EGR 111 Plotting Data

Transcription

1 EGR 111 Plotting Data This lab shows how to import data, plot data, and write script files. This lab also describes the Current Folder, the comment symbol ( % ), and MATLAB file names. New MATLAB Commands: disp, clear, plot, title, label, ylabel, title, legend, figure, subplot, lsread Resources (available on course website): Solar_Panel_Data.ls 1. Download the Files Check to see if there is a folder in your P: drive called MATLAB. If not, create folder called MATLAB in your P: drive. We will use this folder to store all the files for this course. Download the file EGR111_Files.zip by going to the course website, right-clicking on EGR111_Files.zip, and clicking on Save Link As or Save Target As Then save the file in your P:\MATLAB folder. Etract the files by going to your P:\MATLAB folder, doubleclicking on EGR111_Files.zip, click on Etract all Files, change the folder to P:\MATLAB, and click Etract. This process should place Solar_Panel_Data.ls and several other files that we will use in later labs into your P:\MATLAB folder. Check to make sure that these files are in your P:\MATLAB folder before proceeding with the lab. EGR111 - p. 1 of 9 - Plotting_Data_rev2.doc

2 2. MATLAB s Current Folder Start MATLAB. Near the top of the MATLAB window, you will see the name of the Current Folder (see Figure 1 below). For this course, you should always begin your MATLAB session by changing this folder to P:\MATLAB by clicking on the Browse for Folder button. The Current Folder is the default folder that MATLAB uses when saving files and looking for files. If you don t set it to P:\MATLAB, then MATLAB won t be able to find the files in this folder. You can also check your Current Folder by typing the following MATLAB command in the command window: pwd Figure 1: Current Folder EGR111 - p. 2 of 9 - Plotting_Data_rev2.doc

3 3. MATLAB Script Files Typing commands into MATLAB's command window is convenient for performing short computations in MATLAB. However, in many cases you probably will want to save your commands so that you can re-use them without re-typing them. You can save a series of commands in a MATLAB script file. To open a new script file, click on File, New, Script. Type the following command into the editor window that opens up: disp('hello from the script file!') Click File, Save, select the folder P:\MATLAB, and give the file a name that adheres to the following rules: starts with a letter contains only letters, digits, and underscores (the _ symbol), but not spaces or operators like -, +, *, /,!, etc. does not contain spaces ends in ".m" Running the script file has the same effect as typing or pasting the commands from the file into the command window. There are two ways to run the script file. The first way is to click on the green button in the editor window that says "Run" or "Save and Run" when the mouse hovers over it: The second way to run the file is to save the file, and then in the command window, type the filename without the ".m". So if you have saved the file with the filename scriptfile.m, then to run the file you would type: scriptfile, or whatever name you have given to the file (but without the.m). If you get an error that says something like "??? Undefined function or variable 'scriptfile'", then check to make sure that the folder where you saved the file is the same as the Current Folder which is shown at the top of the MATLAB window (see step 1 above). You can always change the Current Folder by using the button near the top of the window. EGR111 - p. 3 of 9 - Plotting_Data_rev2.doc

4 y 4. Plotting MATLAB makes it easy to plot data. For eample, suppose we would like to plot the values 2, 4, 8, 5, and 1. Open a new script file and type the following commands. 8 clear data = [2, 4, 8, 5, 1] plot(data) When you run the script file, a plot should appear in a new figure window. It is good practice to put the clear command as the first command in a script file to clear the variables in the workspace. This practice prevents your script file from relying on variables that are currently in the workspace, but will not be there if you close MATLAB and restart it. The command plot(data) makes a plot of the values in vector. Note that it connects the values by straight lines. For the -values, it uses the values 1, 2, 3, 4, and 5 by default. Net let s plot the function yy = 2. Open a new script file, type the following commands into the file, and run the file. clear = -4:4 y =.* plot(,y) ylabel('y') title('y versus ') y versus The command = -4:4 computes the values of where we want to plot a function using the colon operator. Recall that -4:4 epands to [ ]. EGR111 - p. 4 of 9 - Plotting_Data_rev2.doc

5 y The command y =.* computes the square of each of those values. Why the period before the asterisk (.*) on the second line? Well, MATLAB, which is short for MATri LABoratory, is set up to perform matri operations by default, which is not what we want in this case. Here we want to multiply the first element of by the first element of, then multiply the second element of by the second element of, and so on. We call this operation element-by-element multiplication or array multiplication. In order to tell MATLAB that we want element-byelement multiplication instead of matric multiplication, we put the period before the asterisk (.*). We also use the dot in front of other operations such as / and ^ to indicate that we want elementby-element operations:./ and.^. Note that if we do not include the dot, the command y = * would cause an error. The third line plots the data in an -y graph. The first argument () is plotted on the horizontal scale, and the second argument (y) is plotted on the vertical scale. By default the data points are connected by straight lines. Also notice that the label, ylabel and title commands cause strings to be placed on the -ais, y-ais, and title. Always make sure to label the aes on your graphs! If you wanted to use this graph in a report, you could click on Edit, Copy Figure, and then paste the graph into a word processor or other program. MATLAB has lots of useful options for graphs. For eample, if you don t want the data points to be connected by straight lines, you can have MATLAB mark the data points with plus signs (or other symbols) instead. Modify your script file as follows and run it: clear = -4:4 y =.* plot(,y,'+') % <-- change ylabel('y') title('y versus ') y versus Notice that the new plot overwrites the old one Suppose we want to compare two functions, such as cosine and sine by placing both lines on the same graph. Open a new script file, type the following commands into the file, save it, and run it. EGR111 - p. 5 of 9 - Plotting_Data_rev2.doc

