\)
\(\texttt{mmap}\)	[N/A]	[N/A]	[N/A]	[N/A]	理论 \(1\)

---

第二次测速 （2021.05.19）

三组数据，分别 \(n=10^6,10^7,5\times 10^7\) ，值域 \([0,2^{20})\) 。

cin 关同步流：

data #1, in = read1.in, out = read1.out, statu = Accepted, time used = 0.240 s.
data #2, in = read2.in, out = read2.out, statu = Accepted, time used = 1.698 s.
data #3, in = read3.in, out = read3.out, statu = Accepted, time used = 8.299 s.

快读 + \(\texttt{isdigit}\) ：

data #1, in = read1.in, out = read1.out, statu = Accepted, time used = 0.258 s.
data #2, in = read2.in, out = read2.out, statu = Accepted, time used = 1.827 s.
data #3, in = read3.in, out = read3.out, statu = Accepted, time used = 8.781 s.

\(\texttt{fread}\) ：

data #1, in = read1.in, out = read1.out, statu = Accepted, time used = 0.118 s.
data #2, in = read2.in, out = read2.out, statu = Accepted, time used = 0.297 s.
data #3, in = read3.in, out = read3.out, statu = Accepted, time used = 1.115 s.

\(\texttt{sgetn}\) ：

data #1, in = read1.in, out = read1.out, statu = Accepted, time used = 0.092 s.
data #2, in = read2.in, out = read2.out, statu = Accepted, time used = 0.254 s.
data #3, in = read3.in, out = read3.out, statu = Accepted, time used = 1.043 s.