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matlab中遇到的一些问题_matlab解线性规划问题

matlab培训 cdadata 322℃ 0评论

matlab中遇到的一些问题

1什么是Gaussian拟合?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/9/23, BigGreen/MathTools #

        高斯拟合(Gaussian Fitting)即使用形如:
    
          Gi(x)=Ai*exp((x-Bi)^2/Ci^2)

        的高斯函数对数据点集进行函数逼近的拟合方法。

        其实可以跟多项式拟合类比起来,不同的是多项式拟合是用幂函数系,
        而高斯拟合是用高斯函数系。

        使用高斯函数来进行拟合,优点在于计算积分十分简单快捷。这一点
        在很多领域都有应用,特别是计算化学。著名的化学软件Gaussian98
        就是建立在高斯基函数拟合的数学基础上的。
2如何在给定句柄的axis里绘图?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/12, SMTH/MathTools #

        plot(data,’parent’,haxis);
        或者
        hbar=bar(data);
        set(hbar,’parent’,haxis);

3)由Matlab符号运算得到的公式怎么才能将数据代进去运算?
:#ramjet (德芙)2002/3/3, SMTH/MathTools #

        使用subs(),或先将值赋予一个符号变量,然后用eval_r()
5)在Matlab中如何求最值点?如何求一维数组的极值?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/18, SMTH/MathTools#

        最值:
        一维或多维数组最值用max(data(:))
        如果想返回最值所在的位置,用[Y,I]=max(data)

:#FangQ(Qianqian.Fang@Dartmouth.Edu), 2001/4/21,UESTC/Math#

        极值:
        data是你的数据,
        find(diff(sign(diff(data)))==-2)+1
        找到极大值的位置

        find(diff(sign(diff(data)))==2)+1
        找到极小值的位置

        data(find(diff(sign(diff(data)))==-2)+1)和
        data(find(diff(sign(diff(data)))==2)+1)
        返回的是极大值和极小值
6)Matlab中如何作线性拟合/线性回归/多元线性回归?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21, BigGreen/MathTools #

        即用y=a*x+b来拟合一组数据{{x1,y1},{x2,y2}…{xn,yn}}
        matlab中使用polyfit
        x=data(:,1);
        y=data(:,2);
        p=polyfit(x,y,1);
        p(1)为斜率a,p(2)为截距b

        多元线性回归即用y=a1*x1+a2*x2+..+am*xm来拟合数据点{x1i,x2i,…xmi,yi}

        (i=1~n)

          |x11,x21,…xm1|
        A=|x12,x22,…xm2|
          |……………    |
          |x1n,x2n,…xmn|

        Y={y1,y2,y3,…,yn}’

        则系数{a1,a2,…,am}’=pinv(A)*Y
        在matlab中使用
        coeff=A\Y
        则可以得到最小二乘意义上的拟合系数
7)Matlab中如何作圆回归?
:#Peter Boettcher (boettcher@ll.mit.edu),2002/5/16, comp.soft-sys.matlab#

        Q5.5: How can I fit a circle to a set of XY data?
        =================================================

           An elegant chunk of code to perform least-squares circle fitting
        was written by Bucher Izhak and has been floating around the
        newgroup for some time.   The first reference to it that I can
        find is in:

                 function [xc,yc,R,a] = circfit(x,y)
                 %CIRCFIT   Fits a circle in x,y plane
                 %
                 % [XC, YC, R, A] = CIRCFIT(X,Y)
                 % Result is center point (yc,xc) and radius R.A is an
                 % optional output describing the circle’s equation:
                 %
                 %    x^2+y^2+a(1)*x+a(2)*y+a(3)=0

                 % by Bucher izhak 25/oct/1991

                 n=length(x);   xx=x.*x; yy=y.*y; xy=x.*y;
                 A=[sum(x) sum(y) n;sum(xy) sum(yy)…
                    sum(y);sum(xx) sum(xy) sum(x)];
                 B=[-sum(xx+yy) ; -sum(xx.*y+yy.*y) ; -sum(xx.*x+xy.*y)];
                 a=A\B;
                 xc = -.5*a(1);
                 yc = -.5*a(2);
                 R   =   sqrt((a(1)^2+a(2)^2)/4-a(3));

