Lecture note Data visualization - Chapter 27

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The main contents of the chapter consist of the following: Function M-files, syntax of function M-Files, comments, multiple input and output functions, relational model concepts, relational model constraints and relational database schemas, update operations and dealing with constraint violations.

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Nội dung Text: Lecture note Data visualization - Chapter 27

Lecture 27
Recap Function Mfiles Syntax of Function MFiles Comments Multiple Input and Output Functions
Functions with No Input or No Output Although most functions need at least one input and return at least one output value, in some situations no inputs or outputs are required For example: consider this function, which draws a star in polar coordinates: function [] = star( ) theta = pi/2:0.8*pi:4.8*pi; r = ones(1,6); polar(theta,r) The square brackets on the first line indicate that the
Continued…. There are numerous builtin MATLAB functions that do not require any input. For example: A = clock returns the current time: A = 1.0e+003 * Columns 1 through 4 2.0050 0.0030 0.0200 0.0150 Columns 5 through 6
Determining the Number of Input and Output Arguments There may be times when you want to know the number of input arguments or output values associated with a function MATLAB provides two builtin functions for this purpose The nargin function determines the number of input arguments in either a userdefined function or a builtin function The name of the function must be specified as a string, as, for example: in nargin('sin')
Continued…. When nargin is used inside a userdefined function, it determines how many input arguments were actually entered This allows a function to have a variable number of inputs Recall graphing functions such as surf When surf has a single matrix input, a graph is created, using the matrix index numbers as the x – and y coordinates. When there are three inputs, x , y , and z , the graph is based on the specified x and y –values The nargin function allows the programmer to determine
Continued…. The surf function is an example of a function with a variable number of inputs If we use nargin from the command window to determine the number of declared inputs, there isn’t one correct answer The nargin function returns a negative number to let us know that a variable number of inputs are possible: nargin('surf') ans = 1 The nargout function is similar to nargin , but it determines the number of outputs from a function:
Continued…. An example of a function with multiple outputs is max : nargout('max') ans =2 When used inside a userdefined function, nargout determines how many outputs have been requested by the user Consider this example, in which we have rewritten the function to create a star: function A = star1( ) theta = pi/2:0.8*pi:4.8*pi; r = ones(1,6);
Local Variables The variables used in function Mfi les are known as local variables The only way a function can communicate with the workspace is through input arguments and the output it returns Any variables defined within the function exist only for the function to use For example: consider the g function previously described: function output = g(x,y) % This function multiplies x and y together
Continued…. Just as calculations performed in the command window or from a script Mfi le cannot access variables defined in functions, functions cannot access the variables defined in the workspace This means that functions must be completely self contained: The only way they can get information from your program is through the input arguments, and the only way they can deliver information is through the function output Consider a function written to find the distance an object falls due to gravity: function result = distance(t)
Continued…. The value of g must be included inside the function It doesn’t matter whether g has or has not been used in the main program How g is defined is hidden to the distance function unless g is specified inside the function Of course, you could also pass the value of g to the function as an input argument: function result = distance(g,t) %This function calculates the distance a falling object 
Global Variables Unlike local variables, global variables are available to all parts of a computer program In general, it is a bad idea to define global variables However, MATLAB protects users from unintentionally using a global variable by requiring that it be identified both in the commandwindow environment and in the function that will use it Consider the distance function once again: function result = distance(t) %This function calculates the distance a falling object
Creating ToolBox of Functions When a function is called in MATLAB, the program first looks in the current folder to see if the function is defined If it can’t find the function listed there, it starts down a predefined search path, looking for a fi le with the function name To view the path the program takes as it looks for files, select File > Set Path from the menu bar or type pathtool
Continued…. As more and more functions are created to use in programming, it may be needed to modify the path to look in a directory where personal tools have been stored. For example: suppose you have stored the degreesto radians and radianstodegrees functions created in a directory called My_functions. You can add this directory to the path by selecting Add Folder from the list of option buttons in the Set Path dialog window. You’ll be prompted to either supply the folder location or browse to find it (shown in next slide)
Continued…. MATLAB now first looks into the current folder for function definitions and then works down the modified search path
Continued…. Once a folder is added to the path, the change applies only to the current MATLAB session, unless changes are saved permanently Clearly, permanent changes should never make to a public computer However, if someone else has made changes you wish to reverse, you can select the default button to return the search path to its original settings The path tool allows to change the MATLAB search path interactively; however, the addpath function allows to insert the logic to add a search path to any MATLAB
Anonymous Functions and Function Handles Normally, if there is trouble of creating a function, you will want to store it for use in other programming projects However, MATLAB includes a simpler kind of function, called an anonymous function Anonymous functions are defined in the command window or in a script Mfile and are available—much as are variable names—only until the workspace is cleared To create an anonymous function, consider the following example: ln = @(x) log(x)