机器学习- Matlab 编程常用命令速览 --总结自Ng-ML-class Octave/Matlab Tutorial Coursera from http://blog.csdn.net/yangliuy/article/details/8979793 A、Basic operations and Moving data around 1 在命令行模式用shift + 回车即可附加下一行输出 2 length命令apply到矩阵时返回较高的一维的dimension 3 help + 命令是显示命令的简要帮助信息 doc + 命令 是显示命令的详细帮助文档 4 who 命令 显示 当前所有创建的变量 whos 命令 显示当前所有创建变量的详细信息 5 保存变量到.mat 文件save hello.mat b 以二进制压缩保存数据 save hello.txt v -ascii 以可读形式文件保存 即文本格式 6 :means every elements in this col 7 A([1 3], :) 获取第 1、3两行所有列的数据 8 A = [A, [100; 101; 102]] 在A矩阵后面加一列col vector [100,101,102] 9 size(A) 返回一个1行2列矩阵 表明第1和第2个dimensional 的大小 10 C = [A B]等价于C = [A, B] []为向后面的列添加,连接两个矩阵 [] 为concat 连接矩阵或者字符串 11 C= [A; B] ;号表示向下面行添加,因此会增加相应行数,列数不变 B、Computing on data 12 A.*B是矩阵/向量点乘 A*B是矩阵相乘 13 log(v) 和exp(v)求以e为底的对数和指数 14 abs()求绝对值 15 A‘ 求A的转置矩阵 16 max函数返回矩阵中最大元素的值和索引 [val, ind] = max(a) 17 A < 3 会判断A当中的每一个是否小于3,若小于3,对应位置返回true,否则对于位置返回false 18 find(A<3) 返回矩阵中所有值小于3的索引 19 magic Magic square. magic(N) is an N-by-N matrix constructed from the integers1 through N^2 with equal row, column, and diagonal sums. Produces valid magic squares for all N > 0 except N = 2. 20 [r, c] = find(A >= 7) 返回值大于等于7的element的row及col的索引 21 prod(a) 求矩阵a里面所有元素的乘积 floor(a) 对矩阵a中元素向下取整 ceil(a)对矩阵a中元素向上取整 rand(3) 生成3X3的随机方阵 max(A,[],1) 求矩阵A的每一列的最大值(最后一维是1表明为dimension 1) max(A,[],2) 求矩阵A的每一行的最大值 sum(A, 1) 对矩阵A第一维度(即每列)求和(注意matlab中第一维默认是列,然后是行,再然后依次类推。。。) sum(A, 2) 对矩阵A第二维度(即每行)求和 sum(sum(A.*eye(9))) 求矩阵A的对角线元素之和 22 矩阵翻转操作 flipud Flip matrix in up/down direction. 将矩阵上下翻转, 类似还有左右翻转 fliplr, rot90, flipdim. flipud(X) returns X with columns preserved and rows flipped in the up/down direction. For example, X = 1 4 becomes 3 6 2 5 2 5 3 6 1 4 23 pinv(A) 及inv(A) 求矩阵A 的逆矩阵 C、Plotting data 24 t = [0.1 : 0.01 : 0.98] y = sin(t) plot(t, y) 画正弦曲线 25 hold on; 保留当前曲线,画下一条曲线 26 xlabel 标定x轴说明 27 legend('sin','cos') 添加图例 28 title('my plot') 添加图片标题 29 print -dpng 'myPlot.png' 保存图片 30 线条颜色标注控制 b blue . point - solid g green o circle : dotted r red x x-mark -. dashdot c cyan + plus -- dashed m magenta * star (none) no line y yellow s square k black d diamond w white v triangle (down) ^ triangle (up) < triangle (left) > triangle (right) p pentagram h hexagram 31 subplot 画子图,即将多个图合成一个图 >> subplot(1,2,1) % Divides plot a 1X2 grid, access the 1st element >> plot(t,y1) >> subplot(1,2,2) >> plot(t,y2) 32 grid 加上网格 33 figure(1) >> plot(t, y1)>> figure(2) >> plot(t,y2) 将y1与y2的曲线画到文件名为figure1和figure2的两个文件中 34 axis([0.5 1 -1 1]) 设定x轴范围为0.5~1,y轴范围为-1~1 35 clf Clear current figure. 36 imagesc(A) imagesc Scale data and display as image. 把矩阵A画成彩色小方格 37 imagesc(A), colorbar, colormap gray; colorbar 显示颜色渐变条,表明颜色含义 colormap 设置colormap性质 即RGB三色 A color map matrix may have any number of rows, but it must have exactly 3 columns. Each row is interpreted as a color, with the first element specifying the intensity of red light, the second green, and the third blue. Color intensity can be specified on the interval 0.0 to 1.0. 38 a = 1; b = 2; c=3; 不会打出a b c的值 a = 1, b = 2, c=3 会打出a b c 的值 D、Control statements: for, while, if statements 39 for 循环 for i = 1:10, v(i) = 2 ^i; end; >> v v = 2 4 8 16 32 64 128 256 512 1024 >> indices = 1:10 for i = indices, disp(i) end; 40 while 循环 >> while i <= 5, v(i) = 100; i = i+1; end; >> v v = 100 100 100 100 100 64 128 256 512 1024 >> i = 1; >> while true, v(i) = 999; i = i + 1; if i == 6, break; end; end; >> exit v = 999 999 999 999 999 64 128 256 512 1024 41 if elseif 判断分支语句 v(1)=2 v = 2 999 999 999 999 64 128 256 512 1024 >> if v(1) == 1, disp('The value is one'); elseif v(1) == 2, disp('The value is two'); else disp('The value is not one or two.'); end; The value is two 42 函数的定义和使用 定义函数 squareThisNumber.m文件 function y = squareThisNumber(x) y = x^2; 调用 squareThisNumber(5) ans = 25 43 addpath 增加matlab搜索函数的路径 44 matlab里面定义的函数可以返回多于一个值, 这是其与C C++等编程语言 的不同之处,C\C++里面的函数有唯一返回值 可以允许多个返回值可以带来编程上的方便 function [a,b] = squareAndCubeThisNumber(x) a = x^2; b = x^3; 调用 >> [x1,x2] = squareAndCubeThisNumber(5) x1 = 25 x2 = 125 45 cost function J 函数示例 function J = costFunctionJ(X, y, theta) % X is the "design matrix" containing our training examples. % Y is the class labels m = size(X,1); % number of training examples predictions = X*theta; % predictions of hypothesis on all m examples sqrErrors = (predictions - y).^2; % squared errors J = 1/(2*m) * sum(sqrErrors); 调用 X = [1 1; 1 2; 1 3]; y = [1; 2; 3]; theta = [0;1];>> j = costFunctionJ(X, y, theta) j = 0 % squared errors is 0 in this example >> theta = [0;0] theta = 0 0 >> j = costFunctionJ(X, y, theta) j = 2.3333 % squared errors is 2.3333 in this example % which is (1^2 + 2^2 + 3^2) / (2 * 3) = 2.3333 E、Vectorization Matlab中“向量化”的实现算法,可以减少不必要的循环,对向量所有值的计算 操作一致,这是更快速更高效的算法实现思路,要注意体会matlab中“向量化” “矩阵化”操作变量实现算法与其他编程语言Java , C, C++的不同 |
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