Lecture 19 matlab_script&function_files06

Editor's Notes

• #2: Instructor notes: This set of slides covers script files and function files. These are Matlab’s programming options.
• #3: Instructor notes: There are two types of M-files…script files and function files. We start out with script files. Script files are called M-files because the extension on the file is “.m”. Script files are a collection of Matlab commands that when executed are the same as if the commands had been entered from the keyboard. Variables that are used in script files are equivalent to global variables. A variable that originates in a script file is immediately known in the command window. Variables that originated in the command window are known in the script file. This is one are in which script and function files differ. Function files, like C/C++ functions use local variables which must be passed in and out of the function.
• #4: Instructor notes: This example demonstrates several key things about Matlab script files. We’ve got the script file d13.m. In order to invoke it, Matlab’s current working directory must be the same directory as where d13.m is stored, or the entire pathname to the script file must be entered at the command prompt. The load d13.dat command will open the file, d13.dat. Because the entire pathname to d13.dat is not specified, the current working directory must contain d13.dat. The load will open the file and read the entire file into Matlab creating an array called d13 without the “.dat” extension. Note the semicolon at the end of the line. This will prevent the contents of the array from being echoed to the command window. The second line creates a vector called voltage by reading in all of the rows in column 4 from the d13 matrix. Again, note the semicolon at the end of the line. The third line creates a time vector by multiplying 0.005 times a vector that has elements 1 to the length of the voltage vector. The fourth line plots the time on the x-axis and the voltage on the y-axis opening a new window that contains the plot. Lines 5 and 6 add labels to the x-axis and y-axis respectively. Line seven places a title on the plot and line eight puts a grid on the plot.
• #5: Instructor notes: As the slide indicates, the d13.m script file is executed by entering d13 at the command prompt when the current working directory is set to the same directory in which d13.m exists. If Matlab cannot find the file, it will display an error message. Note, that you can add to Matlab’s default search path so that files do not have to be located in the current directory.
• #6: Instructor notes: If the script file that you want to execute is not in the current directory, then use the cd or chdir commands to move to the correct directory and then execute the script file. Note that there are icons at the top of the command window that allow you to use the mouse to navigate to the desired directory. This may be especially useful when you have some idea of the name or roughly where the file is located rather than knowing the exact name or path. In this example the file is stored on the a: drive at the root level.
• #7: Instructor notes: Here we have another script file example…again this one has a lot packed into it. This script file collects mouse clicks and depending on which button was clicked, places a corresponding symbol on the plot window where the mouse was located at the time of the click. Line 1 issues three commands: clear removes all variables from memory, clf clears the figure window, and hold off turns off the plot holding. When the hold is on and multiple plot commands are issued, the plots will be placed on the same figure. In this script the hold is just being initialized to off. Line 2 sets the axis limits for the x and y axes of the plot window. The axis limits can be set at any time, even after the plot is drawn. It’s equivalent to changing the size of a window looking at the plot and gives you the ability to zoom in or out on different regions if you want. Line 3 turns the hold on, this way any plotting commands that are issued will cause the plotting to be on the same plot. Line 4 plots a box on the graph and line 5 places the text string “Click inside box to quit” inside the box that was plotted, placing the beginning of the text at (1, 1.6). Line 5 starts a while loop that terminates with the second “end” statement. Since the condition of the while loop is “1”, it will operate forever, thus the only way to exit the while loop and terminate the program is to satisfy the final “if” condition within the loop which causes the “break” to execute and the while loop to terminate. Inside the while loop, on line 6, the Matlab function ginput () is used. It returns the x and y coordinates on the plot where the mouse button was clicked as well as which mouse button was clicked. Line 7 is an “if” statement that checks to see if mouse button 1 was clicked. If so, a red “+” is plotted on the graph at the (x,y) location where the mouse click was read. The “end” statement on the line ends this “if” condition. Line 8 is an “if” statement that checks to see if mouse button 2 was clicked. If so, a yellow “o” is plotted on the graph at the (x,y) location where the mouse click was read. The “end” statement on the line ends this “if” condition. Line 9 is an “if” statement that checks to see if mouse button 3 was clicked. If so, a green “*” is plotted on the graph at the (x,y) location where the mouse click was read. The “end” statement on the line ends this “if” condition. Line 10 is the final “if” condition. It checks to see if the (x,y) coordinates of mouse click fall within 1<x<2 and 2<y<3, which are the limits of the text box. If so, then the “break” is executed and the while loop ends. Line 11 is the “end” statement for line 10. Line 12 is the “end” statement for the while loop. Line 13 turns off the plot hold, thus allowing multiple plot windows to be opened again.
