30.08.07
Posted in C/C++ at 6:57 pm by Guy
Few days ago I decided to check what is really the fastest method to iterate over strings in C++. As a string class I chose string class from STL as it is very popular and provides a couple of ways to iterate it. So how can one iterate over an std::string?
- By using indexes. E.g.
str[i] and running i from zero to the length of the string.
- By using the
at method. string::at(size_t pos) provides similar interface to indexes with the exceptions that it checks whether the given position is past the end of the string and then throws an exception. One may see it as the safe version of the regular index.
- Treating the string as a sequence of characters and and iterate over it using iterators.
- Using
string::c_str() to get a pointer to a regular C string representation of the string stored in the std::string and treating it as array, e.g. using indexes to go over it.
- The last way to iterate over the string is to get a pointer to a C string representation using
string::c_str() and advancing the pointer itself to iterate over the string.
The third method is the native method of iterating over objects in STL, and like the last two it can’t be used if the iteration changes the string itself (e.g. inserting or deleting characters). The first and second method are similar to the fourth (treating the pointer to the C string as an array), except that they aren’t so problematic as the latter when changing the string. The second method is the safest as it’s the only one that does range checks and throws exception if trying to access positions which are outside the string.
To benchmark and find out which method is the fastest method to iterate over a string I’ve created a huge string of random characters ranging from ‘a’ to ‘z’ and five executables, each one implementing one of the above iteration methods to do a simple task
(count the number of occurrences of each letter). The string is fifty million characters long which, as the longer the string the less important the overhead becomes.
The executables for the benchmark of every version were compiled with the default setting of g++ (without optimization as the compiler might change the iteration methods when optimizing). The benchmark executables where timed by using the time command and redirecting the executables output to /dev/null. The tests were run both on 64bit Gentoo (with 1 GB RAM) and on 32bit Kubuntu (with 512 MB RAM), to make sure the overall results (which method it better not the runtime itself) isn’t system depended.
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17.08.07
Posted in Projects, Python, radio.py at 1:44 pm by Guy
Download radio-0.3.tar.gz.
Update: radio.py-0.4 is now available.
I like listening to music and radio while working, and fortunately there are numerous ways to do that. Unfortunately, most ways that allow you to listen to radio are very resource consuming/memory hogs (such as listening to streaming-media via web-browsers) or very unfriendly to users (listening via mplayer for example). So, I set out to find a way that will use as little system resources as possible while keeping it user-friendly. One other requirement that I had, that I will be able to do all that from the command-line, so it will work great with GNU Screen and won’t require an X server (if I work without one).
I used for some time mplayer for listening to radio. I had a file with a list of web-radio streams URLs which I would copy and pass to mplayer -playlist. This method answered two of the requirements (minimal resources and command-line interface), but wasn’t really user friendly. So, I wrote a little wrapper script in python around mplayer - radio.py. After quick installation (download and extract the tar archive and copy radio.py to somewhere in you PATH), radio.py will allow you to listen to stations easily, and it will also do couple more things for you.
To listen to a station just call radio.py with the station’s name, e.g. in the command-line enter radio.py BBC1 to listen for BBC radio channel 1. To view a list of know stations run radio.py --list. Currently there aren’t many stations (just stations I thought that are needed or I listen to). You can easily edit radio.py to add new stations (the script is documented and very clear). If you do so, please write a comment or email me so I will be able to add those stations to next release by default.
So, as you seen radio.py allows you to easily listen to radio, as easy as writing the station’s name. But, as I said, it can do more things that I thought should be in a radio script. It has both a sleep feature (that turns off the radio after specified amount of time) and a wake-up feature (that starts the radio after a specified amount of time). This two features can be used together, and practically allow you to use radio.py as an alarm clock.
You can find more information about radio.py options by calling radio.py --help. I hope you will find this script useful as I do.
Download:
radio-0.3.tar.gz.
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10.08.07
Posted in C/C++, Tutorials at 12:22 am by Guy
In this post and its follow ups I intend to cover the basics of CGI programming in C++. There are great performance gain in writing CGIs in C++ compared to interpreted languages such as PHP and it’s usually it’s even faster than PHP scripts which are interpreted via mod_php. On the other hand PHP and other traditional web development languages are well suited for the task, by means of libraries and development time. However developing small highly efficient CGI scripts in C++ is easier than you think.
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