           Tom Davis provided a more sophisticated approach that works
        for more cases in   and Code included.
8)Matlab中如何绘制箭头?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21, SMTH/MathTools #

        到http://www.mathworks.com/matlabcentral/fileexchange/index.jsp
        2-D Plotting and Graphics中查找arrow.m,或者
        http://www.mathworks.com/matlabcentral/spotlight/arrows.shtml
        http://www.math.umd.edu/~jec/matcomp/matcompmfiles/mfiles.html
9)Matlab中如何作二维数据的插值?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21, BigGreen/MathTools #

        对于一维、二维、三维规则数据点阵使用interp1/interp2/interp3,
        二维、三维非规则数据用griddata/griddata3
10)Matlab中如何绘制三维数据阵?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21, BigGreen/MathTools #

        如果使用matlab,打开帮助窗口,在目录树上找到
        MATLAB\Using Matlab\
           3-D Visualization: Volume Visualization Techniques
          
        如果图形复杂,建议使用Tecplot,参见Tecplot手册中数据格式,将你
        的三维数据读入Tecplot,双击zone,可以设置mesh/contour/surface
        transparency等。
       
        在Field菜单中有3D Iso-surface Details和3D Slice Details,可以绘制等值

        面和任意平面的截面图。
11)Matlab中如何注解一大段代码?
:#hyphone,2002/7/6, SMTH/MathTools #

        注释大段代码选中代码,Ctrl+R;取消注释,选中代码,Ctrl+T。
        或者用Edit菜单或者右键弹出中的注释。

:#misc,2002/6/21, SMTH/MathTools #

        if(0)
        大段的代码
        end
12)Matlab中如何计算程序运行的时间?
:#misc,2002/6/21, SMTH/MathTools #

        tic
          your_code;
        toc
        或者使用
        t=cputime;
          your_operation;
        cputime-t
13)Matlab中如何改变默认的工作路径?
:#SindyGong, 2002/4/7, SMTH/MathTools #

        编辑一个startup.m文件,其中cd yourpath
        或者在X:\matlab\toolbox\local\matlabrc.m的最后添加cd yourpath
        参见:
        http://www.mathworks.com/support/solutions/data/25164.shtml
14)Matlab如何改变默认的图形字体?
:#comp.soft-sys.matlab FAQ#

        编辑一个startup.m文件,其中
        set(0,’DefaultObjectnamePropertyName’,Value)
        或者在X:\matlab\toolbox\local\matlabrc.m的最后添加
        set(0,’DefaultObjectnamePropertyName’,Value)
15)如何在Matlab中实现交互操作?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21,BigGreen/MathTools #

        如果只在命令窗口进行交互操作,请参见demo中的例子,主要是
        通过input命令和pause/clear/disp等实现的,还有一些窗口资源可以使
        用:
        uigetfile,uiputfile,uiwait,uisetcolor,uisetfont, uiopen,uisave
        inputdlg,msgbox,helpdlg,questdlg,warndlg,errordlg
16)Matlab中为什么只能在小数点后显示四位?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21,BigGreen/MathTools #

        用format命令来改变命令窗口数字的显示格式和精度,但不会影
        响matlab的计算精度,matlab的矩阵运算默认都是双精度浮点型运算。
17)Matlab如何在命令窗口按照格式输出?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21,SMTHTools #

        fprintf(1,”your_format_string”,var1,var2,…);
18)如何在Matlab中画隐函数曲线?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21,BigGreen/MathTools #

        在http://www.mathworks.com/matlabcentral/fileexchange/index.jsp
        查找implicit,会找到一个Arthur Jutan写的implot.m
        Mathematica中绘制隐函数用ImplicitPlot[]
        或者ImplicitPlot3D[]
        Maple中为implicitplot(),implicitplot3d()
        参见
        http://engineering.dartmouth.edu/~fangq/MATH/download/source/
         ImplicitPlot3D.htm
19)Matlab中什么函数可以删除矩阵的某一行或列?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21,BigGreen/MathTools #

