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• how a pure functional programming language manage without assignment statements?

  - by Gnijuohz

  When reading the famous SICP,I found the authors seem rather reluctant to introduce the assignment statement to Scheme in Chapter 3.I read the text and kind of understand why they feel so. As Scheme is the first functional programming language I ever know something about,I am kind of surprised that there are some functional programming languages(not Scheme of course) can do without assignments. Let use the example the book offers,the bank account example.If there is no assignment statement,how can this be done?How to change the balance variable?I ask so because I know there are some so-called pure functional languages out there and according to the Turing complete theory,this must can be done too. I learned C,Java,Python and use assignments a lot in every program I wrote.So it's really an eye-opening experience.I really hope someone can briefly explain how assignments are avoided in those functional programming languages and what profound impact(if any) it has on these languages. The example mentioned above is here: (define (make-withdraw balance) (lambda (amount) (if (>= balance amount) (begin (set! balance (- balance amount)) balance) "Insufficient funds"))) This changed the balance by set!.To me it looks a lot like a class method to change the class member balance. As I said,I am not familiar with functional programming languages,so if I said something wrong about them,feel free to point out.

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• Why the overhead when allocating objects/arrays in Java?

  - by Gnijuohz

  How many bytes an array occupies in Java? Assume It's a 64bit machine and also assume there are N elements in an array, so all these elements would take up 2*N, 4*N or 8*N bytes for different types of array. And a lecture in Coursera says that it would occupy 2*N+24, 4*N+24 or 8*N+24 bytes for a N element array and the 24 bytes is called overhead, but didn't explain why the overhead is needed. Also objects have overheads, which is 16 bytes. What exactly are these overheads? What are these 24/16 bytes composed of? Also, do these overheads only exist in Java? How about C, C++ and Python?

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• Is the phrase "never reinvent the wheel" suitable for students?

  - by Gnijuohz

  I find myself constantly running into this expression "don't reinvent the wheel" or "never reinvent the wheel" when I ask some questions on SO. They tell you to use some frameworks or existing packages. I know where this attitude is coming from since it's unwise to waste time on something others have already solved. Or it that so? As a student, I find by using some code others wrote to solve my problem I can't learn as much as I'd like to, and I gain less insight. And sometimes I think that phrase is mainly for working programmers facing deadlines and not for students like me. Is it that bad to "reinvent the wheel"? Maybe I'm thinking it wrong? Maybe there is a way I can avoid reinventing the wheel and at the same time learn a lot?

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• Is it easier to develop from scratch or not? [closed]

  - by Gnijuohz

  I am currently reading the book computer science: an overview by J. Glenn Brookshear.And In chapter 7,there is one passage as follows: In fact, it is often within this phase(modification) that a piece of software is discarded under the pretense (too often true) that it is easier to develop a new system from scratch than to modify the existing package successfully. When I read this,an article by Joel occurred to me which mentions how Mozilla shouldn't have written its browser from scratch.(The article is here) So,Is it mostly true that it's easier to develop a new system from scratch than to modify the existing one?Or it's closely related to the complexity of the system?

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• what's the overhead when allocating objects/arrays in Java?

  - by Gnijuohz

  How many bytes an array occupies in Java? Assume It's a 64bit machine and also assume there are N elements in an array, so all these elements would take up 2*N, 4*N or 8*N bytes for different types of array. And a lecture in Coursera says that it would occupy 2*N+24, 4*N+24 or 8*N+24 bytes for a N element array and the 24 byte is called overhead, but didn't explain it. Also objects have overheads, which is 16 bytes. What exactly are these overheads? Also, do these overheads only exist in Java? How about C, C++ and Python?

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• Books or resources about x86 64 assembly written in AT&T style?

  - by Gnijuohz

  I know what I'm asking for is quite specific and many would say if you know x86 assembly, this wouldn't be an issue. But now I'm taking a course that requires me to use x86 64 assembly in AT&T style and I'm not familiar with assembly in the first place. So I think if I can find some books or web resources about x86 64 assembly written in AT&T convention, it'll help me the most. But so far, I haven't found such books or detailed web resources yet. So can anyone point me to some good resouces? Advice on learning Assembly is also appreciated here!

