octave-basic

在coursera上斯坦福的machine learning，lecturer极力推荐开源的编程环境Octave入手，所以我也下载了来试一试吧==

参考链接：http://www.linuxdiyf.com/linux/22034.html

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安装（Ubuntu16.04）:我看了下官网，Ubuntu上已经更新到4.0.3了，不过还是选了stable的，这里应该是4.0.2

$ sudo apt-add-repository ppa:octave/stable
$ sudo apt-get update
$ sudo apt-get install octave

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运行octave：有图形界面 $ sudo octave

或者使用命令行来运行：$ sudo octave-cli

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Octave和Matlab的主要区别：
1、费用方面
Octave是完全免费的（并且是开源的），而Matlab是商业软件
2、占用空间
Octave比较小，安装程序只有几十兆；Matlab庞大，是因为有大量的面向各种应用领域的工具箱，Octave无法相比的。
3、语法方面
Octave最初便是模彷Matlab而设计，语法基本上与Matlab一致，严谨编写的代码应同时可在Matlab及Octave运行，但也有很多细节上差别。
所以真的要写可以在matlab上运行的octave的话，需要考虑兼容性。
4、绘图方面
Octave的画图后台是强大的Gnuplot，有人认为绝对不会弱于Matlab，而且输出格式要远多于Matlab，公式显示也要强大很多。但也有人说Octave绘图速度比Matlab慢。
5、用户界面
Octave的GUI才刚开始有，可能弱一些。

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Octave操作

% one row
x1 = [1, 3, 2]
% three row
x2 = [1; 3; 2]
% complex matrix should use .' for transposition
A.'
% A and B are matrix. Element-wise.
% .*  ./   .^
A ./ B
a = 5
% Cij = a^Bij
C = a .^ B
% Moreover, we can also calculate row-wise multiply or division:
a = [1 2 3; 4 5 6; 7 8 9]
b = [10; 11; 12]
c = b.*a
d = a./b

index:

和Matlab一样，下标从1开始。

x = [1.2, 5, 7.6, 3, 8]
x(2)
% use index list
x([1, 3, 4])
A = [1, 2, 3; 4, 5, 6; 7, 8, 9]
% 1st and 3rd rows and 2nd and 3rd cols
A([1, 3], [2, 3])
% : means all elements
A(2, :)

range:

% start:step:stop
% default step is 1
1:3:10
% end means last element in the row or col. For vector and matrix
% last col of A showing
A(:, end)

special matrix:

x = [1,2,3;4,5,6;7,8,9]
% lower triangular part of x. Diagonal included.
tril(x)
% upper triangular part of x. Diagonal included.
triu(x)
% indentity matrix. eye(m, n) for m*n. filled by '0'
eye(n)
ones(m, n)
zeros(m, n)
% elements are in [0,1). uniformly drawn
rand(m, n)
% normaly distributed. negative is ok
randn(m, n)
% return a random permutation which is a row vector
randperm(n)
v = [1,2,3]
% diagonal are from v. other places are '0'
diag(v)
% return a vector contain elements from diagnoal of x
diag(x)

% n elements in [a, b]. avg. n, is optional with default value 100.
linspace(a, b, n)
% n elements in [10^a, 10^b]. n is optional with default value of 50
logspace(a, b, n)

% left-right exchange
fliplr(x)
% up and down exchange
flipud(x)
% returns a copy of matrix A that has been rotated by (90n)° counterclockwise
rot90(x, n)
% sort and in increasing order
sort(x)
% rearrange x to m*n matrix. Selection start from x11 to xm1, then x21 to xm2....
reshape(x, 2, 6)