吴恩达 ML 公开课笔记(4)-Octave abc

Too lazy to explain some of the commands…orz

Basic

• PS1('sign') Change prompt to ‘sign’
• Matrix Assignment
  • v = [1 2; 3 4; 5 6]
  • v = [1.1 1.2 1.3] = [1:<0.1:>1.3] (default step is 1)
• Matrix Generation commands
  • ones(2,3) = [1 1 1; 1 1 1] (ones/zeros/rand/randn)
  • eye(n): generate n by n identical matrix
  • magic(n): generate n by n magic matrix
• size(M, n): return the size of the n=1:row n=2:coloum
• length(M): max dimention of M

Moving data around

• pwd: show current path
• load('xxx') load <filename>
• who: display current scope. whos: details
• clear <variable>: clear the space of the variable
• M(start:end): count from start to end coloum by coloum
• save <filename> <variable> <encode>: save variable to the file using enconde like ascii

Manipulate

• M(x,y): indexing
• M(r1:r2,c1:c2) sub-matrix block: coloumn1-2/row1-2.
• M(rn, :) = [a b c]: assign [a b c] to row n of M
• M([r1 r2], :): copy row1 and row2
• M(M, [a b c]): append [a b c] to M
• M(:): put all elements of M into a single vector
• M = [X Y] = [X,Y]: concatenating X & Y into M(horizontally)
• M = [X;Y]: concatenating X & Y into M (vertically)

Computing on data

• M': transpose
• pinv(M): inverse of M
• Element-wise calculation
  • A*b, A .* B: element-wise multiply, as well as .^, ./, log(v), exp(v), abs(v) and so on
  • M < a: element-wise comparison, which returns a boolean matrix
• max()
  • [value, index] = max(M)
  • max(M,N): M and N are of the same size, do element-wise comparison, then generate a new matrix
  • max(M) = max(M,[],1): 1->coloum-wise; 2->per row-wise.
• [r, c] = find(M < a): returns the index of the elements
• sum()
  • sum(M): summarize all the elements
  • sum(M,1): summarize each coloum
  • sum(M,2): summarize each row
• prod(M) = prod(M,1): multiply each coloum. 1->coloum-wise; 2->per row-wise.
• floor(M): rounds down
• ceil(M): rounds up
• flipud(M): flip upside-down

Plotting data

• hist(v, n): plot v with n buckets histogram
• plot(x, y, <color>)
• plot(x,y1) hold; plot(x,y2)
• xlabel('X') ylabel('Y')
• legend('y1','y2') title('title')
• print -dpng 'filename.png'
• close: close figure window
• figure <number>; plot(x,y1);figure <number>; plot(x,y2); …
• subplot(x,y,index): access the index
• axis([xmin xmax ymin ymax])
• clf: clear all figure
• imagesc(M), colorbar, colormap gray;

Control statements

• for i=1:10, v(i) = 2^i; end;
• break continue
• while i<=5, v(i)=100; i=i+1; end;
• if i==6, <stat>; elseif i==7, <stat2>; else <stat3>; end;

Functions

• <functionname.m>: format of the function file
• Example:

1
2
3

function [y1, y2] = squareAndCubeThisNumber(x)
y1 = x^2;
y2 = x^3;

• addpath(‘path’)