1. matlab中gradient函数求f=2x2+3y3的梯度的实例 # http://www.zdh1909.com/html/matlab/9930.html
matlab中gradient函数求f=2x2+3y3的梯度的实例如下：
在上MATLAB课的时候,有学生问怎么用gradient函数求f=2x2+3y3的梯度啊,不懂得怎么写格式,试了很多次都不行.解答如下:
在MATALB中,求梯度只能是求数值梯度,所以必须将函数f离散化,用差分代替微分,精度取决于差分步长,因为现在计算机速度足够快,所以差分可以取得足够小,也不影响计算速度和计算精度.方法如下:
>>X=-6:0.6:6; %计算区间是[-6 6],步长0.6
>>Y=X;
>>[x,y]=meshgrid(X,Y) %生成计算网格
>>f=2.*x.^2+3.*y.^3 %计算网格结点上的函数值
>>[Dx,Dy]=gradient(f) %用数值方法求函数梯度
>>quiver(X,Y,Dx,Dy) %用矢量绘图函数绘出梯度矢量大小分布
>>hold on
>>contour(X,Y,f) %与梯度值对应,绘出原函数的等值线图
2. Using errorbar() with semilogy() in MATLAB? # http://stackoverflow.com/questions/3550241/using-errorbar-with-semilogy-in-matlab
h = errorbar(x,y,ebar); set(get(h,'Parent'), 'XScale', 'log') `</pre> or <pre>`ax = axes(); errorbar(ax, x,y,ebar); set(ax, 'XScale', 'log'); 3. Multi y axis # [http://stackoverflow.com/questions/6573862/matlab-two-y-axis-with-the-same-x-axis](http://stackoverflow.com/questions/6573862/matlab-two-y-axis-with-the-same-x-axis)
1. 计算机领域学习路线图 # from: http://goo.gl/RZtPC
 目标 # 帮助计算机领域的初学者, 从零开始, 一步步成为一个靠谱的计算机领域的专家. 这里面只包含偏软件的部分.
我们需要达成以下目标:
了解计算机领域的基础知识. 对背景的了解是基础. 拥有计算机科学家的思维和解决问题的能力 能编程! 编程是我们解决问题的主要方式. 使用开源系统, 加入到开源社区中去 计算机基础课程学习 # 想要学习计算机领域的基础知识, 我选择使用mit公开课里面的内容. 下面我挑了一些必读课程:
入门课程.
6.00 入门课.
6.001 老牌入门课.
6.087 C语言是必须要掌握的..
6.189 python也是必须要掌握的.
基础课程.
6.042J 恩, 一些计算机领域必须的数学基础.
6.046J 算法基础.
6.002 计算机组成原理.
6.828 操作系统
6.035 编译原理.
网络部分.
6.033 Computer System Engineering
6.171 Software Engineering for Web Applications
上面是必读, 当然还有其他的课程, 感兴趣可以学学, 都在: http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/
可做和必做事项 # 下面我整理了一些必须做和可做的事情, 我觉得它们对于提高实力非常重要.
开源社区
安装和使用linux, 在linux上面做开发
...
skills: # shortcut of “echo”: <?=$foo?>
phpMyadmin timeout / maxlifttime / cookie validity: # ref:http://birdchan.com/home/2011/06/06/phpmyadmin-timeout-after-1440-seconds/
vim /etc/phpmyadmin/config.inc.php : add:
$cfg[‘Servers’][$i][‘LoginCookieValidity’] = 60 * 60 * 4; //4 hours
vim /etc/php5/cli/php.ini : modify:
session.gc_maxlifetime = 14400
Add / Create a User: # CREATE USER <userName>@localhost IDENTIFIED BY "<newPassWd>"; CREATE DATABASE <newDbName> DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci; GRANT ALL ON <newDbName>.* TO <userName>@localhost;
Super PRIVILEGES:
GRANT ALL <span style="text-decoration: underline;">PRIVILEGES</span> ON *.* TO <userName>@localhost WITH GRANT OPTION;
Revoke:
REVOKE INSERT ON *.* FROM <userName>@localhost; REVOKE ALL PRIVILEGES, GRANT OPTION FROM <userName>@localhost;
Reset the password: # SET PASSWORD FOR <userName>@localhost = PASSWORD('<newPassWd>');
reset root password: http://goo.gl/jGXR6
...