• #8: Instructor notes: Now that we’ve seen a script file, we’re going to move to function files. Function files in Matlab are very much like functions in C/C++. The key difference between function files and script files is that variables in function files are local. Any variables that are used within a function file need to be specified within the function or passed in as arguments in the argument list. You may want to point out the similarities between C/C++ functions and Matlab function files. One difference between the two is that while a C/C++ function can only “return” one value, Matlab functions can return as many as desired. The function file should have the same name as the function itself with the “.m” extension added to it. Unlike C/C++ functions which can only be called from within a program, Matlab function files can be called directly from the command window, from a script file, or from another function file.
• #9: Instructor notes: This first bullet points out the reason that variables inside of function files are local and not known to the command window is that executing a function file causes control to switch out of the Matlab workspace into the function file’s workspace. As the second and third bullet points indicate, the function file is not part of the Matlab workspace and therefore those variables that exist within the function file and their corresponding values are not known to the command window when control is returned from the function file. In order to transfer the values of desired variables back to the Matlab workspace, they must be passed back according to the function call syntax. What this boils down to is that variables that are being passed into the function need to appear in parentheses to the right of the function name during the function call and variables that are being passed back from the function to the Matlab workspace must appear in square brackets to the left of the equals sign in the function call and function definition.
• #10: Instructor notes: Most of the information on this slide has been previewed on the previous slide. Arguments that are going into the function appear within parentheses to the right of the function name when the function is called. This is how you would get variables from the Matlab workspace or another function file into the called function. Values that are being determined within the function and returned by the function appear in square brackets prior to the equal sign on the line on which the function is called. These values will be returned by the function back to the Matlab workspace of the workspace of another function if this function were called within another function. Just as in C/C++, it’s the value of the variables that’s being passed. Addresses are NOT passed between workspaces.
• #11: Instructor notes: Using the same variable names in the return list of a Matlab function call as in the argument list has the effect of assigning new values to those variables.. This is a key idea to get across.
• #12: Instructor notes: Here we have an example of a swap function that has the same variables in its argument list as it does in its returned values list. In this example a and b are passed into the function. Within the function, temp is assigned a’s value, a is assigned b’s value and temp, which is a’s original value, is assigned to b. So, by the end of swap, the values of a and b have been swapped. Then, the swapped a and b are returned. To actually have the swap of a and b have an effect in the workspace that called swap requires that in the call to the function, the same variables were used in the calling “return” list and the calling “argument” list. A side note here…the comments that occur immediately after the first line of the function definition will be displayed in the command window if the user types help function_name at the command prompt. So, in this example, if the user were, at the command prompt, to type help swap, then the six comment lines that appear in the function definition above, would be displayed in the command window.
• #13: Instructor notes: Here we see the result of calling the swap function. Note that the variables in the function call do not have to be a and b, since a and b are local to the function definition. What is necessary is that the variables in square brackets and in parentheses have the same name. When called with initial values of x=5 and y=6, swap switches the values of x and y in the calling workspace. Hopefully some carryover from C/C++ explanation of functions will help out here, but this is somewhat of an abstract point to get a grasp of and quite possibly should be gone over slowly on the board or with an example that demonstrates visually what is taking place. It is slightly different than what happens in the C/C++ examples of swap.
• #14: Instructor notes: As quickly mentioned on an earlier slide, when entering help function_name at the cojmmand prompt, the comment information that appears immediately beneath the first line of the function will be displayed as help information in the command window.
• #15: Instructor notes: A heads up for the students and a quick reminder…the Matlab function that is being called must exist in the current working directory, or be in a directory that is specified as part of Matlab’s default search path, otherwise Matlab will not be able to find the function and will return an error message. Not finding the function and finding the function but with a “signature” that does not match that of the function file, will result in an error message. That error message may be somewhat obtuse and may be along the lines of that given in the slide. So, it’s a good idea for both student and instructors to be on the lookout for error messages that do not appear to have anything to do with the problem at hand.
• #16: Instructor notes: A reminder to make sure you change your working directory to the directory in which the function file exists prior to executing the function file. Either the cd or chdir commands can be used, as well as the icons at the top of the command window.
• #17: Instructor notes: As the slide indicates, Matlab is more flexible in dealing with variables changing their data types on the fly than C would be. All that is required to change the data type of a variable, is to assign the variable to a new value. After having gotten used to the strict data typing of C this might take students some practice to get used to.
• #18: Instructor notes: Here we see the factorial function. It takes in argument “k” and returns value “n”. The main point here is that if k is less than zero, the factorial cannot be computed and n is assigned a string error message as its value. Otherwise, if the factorial can be computed, n is assigned the value of the factorial. So, on the fly Matlab is able to handle assigning to the same variable, different types of values.