20.11 M文件例子
	精通MATLAB工具箱含有许多实用函数，它们可以验证本章的许多概念。这些函数的基本部分已经在二维和三维图形这些章阐述过了。有了前面对句柄图形的讨论，我们现在可以更彻底地讨论这些函数。
	最简单的精通MATLAB工具箱的函数之一提出了一个共同的问题。MATLAB函数gcf返回当前图形的句柄。但是，它有一个副作用。如果图形不存在，gcf就创建一个，并返回它的句柄。如果想寻找一个图形是否存在于头一个位置，要是没有，又不得不创建，怎么办？函数mmgcf正好实现由其内容所描述的工作。
	function HF=mmgcf
	%MMGCF Get Current Figure if it Exists.
	%  MMGCF returns the handle of the current figure if it exists.
	%  If no current figure exists,MMGCF returns an empty handle.
	%
	%  Note that the function GCF is different.It creates a figure and returns its handle if it does not 	%  exist.
	%  Copyright  (c)  1996 by Prentice-Hall,Inc.
	Hf=get(0, ‘Children’ );  %  check for figure children
	if isempty(Hf)
		return
	else
		Hf=get(0, ‘CurrentFigure’ );
	end
	函数mmgcf首先检查根对象的子对象的图形是否存在，如至少有一个图形对象时，根对象的 ‘CurrentFigure’ 属性就返回当前的图形。
	函数mmgca为坐标轴对象执行同样的功能，如同在它的M文件内所描述的那样。
	function Ha=mmgca
	%MMGCA Get Current Axes if it exists.
	%  MMGCA returns the handle of the current axes if it exists.
	%  If no current axes exists,MMGCA returns an empty handle.
	%
	%  Note that the function GCA is diffent.It create a figure and an axes and returns the axes 	%  handle if they do not exist.
	%  Copyright  (c)  1996 by Prentice-Hall,Inc.
	Ha=findobj(0, ‘Type’ , ‘axes’ );
	if isempty(Ha)
		return
	else
		Ha=get(get(0, ‘CurrentFigfure’ ), ‘CurrentAxes’ );
	end
	由于函数gco已经表现出当对象不存在时返回空矩阵的行为特性，就不需要函数mmgco了。
	在精通MATLAB工具箱中的另一个函数是mmzap，在二维图形那一章里已作过介绍。如下M文件中所示，它使用mmgcf作错误检查，与findobj和get一起删除一个指定的图形。
	function mmzap(arg)
	%MMZAP Delete graphics object using mouse.
	%  MMZAP waits for a mouse click on an object in a figure window and deletes the object.
	%  MMZAP or MMZAP text erases text objects.
	%  MMZAP axes erases axes objects.
	%  MMZAP line erases line objects.
	%  MMZAP surf erases surface objects.
	%  MMZAP patch erases patch objects.
	%
	%  Clicking on an object other than the selected type or striking a key on the keyboard aborts 	%  the command.
	%  Copyright  (c)  1996 by Prentice-Hall,Inc.
	if nargin 1,arg= ‘text’ ;end
	Hf=mmgcf;
	if isempty(Hf),error( ‘No Figure Available.’ ),end
	if length(findobj(0, ‘Type’ , ‘figure’ ))==1
		figure(Hf)  %  bring only figure forword
	end
	key=waitforbuttonpress;  %  pause untill user takes some action
	if key  %  key on keyboard pressed
		return  %  take no action
	else  %  object selected
		object=gco  %  get object selected by buttonpress
		type=get(object, ‘Type’ );
		if all(type(1:4)==arg(1:4))  %  delete only if  ‘Type’  is correct
			delete(object)
		end
	end
	在编写句柄图形函数的M文件时，函数mmzap描述了一种很有用的技术。它利用函数waitforbuttonpress和gco的结合用鼠标来获取所选定对象的句柄。waifortbuttonpress是一个MATLAB内置函数，它的功能是等待鼠标点击或按键。它的帮助文本如下：
	? help waitforbuttonpress
	WAITFOR BUTTONPRESS Wait for key/buttonpress over figure.
		T= WAITFOR BUTTONPRESS stops program execution untill a key or mouse button is 		pressed over a figure window.Returns 0 when terminated by a mouse buttonpress,or 1 		when terminated by a keypress.Additional information about the terminating event is 			available f
MATLAB中M文件例子.doc