persistent (表面意思是 持续的，连续的)

Define persistent variable  %%声明静态变量

Syntax            %% 语法

persistent X Y Z        %%定义局部变量 X  Y   Z

Description  描述

其实就是定义局部静态变量的语法，变量生命期为全局，但是可见性在其被声明的函数内部，

persistent X Y Z defines X, Y, and Z as variables that are local to the function in which they are declared; yet their values are retained in memory between calls to the function.

定义局部静态变量X Y Z，在函数范围内声明X Y Z，它们的值局限在这个函数内。

Persistent variables are similar to global variables because MATLAB creates permanent storage for both. They differ from global variables in that persistent variables are known only to the function in which they are declared. This prevents persistent variables from being changed by other functions or from the MATLAB command line.

Persistent variables(局部变量)与global variables(全局变量) 很类似，因为matlab为它们都有固定的内存。不同的是Persistent variables仅仅在声明的函数范围内识别。与其他函数的变量或MATLAB 命令行所区别。

Persistent variables are cleared when the M-file is cleared from memory or when the M-file is changed. To keep an M-file in memory until MATLAB quits, use mlock.

当M文件被从内存中清除或当M文件被改变时，Persistent变量被清除。要保持M文件在内存中，请使用mlock

If the persistent variable does not exist the first time you issue the persistent statement, it is initialized to the empty matrix.

如果Persistent变量在声明时没有赋值，将被初始化为空矩阵。

It is an error to declare a variable persistent if a variable with the same name exists in the current workspace.

如果在当前工作空间中有同名的变量存在，则声明同名的Persistent变量将会报错。

Remarks   备注

There is no function form of the persistent command (i.e., you cannot use parentheses and quote the variable names).

Matlab不允许将Persistent变量声明为函数参数。

Example  举例

This function prompts a user to enter a directory name to use in locating one or more files. If the user has already entered this information, and it requires no modification, they do not need to enter it again. This is because the function stores it in a persistent variable (lastDir), and offers it as the default selection. Here is the function definition:

这个函数提示你输入一串字符定位文件。如果你已经输入了确切信息，就不必要再改变了。

这个函数定义了一个局部变量lastDir，并把默认选择赋值给它lastDir。下面是函数列：

function find_file(file)%%头文件

persistent lastDir  %定义persistent variable

if isempty(lastDir)

prompt = 'Enter directory:  ';      %初始状态为'Enter directory:

else

prompt = ['Enter directory[' lastDir ']:  '];  %默认路径名

end

response = input(prompt, 's');           %输入路径名

if ~isempty(response)

dirName = response;

else

dirName = lastDir;

end

dir(strcat(dirName, file))            %查找文件

lastDir = dirName;                 %赋值

Execute the function twice. The first time, it prompts you to enter the information and does not offer a default:

cd(matlabroot)

find_file('is*.m')

Enter directory:  toolbox/matlab/strfun/

iscellstr.m  ischar.m     isletter.m   isspace.m    isstr.m

isstrprop.m

The second time, it does offer a default taken from the persistent variable dirName:

find_file('is*.m')

Enter directory[toolbox/matlab/strfun/]:  toolbox/matlab/elmat/

isempty.m               isfinite.m              isscalar.m

isequal.m               isinf.m                 isvector.m

isequalwithequalnans.m  isnan.m