Question

I’m using Windows 7 using CPython for Python 3.22 and MinGW‘s g++.exe for C++ (which means I use the libstdc++ as the runtime library). I wrote two simple programs to compare their speed.

Python:

x=0
while x!=1000000:
    x+=1
    print(x)

C++:

#include <iostream>

int main()
{
    int x = 0;
    while ( x != 1000000 )
    {
        x++;
        std::cout << x << std::endl;
    }
    return 0;
}

Both are not optimized.

I ran C++ first, and then i ran Python through the interactive command line, which is much slower than directly starting a .py file.

However, Python outran C++, and it turned out to be more than twice as fast. Python took 53 seconds, and C++ took 1 minute and 54 seconds.

Is it because Python has some special optimization done to the interpreter or is it because C++ has to refer to <iostream> and std which slows it down and makes it take up RAM?
Or is it some other reason?

I tried again, with \n instead of std::endl, and compiling with the -O3 flag. This time it took 1 minute to reach 500,000.

Answer 1

There isn’t anything obvious here. Since Python’s written in C, it must use something like printf to implement print. C++ I/O Streams, like cout, are usually implemented in a way that’s much slower than printf. If you want to put C++ on a better footing, you can try changing to:

#include <cstdio>
int main()
{
    int x=0;
    while(x!=1000000)
    {
        ++x;
        std::printf("%d\n", x);
    }
    return 0;
}

I did change to using ++x instead of x++. Years ago people thought that this was a worthwhile “optimization.” I will have a heart attack if that change makes any difference in your program’s performance (OTOH, I am positive that using std::printf will make a huge difference in runtime performance). Instead, I made the change simply because you aren’t paying attention to what the value of x was before you incremented it, so I think it’s useful to say that in code.