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Advanced use of functionsPowerPoint Presentation

Advanced use of functions

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0. Advanced use of functions. Anonymous functions function handles subfunctions and nested functions. Function handle. 0. Useful as a parameter to other functions Can be considered as an alternate name for a function – but with more capabilities Example: sine_handle = @sin

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Function handle

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- Useful as a parameter to other functions
- Can be considered as an alternate name for a function – but with more capabilities
- Example:
- sine_handle = @sin
- sine_handle(x)
- has same values as sin(x) for all x

Three ways of plotting sin(x)

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- x = [0 : 0.01 : 2*pi] ;
- y = sin( x );
- plot(x,y)
- plot( x, sin(x) )
- plot( [0 : 0.01 : 2*pi] , sin( [0 : 0.01 : 2*pi] )) ;
- Last method has the advantage that no permanent storage is needed for x and/or y

Function handle continued

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- In last example everything is on one line
- but it requires writing the interval twice
- It would be more convient to write
- gen_plot( function_handle, interval )

- The first parameter has to be a function handle and not just the name of a function
- gen_plot( sin, [0 : 0.01 : 2*pi ] ) does not make sense to Matlab, but the following does
- gen_plot( sine_handle, [0 : 0.01 : 2*pi] )

Using a function handle

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- When plotting lots of functions it may be useful to have a function with the name gen_plot available
- function [] = gen_plot( func_handle, interval ) ;
- plot( interval, func_handle(interval) ) ;

- The example shows how to pass functions as parameters.
- gen_plot( sine_handle, [0 : 0.01 : 2*pi] )

Anonymous functions

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Assume the user needs to work temporarily with the function x3+3*x – 1

- Instead of writing the function
- function y = mypoly(x) ;
- y = x.^3+3*x-1

- and storing it as mypoly.m in subdirectory work we can use an anonymous function with the function handle mypoly
- mypoly = @(x) x.^3+3*x-1

Using anonymous functions

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- With a function handle an anonymous function can be used like any other
- gen_plot( mypoly, [-10 : 0.01 : 10] )

- or try to find a zero near 1.5
- fzero( mypoly, 1.5 )

- Without the function handle the anonymous function can also be inserted directly as a parameter
- gen_plot( @(x) x.^3+3*x-1, [-10 : 0.01 : 10] )

More examples Assume f1 had been defined as a function and kept in f1.m then Matlab used an alternate method in the past. In order to be backward compatible it is still available, but the use is not recommended: fzero( 'f1', 0 ) Use function handles instead!

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- f1 = @(x) x + 2* exp(-x) -3
- fzero( f1, 0 )
- fzero( f1, 1 )

- fzero( f1, 0 ) would be in error

- fzero( 'sin', 0 )
- fzero( 'x.^3', 0 ) need to use default variable name x

Commands of Matlab: clear, dir, which, cd, …

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- they can be used with a parameter, i.e.
- clear functions
- dir C:\MATLAB_SV701\toolbox\matlab
- which clear
- cd E:\work

- all are builtin functions
- the parameter is interpreted as a character string. A blank terminates the character string
- equivalent calls
- clear('functions')
- dir('C:\MATLAB_SV701\toolbox\matlab')
- which('clear')
- cd(' e:\work')

Remark:

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- Builtin functions can be called like a command
- median [1,2,100]

- instead of
- median([1,2,100])

- Matlab gives no warning in the first case and returns 1
- [1,2,100] is treated as a character string
- median('[1,2,100]') also returns 1

Subfunctions, example

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- function [avg,med] = mystat(u)
- n = length(u) ;
- avg = mymean( u,n ) ;
- med = mymedian( u,n ) ;
- end % function mystat
- function a = mymean( v,n )
- a = sum(v)/n;
- end % function mymean
- function m = mymedian( v,n ) ;
- w = sort(v) ;
- if rem(n,2) ==1
- m = w((n+1)/2)
- else
- m = (w(n/2)+ w(n/2+1))/2 ;
- end
- end % mymedian

Subfunctions

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- subfunctions are stored in the same file as the main function and can only be called in that file
- the scope of subfunctions is restricted to the file in which they are defined
- the example given is for illustration only
- the example uses modular design, but carries it to an extreme
- the overhead of calling a function outweighs any benefit in this case
- if a function mystat has to be written the following would be acceptable

Avoid unnecessary calculations

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- function [avg,med] = mystat2(u)
- n = length(u) ;
- avg = sum(u)/n ;
- if nargout == 2 % only compute if requested
- w = sort(u) ;
- if rem(n,2) ==1
- med = w((n+1)/2) ;
- else
- med = (w(n/2)+ w(n/2+1))/2 ;
- end
- end
- end % mystat2

Nested functions

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- main_function
- nested_function_1
- …
- end % nested_function_1
- nested_function_2
- …
- end % nested_function_2
- …
- …
- end % main_function

Nested function

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- When using nested functions all functions need a matching end statement!
- subfunction versus nested functions
- nested functions have access to all variables defined in the main function!
- avoids passing parameters or using global variables

- For a structured design use subfunctions. Avoid nested functions or use them sparingly

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