Thursday, April 19, 2018

Why does text processing in Python seem faster than in C++ in the following two examples? is it an I/O issue?

Hereunder are two short code snippets, one in C++ and one in Python, doing in principle the same simple text processing.



When running both on a large text file (i.e. Shakespeare' integral words as an UTF-8 or ASCII file from Gutenberg.org, it appears that the Python code runs five times faster, under Mac with Python 2.7.10 and g++ from Xcode (2016-11)).



Question: Did I make an ostensible mistake in the C++ test hereunder?




#include 
#include 
#include 
#include 

// C++ example.

std::map  M;

void word_frequency(std::string word) {

    M[word] = ++M[word];
}

void countwords(std::string fname)
{
    std::ifstream fin(fname);
    int count=0;
    std::string word, line;

    while (std::getline(fin, line)) {

        std::istringstream iss(line);
        while (iss >> word) {
            //std::cout << word << std::endl;
            word_frequency(word);
        }
    }
    //std::cout << count << std::endl;
}

int main(int argc, char **argv) {


    if (argc < 2) {
        std::cerr << "Usage: count_words \n";
        return EXIT_FAILURE;
    }
    else {
        countwords(argv[1]);
    }

    for (auto item : M)  std::cout << item.first << " " << item.second << std::endl;

}


#
# Python example
#

import sys

def main():

    lines = (l for l in file(sys.argv[1]))
    m = dict()
    for l in lines:
        for w in l.split(" "):
            if w in m:
                m[w] = m[w]+1
            else:
                m[w] = 1
    print m



if __name__ == "__main__":
    main()
-

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