Question

The following code in python takes very long to run. (I couldn’t wait until the program ended, though my friend told me for him it took 20 minutes.)

But the equivalent code in Java runs in approximately 8 seconds and in C it takes 45 seconds.

I expected Python to be slow but not this much, and in case of C which I expected to be faster than Java was actually slower. Is the JVM using some loop unrolling technique to achieve this speed? Is there any reason for Python being so slow?

import time
st=time.time()
for i in xrange(0,100000):
    for j in xrange(0,100000):
        continue;
print "Time taken : ",time.time()-st

Answer 1

gcc 4.2 with the -O1 flag or higher optimize away the loop and the program takes 1 milli second to execute.
This benchmark is not very representative as it is very far from any real world use.
You’re doing a nested loop for a reason, and you never leave it empty.
Python doesn’t optimize away the loop, although I see no technical reason why it couldn’t.

Python is slower than C because it’s further from the machine language. xrange is a nice abstraction but it adds a heavy layer of machine code compared to a simple C loop.

C source:

int main( void ){
        int i, j;
        for (i=0;i<100000;i++){
                for (j=0;j<100000;j++){
                        continue;
                }
        }
        return 0;
}