我想用Python和C++来比较STDIN中的字符串输入的读取行,并震惊地看到我的C++代码运行速度比等效的Python代码慢了一个数量级.因为我的C++生疏了,我还不是一个专家,请告诉我,如果我做错了什么,或者我误解了什么.
(TLDR answer:)包含以下语句:cin.sync_with_stdio(false)
或仅使用fgets
.
TLDR results:.一直向下滚动到问题的底部,并查看表格.)
C++ code:
#include <iostream>
#include <time.h>
using namespace std;
int main() {
string input_line;
long line_count = 0;
time_t start = time(NULL);
int sec;
int lps;
while (cin) {
getline(cin, input_line);
if (!cin.eof())
line_count++;
};
sec = (int) time(NULL) - start;
cerr << "Read " << line_count << " lines in " << sec << " seconds.";
if (sec > 0) {
lps = line_count / sec;
cerr << " LPS: " << lps << endl;
} else
cerr << endl;
return 0;
}
// Compiled with:
// g++ -O3 -o readline_test_cpp foo.cpp
Python Equivalent:
#!/usr/bin/env python
import time
import sys
count = 0
start = time.time()
for line in sys.stdin:
count += 1
delta_sec = int(time.time() - start_time)
if delta_sec >= 0:
lines_per_sec = int(round(count/delta_sec))
print("Read {0} lines in {1} seconds. LPS: {2}".format(count, delta_sec,
lines_per_sec))
Here are my results:个
$ cat test_lines | ./readline_test_cpp
Read 5570000 lines in 9 seconds. LPS: 618889
$ cat test_lines | ./readline_test.py
Read 5570000 lines in 1 seconds. LPS: 5570000
I should note that I tried this both under Mac OS X v10.6.8 (Snow Leopard) and Linux 2.6.32 (Red Hat Linux 6.2). The former is a MacBook Pro, and the latter is a very beefy server, not that this is too pertinent.个
$ for i in {1..5}; do echo "Test run $i at `date`"; echo -n "CPP:"; cat test_lines | ./readline_test_cpp ; echo -n "Python:"; cat test_lines | ./readline_test.py ; done
Test run 1 at Mon Feb 20 21:29:28 EST 2012
CPP: Read 5570001 lines in 9 seconds. LPS: 618889
Python:Read 5570000 lines in 1 seconds. LPS: 5570000
Test run 2 at Mon Feb 20 21:29:39 EST 2012
CPP: Read 5570001 lines in 9 seconds. LPS: 618889
Python:Read 5570000 lines in 1 seconds. LPS: 5570000
Test run 3 at Mon Feb 20 21:29:50 EST 2012
CPP: Read 5570001 lines in 9 seconds. LPS: 618889
Python:Read 5570000 lines in 1 seconds. LPS: 5570000
Test run 4 at Mon Feb 20 21:30:01 EST 2012
CPP: Read 5570001 lines in 9 seconds. LPS: 618889
Python:Read 5570000 lines in 1 seconds. LPS: 5570000
Test run 5 at Mon Feb 20 21:30:11 EST 2012
CPP: Read 5570001 lines in 10 seconds. LPS: 557000
Python:Read 5570000 lines in 1 seconds. LPS: 5570000
微小的基准附录和重述
为了完整性,我想我会用同一个文件更新原来的(同步)C++代码的读取速度.同样,这是一个快速磁盘上的100M行文件.以下是几种解决方案/方法的比较:
Implementation | Lines per second |
---|---|
python (default) | 3,571,428 |
cin (default/naive) | 819,672 |
cin (no sync) | 12,500,000 |
fgets | 14,285,714 |
wc (not fair comparison) | 54,644,808 |