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PO
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2011-12-16T21:47:48.020
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2011-12-16T22:04:34.727
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2011-12-16T22:04:34.727
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text
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<p>Try using the scanf libraries:</p> <h3>r1.cpp</h3> <pre><code>> cat r1.cpp #include <iostream> int main() { double x; long count = 0; while(std::cin >> x) { ++count; } std::cout << count << ": " << x << "\n"; } </code></pre> <h3>r2.cpp</h3> <pre><code>> cat r2.cpp #include <iostream> #include <stdio.h> int main() { double x; long count = 0; while(fscanf(stdin, "%lf", &x) == 1) { ++count; } std::cout << count << ": " << x << "\n"; } </code></pre> <h3>Results Serial</h3> <pre><code>> g++ -O3 r1.cpp -o r1 > time (cat t | ./r1) 40000000: 9.36e+08 real 0m57.669s user 0m56.992s sys 0m1.688s > g++ -O3 r2.cpp -o r2 > time (cat t | ./r2) 40000000: 9.36e+08 real 0m14.419s user 0m13.897s sys 0m1.352s </code></pre> <p>So it took onger than I expected about 60 seconds reading 40,000,000 numbers using IOstream. While only 15 seconds using scanf. So about 4 times faster.</p> <p>I did the same things but just writing the binary value of the doubles to the file.<br> Note you have to write them as binary and of course you loose all type safety and portability.</p> <pre><code>double x; std::cout.write((char*)&x, sizeof(x)); </code></pre> <h3>r1b.cpp</h3> <pre><code>> cat r1b.cpp #include <iostream> int main() { double x; long count = 0; while(std::cin.read((char*)&x, sizeof(double))) { ++count; } std::cout << count << ": " << x << "\n"; } </code></pre> <h3>r2b.cpp</h3> <pre><code>> cat r2b.cpp #include <iostream> #include <stdio.h> int main() { double x; long count = 0; while(fread(&x, sizeof(double), 1, stdin) == 1) { ++count; } std::cout << count << ": " << x << "\n"; } </code></pre> <h3>Result binary</h3> <pre><code>> time (cat t2 | ./r1b ) 40000000: 9.36e+08 real 0m3.930s user 0m3.592s sys 0m0.984s > time (cat t2 | ./r2b ) 40000000: 9.36e+08 real 0m2.110s user 0m1.840s sys 0m0.804s </code></pre>
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1. POQuickly loading numerical data in C++
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1. COThat's great, thanks.
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