Matlab02: structured programming and function definition "suggestions collection"

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MATLAB02: Structured programming and function definition

• Structured programming
  • Process control statements and logical operators
    • Example of process control statement
    • Try to pre allocate memory space when using circular statements
  • Problems that should be paid attention to when writing scripts
    • Add a statement at the beginning of the script to empty the workspace
    • Semicolons should be added after operations and assignment statements `;` Suppress output
    • Use ellipsis `…` Splice multiple lines
• function
  • See built-in functions
  • With ` Function name .m` File form definition function
    • MATLAB Built in function parameters
    • MATLAB Function definition example 1
    • MATLAB Function definition example 2
  • Define a function in the form of a function handle

The best way to learn a technology is to read official documents , You can see MATLAB Official documents

Structured programming

Process control statements and logical operators

Same as most programming languages ,MATLAB There are the following flow control statements :

Flow control statement	effect
if, elseif, else	if if The statement is true , Execute clause
switch, case, otherwise	according to switch The content of the statement determines which clause to execute
while	Repeat the clause until while The condition in is false
for	Execute the clause a fixed number of times
try, catch	Execute clauses and catch exceptions during execution
break	Out of the loop
continue	Go directly to the next cycle
end	End clause
pause	Suspend program
return	Return to the calling function

All the above loops and conditional statements should end with end closed .

MATLAB There are also the following logical operators :

Operator	significance
<	Less than
<=	Less than or equal to
>	Greater than
>=	Greater than or equal to
==	be equal to
~=	It's not equal to
&&	And
||	or

&& and || The operator supports the function of logical short circuit .

Example of process control statement

The following is a demonstration of each process control statement :

Try to pre allocate memory space when using circular statements

If the memory space required by a variable is a predictable fixed value , We should try to allocate memory space for it in advance .

Take the following two procedures as examples , Demonstrate this :

You can see , Program one takes longer than program two . This is because : For program one , There is no pre variable A Allocate memory , So whenever A When the shape of the changes , All need to be re A Allocate memory address , It takes more time .

Problems that should be paid attention to when writing scripts

Add a statement at the beginning of the script to empty the workspace

At the beginning of each script , The following statement should be added , Clear the workspace cache and traces of previous program runs :

clear all	%  Clear variables in the workspace memory 
close all 	%  Close the image drawn by the program before 
clc			%  Clear the output of the program on the terminal before 

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Semicolons should be added after operations and assignment statements ; Suppress output

Semicolons should be added to all operations and assignment statements ; Suppress output , If you need to output a variable to the terminal , It should be called disp Method .

Use ellipsis ... Splice multiple lines

stay MATLAB in , Ellipsis ... You can splice multiple lines of statements into one line , Using this statement flexibly can improve the readability of the code .

annPoints_sampled = annPoints(annPoints(:,1)>x1 & ...
    annPoints(:,1) < x2 & ...
    annPoints(:,2) > y1 & ...
    annPoints(:,2) < y2);

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function

Similar to other languages ,MATLAB You can also define functions . Similar to scripts , Functions can be stored Function name .m In file , It can also be defined in memory in the form of a function handle .

See built-in functions

We can use which Command to view the location of the built-in function source code file , And edit Command can be combined to view the source code of built-in functions .

Run the following statement to open MATLAB Built in mean Function source file :

edit(which('mean.m'))

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You can see in the editor mean The source code of the function is as follows :

With Function name .m File form definition function

stay MATLAB The format of the function defined in the file is as follows :

function [ Output variable name ] =  Function name ( Enter variable name ) 
%  Documentation for functions 

 function code 

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• function It's a keyword , Declare that what is saved in this file is a function .
• The input variable and Output variables nonessential , Functions can have no input variables , You can also have no output variables .
• Function name Should and .m Same file name , And does not contain special characters ( It's better not to have Chinese ).

MATLAB Built in function parameters

stay MATLAB in , Some built-in function parameters are as follows :

Function parameter	significance
imputname	Enter a list of variable names
mfilename	Function source code file name
nargin	Enter the number of variables
nargout	Number of output variables
varargin	Variable length input parameter list
varargout	Variable length output parameter list

MATLAB It does not provide the syntax of specifying default parameter values and function overloading in other high-level languages , But use the built-in function parameters mentioned above flexibly , You can specify the default parameter value and method overloading to a certain extent :

function [volume]=pillar(Do,Di,height)
if nargin==2,
	height=1;
end
volume=abs(Do.^2-Di.^2).*height*pi/4;

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MATLAB Function definition example 1

The following program is used to calculate the displacement of a free falling body : x = x 0 + v 0 t + 1 2 g t 2 x = x_0 + v_0 t + \frac{1}{2} g t^2 x=x0​+v0​t+21​gt2

function x = freebody(x0,v0,t)
% calculation of free falling
% x0: initial displacement in m
% v0: initial velocity in m/sec
% t: the elapsed time in sec
% x: the depth of falling in m
x = x0 + v0.*t + 1/2*9.8*t.*t;

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This function demonstrates a MATLAB Programming skills : Try to use .* Instead of *, Because the former is not only for parameters t Scalar case is available , Also for variables t In the case of vectors or matrices .

freebody(0, 0, 2)			%  obtain  19.6000
freebody(0, 0, [0 1 2 3])	%  obtain  [0 4.9000 19.6000 44.1000]
freebody(0, 0, [0 1; 2 3])	%  obtain  [0 4.9000; 19.6000 44.1000]

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MATLAB Function definition example 2

The following function realizes the conversion from Fahrenheit temperature to Celsius temperature , This function can identify the number of input samples to be converted , When the number of samples to be converted is 0 when , Exit function .

function F2C()
while 1
    F_degree = input('tempreature in Fahrenheit: ', 's');
    F_degree = str2num(F_degree);
    if isempty(F_degree)
        return
    end
    C_degree = (F_degree-32)*5/9;
    disp(['tempreature in Celsius: ' num2str(C_degree)])
end

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( Need to press Ctrl+C Exit procedure )

Define a function in the form of a function handle

We can also define functions in the form of function handles , This is closer to the mathematical definition of function , The syntax is as follows :

 Function handle  = @( The input variable )  Output variables  

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You can call this method directly through the function handle .

f = @(x) exp(-2*x);
x = 0:0.1:2;
plot(x, f(x));

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