        A(j,:)=[];   %删除A的第j行
        A(:,i)=[];   %删除A的第i列
20)Matlab中能开的最大数组是由什么决定的?
:# chenft (mike),2002/6/1, SMTH/MathTools #

      I have had similar problems. Below is an explanation I received from  
      Ian Boyd
      from Mathworks (just giving credit where credit is due) that explains
      what’s happening. You solution is to run matlab with the -nojvm mode.
      “The heap memory system in JAVA consists of data and handle elements.
      When you allocate a variable you get a handle and data. As long as  
      data has an associated handle, the JVM considers it valid and
      will not clean it up.

      However, when you call the clear function in MATLAB, all handles are
      destroyed, and the data associated is now invalid. This means that  
      the JAVA engine can free up that data (garbage collection), but does
      not mean that it will clean it up at that moment.

      Calling the PACK command encourages JAVA to run the garbage collector
      and de-fragment the memory. But it does not force it to (This is part  

      of the JAVA design). Even though the memory is ‘freed’ on the heap,  
      it is not actually free to the OS, it is only free to the JVM. Here  
      is one way to think of it:

      [MATLAB]
      [JAVA]
        [OS]
      MATLAB runs on JAVA (virtual machine), and Java runs on the OS
      (physical machine). So when MATLAB is running in JAVA mode memory
      allocations   are requested from the JRE, not the OS.

      One problem you may be running into is that the default maximum
      JAVA heap size is relatively low ( <= 64 MB), so that is all the
      memory one session of MATLAB will ever get on your system.

      The good news is that you can increase this value. You will need
      to create a java.opts file in $MATLAB/bin/$ARCH (or in the current
      directory when   you start MATLAB) and put the following command:

      %%%BEGIN CODE%%%
      maxHeapSize = 268435456
      %%%END CODE%%%

      This will give you 256MB of JVM memory and you can adjust the
      parameter as needed.

      Note: $MATLAB is the root directory and $ARCH is your system
      architecture. This solution works on Windows as well as Solaris,
      Linux,Alpha, and SGI. A similar operation is possible on IBM and
      HPUX, but with a different syntax.

      For the 1.1.8 JVM (Windows, Linux, Solaris, Alpha, SGI) our
      defaults are:
     
         minHeapSize = 16000000
         maxHeapSize = 64000000

      These are the structure field names in   that correspond to
      -ms and -mx, and the settings above are roughly 16MB and 64MB.
      To investigate the Java heap a bit, ask via the following:
         >> java.lang.Runtime.getRuntime.totalMemory
         >> java.lang.Runtime.getRuntime.freeMemory

      When the free memory hits zero, Java will double the heap size
      (up to the maximum setting).

      If you choose to run without Java, you will remove the overhead
      of the middle man, but you will also lose some MATLAB functionality
      (mostly graphics and the Editor). You will still have most
      of the computational power though.

      Without JAVA, memory management will come directly from the OS,
      and a CLEAR operation will result in memory being freed back to
      the OS.

21)如何在Matlab中添加新的工具箱?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21,BigGreen/MathTools #

        如果是Matlab安装光盘上的工具箱,重新执行安装程序,选中即可。
        如果是单独下载的工具箱,一般情况下仅需要把新的工具箱解压到某
        个目录,然后用addpath(对于多个目录的使用genpath())或者pathtool添
        加工具箱的路径,然后用which newtoolbox_command.m来检验是否可
        以访问。如果能够显示新设置的路径,则表明该工具箱可以使用了。
        具体请看工具箱自己代的README文件。
22)如何读写Matlab的.mat文件?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21,BigGreen/MathTools #

        文件结构参见:
        http://www.mathworks.de/access/helpdesk/help/pdf_doc/matlab/
          matfile_format.pdf
        http://www.mathworks.com/support/solutions/data/8757.shtml
        ftp://ftp.mathworks.com/pub/tech-support/solutions/s8757/
          readmemat.txt