see also tcpdump
Install Wireshark # sudo add-apt-repository ppa:wireshark-dev/stable sudo apt update sudo apt install wireshark Config For Non-Root (Normal) Users # for 1 user # chgrp ats /usr/bin/dumpcap chmod 4750 /usr/bin/dumpcap # chmod u+s for a user group # groupadd wireshark usermod -a -G wireshark userName1 chgrp wireshark /usr/bin/dumpcap chmod 4750 /usr/bin/dumpcap ref:
http://anonscm.debian.org/viewvc/collab-maint/ext-maint/wireshark/trunk/debian/README.Debian?view=markup
Update: 2017-01-06
Warn 1: for non-English, use samba/sshfs instead. unless win has NFS v4 or above which can solve non-UTF8 problems.
Warn 2: svchost consumes 1.5GB ram, don’t know why.
See Also Links
SSHFS/SFTP
CIFS/SMB/Samba
OBS # Combining simlink (ln -s) with NFS may cause weird file path problems. We tried to have a link at NFS to Dropbox in nfs, also tried to have a link at home to mount point at /mnt.
...
See also: /iptables
FreeBsd 7 Local Router (Like a Switch ???) # (local only, no NAT)
vim /etc/rc.conf
ifconfig_em0="inet 10.0.0.12 netmask 255.255.255.0" ifconfig_em1="inet 10.0.1.13 netmask 255.255.255.0" notice: will get route problem if using the same net-addr.
permanent # vim /etc/sysctl.conf net.inet.ip.forwarding=1 # reboot temp sysctl net.inet.ip.forwarding=1 # or echo 1 > /proc/sys/net/ipv4/ip_forward // problem: ping test: could get reply since icmp packet 113. where are the first 112 packets???
FreeBsd 7 Bridge # ref: shaper freeBsd-v7 : bridge
...
Tested on platform: # vmware
Linux ubuntu 2.6.32-38-generic #83-Ubuntu SMP Wed Jan 4 11:13:04 UTC 2012 i686 GNU/Linux
DISTRIB_ID=Ubuntu; DISTRIB_RELEASE=10.04; DISTRIB_DESCRIPTION=“Ubuntu 10.04.4 LTS”
httpd-2.0.64.tar.bz2; net-snmp-5.4.4; mod_ap2_snmp_1.04.
sudo -s;
apt-get remove lamp-server^ snmpd # if you have already installed apache or snmp-server.
Just do it: # http://mod-apache-snmp.sourceforge.net/english/docs.htm
(below is a summary)
1) Install Apache Web Server with DSO support enabled.
./configure --enable-so; make; make install 2) Unzip NET-SNMP sources.
3) Unzip Mod-APACHE-SNMP sources.
...
The path "" is not valid path to the gcc binary. Would you like to change it? [yes] when typed in "/usr/bin/gcc" manually,
The path "/usr/bin/gcc" is not valid path to the gcc binary. Would you like to change it? [yes] I found solution:
http://superuser.com/questions/192047/gcc-path-not-valid-when-installing-vmware-tools-from-vmware-workstation-7-1-1-bu
ie: sudo apt-get install linux-source linux-headers-uname -r libglib2.0-0
// Got “If you wish to have the shared folders feature, you can install the driver by running vmware-config-tools.
...
XX的kde, 你以为你是微软啊, 乱给用户装东西!!!
卸载了半天, 一直找不到该卸载什么,最后只好这么办:
apt-get install kwalletmanager
运行 kwalletmanager, 设置里面 uncheck “enable kwallet subsys”.