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• configure squid3 to set up a web proxy in ubuntu12.04

  - by Gnijuohz

  I am in a LAN and have to use a proxy given to access the web in a very limited way. I can't even use google, github.com or SE sites. However I can use ssh to log into a server, which I have root access so basically I can do anything I want with it. So I was thinking that maybe I could use that server as a proxy so I can visit sites through it. I tested it using ssh -vT [email protected] which gave a proper response. And In my computer I can't do this. Also I tried downloading something from the gun.org using wget, which can't be done in my computer too. And it succeeded on that server. I don't know if that's enough to say that this server have full access to the Internet. But I assumed so and I installed squid3 on it. After trying some while, I failed to get it working. I got this after I run squid3 -k parse 2012/07/06 21:45:18| Processing Configuration File: /etc/squid3/squid.conf (depth 0) 2012/07/06 21:45:18| Processing: acl manager proto cache_object 2012/07/06 21:45:18| Processing: acl localhost src 127.0.0.1/32 ::1 2012/07/06 21:45:18| Processing: acl to_localhost dst 127.0.0.0/8 0.0.0.0/32 ::1 2012/07/06 21:45:18| Processing: acl localnet src 10.1.0.0/16 # RFC1918 possible internal network 2012/07/06 21:45:18| Processing: acl SSL_ports port 443 2012/07/06 21:45:18| Processing: acl Safe_ports port 80 # http 2012/07/06 21:45:18| Processing: acl Safe_ports port 21 # ftp 2012/07/06 21:45:18| Processing: acl Safe_ports port 443 # https 2012/07/06 21:45:18| Processing: acl Safe_ports port 70 # gopher 2012/07/06 21:45:18| Processing: acl Safe_ports port 210 # wais 2012/07/06 21:45:18| Processing: acl Safe_ports port 1025-65535 # unregistered ports 2012/07/06 21:45:18| Processing: acl Safe_ports port 280 # http-mgmt 2012/07/06 21:45:18| Processing: acl Safe_ports port 488 # gss-http 2012/07/06 21:45:18| Processing: acl Safe_ports port 591 # filemaker 2012/07/06 21:45:18| Processing: acl Safe_ports port 777 # multiling http 2012/07/06 21:45:18| Processing: acl CONNECT method CONNECT 2012/07/06 21:45:18| Processing: http_port 3128 transparent vhost vport 2012/07/06 21:45:18| Starting Authentication on port [::]:3128 2012/07/06 21:45:18| Disabling Authentication on port [::]:3128 (interception enabled) 2012/07/06 21:45:18| Disabling IPv6 on port [::]:3128 (interception enabled) 2012/07/06 21:45:18| Processing: cache_mem 1000 MB 2012/07/06 21:45:18| Processing: cache_swap_low 90 2012/07/06 21:45:18| Processing: coredump_dir /var/spool/squid3 2012/07/06 21:45:18| Processing: refresh_pattern ^ftp: 1440 20% 10080 2012/07/06 21:45:18| Processing: refresh_pattern ^gopher: 1440 0% 1440 2012/07/06 21:45:18| Processing: refresh_pattern -i (/cgi-bin/|?) 0 0% 0 2012/07/06 21:45:18| Processing: refresh_pattern (Release|Packages(.gz)*)$ 0 20% 2880 2012/07/06 21:45:18| Processing: refresh_pattern . 0 20% 4320 2012/07/06 21:45:18| Processing: ipcache_high 95 2012/07/06 21:45:18| Processing: http_access allow all I deleted some allow and deny rules and added http_access allow all so that all the request would be allowed. After configuring my computer, I got this error: Access control configuration prevents your request from being allowed at this time. Please contact your service provider if you feel this is incorrect. And the log in the server showed that my TCP requests had all been denied. So, first of all, is what I am trying to do achievable? If so, how to configure the squid in the server so that I use it as a proxy to surf the Internet? My computer and the server both run Ubuntu11.04. Thanks for any help~

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• How can I remove the ssh last login info?

  - by Gnijuohz

  Whenever I log into a server using ssh. The prompt gives me "last login" information. I was wondering where this information comes from. How can I remove this record so when someone else log into the same server, the person would see my login info with my ip in it? So how can I do this? For the record, I am not hacking someone's computer and the server runs Ubuntu 12.04. EDIT: which file logs this kind of information? If I find the file, then I can do anything to it as root. Thanks.

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• Unpack this directory into your PYTHONPATH?

  - by Gnijuohz

  I want to install qt4reactor so I went to the github page, it says: Unpack this directory into your PYTHONPATH I don't know what that means. I unziped the file and I tried to add the path to PYTHOPNPATH in the .bashrc file like this: export PYTHONPATH=/home/jacos/qtreactor But I can't import it whatsoever. What I want to ask is where exactly is my PYTHONPATH? How can I find that out? What's the right way to do this?

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