        建议使用matlab自己提供的函数来读写简单安全,或者参考:
        http://engineering.dartmouth.edu/~fangq/MATH/download/
           source/mat_file.txt
        来自matlab的c math library
23)如何得到contour线上的坐标点?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21,BigGreen/MathTools #

        lcount=5;
        [c,h]=contour(peaks,lcount);
        x=get(h,’xdata’);
        y=get(h,’ydata’);

        这里得到的x和y都是cell数组,用x{1}/y{1}来得到每条线上的坐标对,
        注意,每条线的最后一个数据是NaN
24)如何将Matlab绘制的三维网格图帖到word里?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21,BigGreen/MathTools #

        如果需要位图,好处是所见即所得,坏处是图像精度差,不能放缩:
                1.用拷屏 Alt+PrintScreen
                2.在图形窗口菜单Edit\Copy Options….\选择Bitmap,可以
                  选择透明背景,然后Edit\Copy Figure

        如果需要拷贝矢量图:
                在图形窗口菜单Edit\Copy Options….\选择Metafile,然后
                Edit\Copy Figure,在Word中粘贴

        经常地,按照Metafile方式粘贴的图片曲线会出现锯齿,最好的方式是
        使用eps文件:
                1.将需要拷贝的图作为当前窗口
                2.再转换到matlab命令窗口,print -deps filename.eps
                3.-deps还可以用depsc,deps2,depsc2
                4.在word中插入图片,选中该eps,如果是word 2000以前版本
                  ,不会显示图片内容,但可以打印,word XP即可显示,又可打印。

                5.如果不满意,可以在word中双击编辑,如果安装有Adobe
                  Illustrator等矢量图像编辑软件,也可以进行编辑。
25)请问可以查看Matlab中函数的源代码吗?
:#FangQ(Qianqian.Fang@Dartmouth.Edu),2002/6/21,BigGreen/MathTools #

        Matlab除了buildin函数和mex/dll文件看不到原码,其他如工具箱等都可
        以直接看到代码,首先确认该文件安装在matlab中,即which
        filename.m存在,然后可以edit filename.m
26).Matlab有没有求矩阵行数/列数/维数的函数?
:#FangQ(Qianqian.Fang@dartmouth.edu),2002/6/21,BigGreen/MathTools #

         ndims(A)返回A的维数
         size(A)返回A各个维的最大元素个数
         length(A)返回max(size(A))
         [m,n]=size(A)如果A是二维数组,返回行数和列数
         nnz(A)返回A中非0元素的个数
27).Matlab中如何中断运算?
:#FangQ(Qianqian.Fang@dartmouth.edu),2002/6/21,BigGreen/MathTools #

         在命令窗口按Ctrl+C,在UNIX/LINUX会立即中断运算,在Windows可
         能由于操作系统的原因,有时会出现死机和等待的情况。
28).Matlab中有没有画圆或椭圆的函数?
:#FangQ(Qianqian.Fang@dartmouth.edu),2002/6/21,BigGreen/MathTools #

         没有,Matlab没有提供直接绘圆的图元函数,需要自己写代码,其实
         就两句:
         sita=0:pi/20:2*pi;
         plot(r*cos(sita),r*sin(sita)); %半径为r的圆

         plot(a*cos(sita+fi),b *sin(sita+fi)); %椭圆

         如果是单位圆,可以使用rectangle(‘Curvature’, [1 1])
29).Matlab下如何定义整形
:#修改:fhorse (马不停蹄),2002/6/21,SMTH/MathTools #

         Matlab默认的矩阵数据结构都是双精度浮点型,即64位来表示一个数

         字,大多数的函数和操作都定义在double数据结构,如果你需要
         把double的数据转换为整形,然后再参与运算,需要使用
         double(int32(x))或者floor/round/ceil等函数

         如果为了节省内存,只进行赋值、打印等简单操作,可以参
         见uint8/uint16/uint32命令的帮助

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