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persistent - Define persistent variable

Syntax

persistent X Y Z

Description

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. Persistent variables are similar to global variables because the MATLAB software 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.

Whenever you clear or modify a function that is in memory, MATLAB also clears all persistent variables declared by that function. To keep a function in memory until MATLAB quits, use mlock.

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

It is an error to declare a variable persistent if a variable with the same name exists in the current workspace. MATLAB also errors if you declare any of a function's input or output arguments as persistent within that same function. For example, the following persistent declaration is invalid:

function myfun(argA, argB, argC)
persistent argB

Remarks

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

Example

This function writes a large array to a spreadsheet file and then reads several rows from the same file. Because you only need to write the array to the spreadsheet one time, the program tests whether an array can be read from the file and, if so, does not waste time in repeating that task. By defining the dblArray variable as persistent, you can easily check whether the array has been read from the spreadsheet file.

Here is the arrayToXLS function:

function arrayToXLS(A, xlsfile, x1, x2)
persistent dblArray;

if isempty(dblArray) 
    disp 'Writing spreadsheet file ...'
    xlswrite(xlsfile, A);
end

disp 'Reading array from spreadsheet ...'
dblArray = xlsread(xlsfile, 'Sheet1', [x1 ':' x2])
fprintf('\n');

Run the function three times and observe the time elapsed for each run. The second and third run take approximately one tenth the time of the first run in which the function must create the spreadsheet:

largeArray = rand(4000, 200);

tic,  arrayToXLS(largeArray, 'myTest.xls','E254', 'J256'),  toc
Writing spreadsheet file ...
Reading array from spreadsheet ...
dblArray =
    0.0982    0.3783    0.1264    0.7880    0.1902    0.5811
    0.2251    0.2704    0.5682    0.7271    0.8028    0.2834
    0.6453    0.5568    0.8254    0.4961    0.9096    0.5402

Elapsed time is 8.990525 seconds.


tic,  arrayToXLS(largeArray, 'myTest.xls','E257', 'J258'),  toc
Reading array from spreadsheet ...
dblArray =
    0.4620    0.3781    0.6386    0.5930    0.0946    0.4865
    0.1605    0.1251    0.8709    0.5188    0.6702    0.2138

Elapsed time is 0.912534 seconds.


tic,  arrayToXLS(largeArray, 'myTest.xls','E259', 'J262'),  toc
Reading array from spreadsheet ...
dblArray =
    0.7015    0.6588    0.4023    0.0359    0.4512    0.6097
    0.1308    0.6441    0.0431    0.6396    0.7481    0.8688
    0.8278    0.2686    0.5475    0.8550    0.5896    0.1080
    0.9437    0.1671    0.0505    0.1203    0.2461    0.7306

Elapsed time is 0.928843 seconds.

Now clear the arrayToXLS function from memory and observe that running it takes much longer again:

clear functions

tic,  arrayToXLS(largeArray, 'myTest.xls','E263', 'J264'),  toc
Writing spreadsheet file ...
Reading array from spreadsheet ...
dblArray =
    0.6292    0.7788    0.0732    0.6481    0.9299    0.8631
    0.7700    0.5181    0.9805    0.5092    0.8658    0.4070

Elapsed time is 7.603461 seconds.

See Also

global, clear, mislocked, mlock, munlock, isempty

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