Re: Java vs C++ speed (IO & Sorting)

Thu, 20 Mar 2008 05:53:57 -0700 (PDT)
On 20 Mrz., 11:01, peter koch <> wrote:

First of all, I believe this is a bad test. A lot of the time will be
involved with I/O which the compilers cant really affect.

Quick check, comment out the std::sort() call:
with std::sort(): 375ms
w/o : 281ms
I seem to have a similar machine in terms of performance, compared to
the original poster :-)

The program's runtime is dominated by the I/O, executed in both cases
by the same OS back-end functions.
The rest of the program is mainly comparing and shoving around memory
segments, I assume in both cases executed by library *machine* code.
Its only natural to me, the execution time is near identical.
Memory allocation seems to be no issue, at least not for C++ - if I
comment out the buf.reserve() call, no change in runtime is
However, the Java code pre-allocates 5000 lines, the C++ version
50000. Somebody with a Java environment may check out what happens if
the number is adjusted.
(The example text is ~31,000 lines).

One more thing caught my eye: The bible file contains a single empty
line that is processed by the C++ version but not by the Java version.
One extra empty line is not much, but induces O(log n) extra steps for
the sorting. If I modify the C++ program to disregard the empty line,
computing time goes down to 358ms (or 94ms --> 77ms for the sorting

I also
notice that the time included does not involve releasing memory used
by the Java-program which is unfair as this time was measured in the C+
+ version.

Plus, the considerable effort for loading and initialization, and
garbage collection of the Java VM is not included.

Be that as it is, I notice that the C++ version is fifty percent
shorter which suggests that developing with C++ will be quite a lot

While I agree in part, IMHO you are not referring to the right reason.
The physical typing of the programs should not make a big difference -
in C++ you can use really nifty constructs that save plenty of source
bytes. However, most other developers will have problems reading your
code - even you yourself may not be able to explain a code snippet you
wrote "ad hoc" only one week later.
Just as example, the infamous Ackerman function:

int ack( const int m, const int n ) {
  return m?n?ack(m-1,ack(m,n-1)):ack(m-1,1):n+1;

This *is* valid C++ code - a real space-saver, horrible style. I would
prefer the more typing intensive, but better manageable version:

int ack( const int m, const int n ) {
   if ( m == 0 ) return n+1;
   if ( n == 0 ) return ack( m-1, 1 );
   return ack( m-1, ack( m, n-1 ) );

Both versions are not identical in run-time efficiency: the "nifty"
version takes 5,4s on my system for ack(4, 1), the lengthy one 3,6s
only. I have no quick explanation for the difference, though.
For my feeling, too, the Java version looks clumsy style - it is
harder to understand.

just my EURO.02,


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