6 clear figure = linspace(,2*pi); y = cos(); z = sin(); plot(,y,,z) ylabel('cos() and sin()') title('cos() and sin() versus ') legend('cos()','sin()') cos() and sin() cos() and sin() versus cos() sin() The figure command opens a new figure window. The plot(,y,,z) command specifies the and y values for two lines, so both are plotted on the same plot. You can add an arbitrary number of lines to a plot by specifying more pairs of data. The linspace command was introduced in the previous lab (page 6). The command linspace(,2,5) creates a vector of five equally-space numbers:,.5,1.,1.5,2.. What does linspace(,2*pi) create? Use help linspace to learn the answer. The legend command in used with plots with more than one line so we can tell which line is which. We give the legend command a string that describes each of the lines. Also, if you do not like where MATLAB has placed the legend in the figure window, you can click on the legend and drag it to a new location. We can also break a figure window into multiple independent plots using the subplot command. Open a new script file, and enter the following commands into the file, save it, and run it. clear figure = linspace(,2*pi); y = cos(); z = sin(); subplot(2,1,1) plot(,y) ylabel('cos()') title('cos() versus ') subplot(2,1,2) plot(,z) ylabel('sin()') title('sin() versus ') cos() sin() cos() versus sin() versus EGR111 - p. 6 of 9 - Plotting_Data_rev2.doc

7 The first argument of the subplot command is the number of rows of subplots, the second argument is the number of columns, and then the third argument a number that indicates which of the subplot windows to make active. So, in this case, for the first plot we use subplot(2,1,1) to get 2 rows, 1 column, and the first subplot window. The second time we use subplot(2,1,2) to get 2 rows, 1 column, and the second subplot window. We can also specify the line color and line type (solid, dashed, etc.). To see all of the options, type: help plot 5. Importing Data from Ecel Files Open the file "Solar_Panel_Data.ls" using Microsoft Ecel, which should be in your P:\MATLAB folder. If it isn t there, see the instructions in Section 1 of this lab. This file contains data taken from the solar panels on top of Shiley Hall from each of seven days. The goal is to import this data and plot it in MATLAB. Copy or type the following command into MATLAB's command window. [data, tt, raw] = lsread('solar_panel_data.ls'); If you get an error message that says something like Error using lsread XLSREAD unable to open file 'Solar_Panel_Data.ls'., check to make sure that you have saved the file Solar_Panel_Data.ls in P:\MATLAB and that you have changed the Current Folder to P:\MATLAB. Also note that MATLAB uses single-quotes (') around strings of characters, not doublequotes ( ). The lsread command places the numerical values from the Ecel document into the first output variable (data), and the header values in the second output variable (tt). The third output variable (raw) will contain unprocessed numbers and tet. Net, print out the data by typing the following into MATLAB's command window: data >> data data = Notice that the eight columns of numerical data from the Ecel file are now stored in the MATLAB variable called data. The first column is the number of hours since midnight, so.25 hours after midnight is 12:15AM, and hours is 1:45pm. The second column is the average EGR111 - p. 7 of 9 - Plotting_Data_rev2.doc

8 power in kilowatts (kw) produced by the solar panels on October 27, 213. The third column is the average power from October 28, 213, and so on. In order to make it easy to plot the data from the first day (October 27, 213), let's separate the hours from the power as follows: hours = data(:,1); % copy first column to hours power = data(:,2); % copy second column to power Then we can plot the data. plot(hours,power) label('hours since Midnight') ylabel('average Power (kw)') Note the use of the percent symbol % above. You will use this important symbol to include informative comments (for your own benefit or for the benefit of other readers). These comment are ignored by the MATLAB interpreter. That is MATLAB ignores everything on a line after the % symbol. Finally, we could use our own alternative names for data, tt, and raws, such as the following: [Weasely,Potter,Granger]=lsread('Solar_Panel_Data.ls'); Data Cursor In order to determine the maimum power and the time that it occurred, click on the Data Cursor button in the figure window, and then click on the graph at the maimum (see figure above). EGR111 - p. 8 of 9 - Plotting_Data_rev2.doc

9 Eercise 1: Write a script file to clear the workspace, load the file "Solar_Panel_Data.ls" into MATLAB, and graph the power versus hour curves for both October 28 and November 2 on the same plot. The result should look something like the graph below. Make sure you label your aes and include a legend. Why do you think the curves look so different? Checkpoint1: Show the instructor your plot from Eercise 1. Note: In addition to using the lsread command, Ecel files can also be imported into MATLAB by clicking on File, Import Data. The lsread command was presented in this lab because it allows us to import data from within a script file or function. EGR111 - p. 9 of 9 - Plotting_Data_rev2.doc