MATLAB Part 1. Introduction

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1 MATLAB Part 1 Introduction MATLAB is problem solving environment which provides engineers and scientists an easy-to-use platform for a wide range of computational problems. In general, it is useful for problems which are either too large or too complex to be efficiently solved in Excel. Advantages of MATLAB over Excel are, It is much more efficient for problems involving matrix algebra. It is supported by a wider range of intrinsic functions and toolboxes. It is more versatile for problems involving loops and decisions. In some ways, the graphics are better. MATLAB is an interpreted programming language, unlike Fortran and C++ which are compiled languages. This means that you do not have to compile your program (i.e. convert it into computer binary code) before running it. Instead, the compiling is done automatically during the execution of the program. This makes MATLAB easy to use, but it also means that MATLAB can be slower than some other languages. This tutorial is designed for someone with no experience of programming. It is based around the tutorials provided in MATLAB s Help pages - this workbook guides you to the most relevant pages and gives you practical exercises to do. In this tutorial the following topics are covered, Fundamentals of programming in MATLAB Using the MATLAB Help facility. Working with matrices Importing and exporting data.

2 Starting MATLAB Start MATLAB using the Start Menu Programs MATLAB 7.0 MATLAB 7.0. When MATLAB has started you may be faced with several windows: a Command window, a Workspace window, a Command History window; and maybe Launch Pad, Current Directory and Help windows as well. For example;

3 Workspace window Current directory box Command history window Command window Close down all the windows except the Command window by hitting the cross at their top right hand corners. For now, you only need the Command window (if you want to view the other windows then you can retrieve them by using the Desktop menu). The Command window is where you type commands which load data, execute MATLAB functions, etc. Change the working directory to your H: by using the browse button at the right hand side of the Current directory Box.

4 Fundamentals of programming in MATLAB A program is a series of commands, functions and variables stored in the memory of the computer. Programming is the procedure of, 1. Defining the problem which your program will solve. 2. Designing the program (deciding which commands you need to solve the problem and where to store them). 3. Writing and storing the commands and functions. 4. Testing that the program works. 5. Documenting the program (giving instructions so that someone else can use it). Programming is easy for people of your intelligence - it only needs understanding of some general concepts, and practice. General concepts you need to understand are assigning variables, variable types, and operator priorities. Assigning variables Type in the MATLAB command window, >> A=5 You have assigned a value of 5 to the variable A. The computer creates a space in the memory called A, and stores 5 in that space. In MATLAB, all variables are stored as matrices. A is an example of the smallest possible matrix with dimensions 1 1. If you type, >>B=[1 2; 3 4] you have assigned a 2 2 matrix to a space in the memory called B. Type >>C= Imperial you have assigned a text string which is stored in a 1 1 matrix called C. We can visualise the memory of this MATLAB workspace as, A= B= C= Imperial Assignments will be over-written if you assign a new value to a variable. For example, type >> A=6 This will over-write your previous value of 5. Now type, >> A=A-1 You have over-written A again, returning it to the value 5.

5 Variable types. There are three important types of variables used in programming. Real numbers (also called floating point numbers) have decimal places. 5.0 is a real number. 5.0e6 is a real number in scientific notation, equal to 5 million. Integers are numbers without decimal places. 5 and -100 are integer values. 5.0 is not an integer. String (also called text) variables are a series of characters. Imperial is a string. A string variable is converted in the computer into number format. However normally you only see the string format. When you assign a numeric value, MATLAB automatically interprets it as a real number. For example, >>A = 5 is stored as a real number with approximately 16 decimal places. If you want to store it as an integer or as text you need to be specific, for example, or >>A = int8(5) >>A = five Operator priority Examples of operators are, ^ (raise to the power of) Highest priority * (multiply) / (divide) + (add) - (minus) Lowest priority The priority means which operations are performed first in a command. For example, >>A+2^2 the ^ operation is performed first so the answer is 9. Parenthesis can be used to specify priority, >>(A+2)^2 gives an answer of 49. When choosing between two operations with equal priority, the computer implements them leftto-right. For example, >>A = 10 / 5 * 7 It will first divide 10 by 5 to yield 2. Then it will multiply 2 * 7 to give the final answer 14. Notice how going left-to-right would give an entirely different answer.

6 Using MATLAB Help Go to the Help Menu on the top toolbar and select MATLAB Help, and the Help window will appear. Note the three useful ways to move around the Help files and find the help you need go back (or forward) one page, search, and the pull-down menu which lists all the pages you have already visited. Select MATLAB Getting started, then click Matrices and Arrays, then Matrices and Magic Squares. Working with MATRICES Work through the help pages, following their instructions, until you have covered 12 pages, when you will reach a page entitled The load function (don t go through this page yet). Move through the pages by using the turn page arrows, (you don t need to know about imaginary numbers so don t get stuck on that page)

7 TIPS MATLAB is case sensitive so a A. After you enter a command in the command window, your answer will be displayed. Sometimes you do not want the answer displayed for example if it is a matrix of dimensions there is no point trying to display it in your window. In such a case, type ; after your command, and the answer will be recorded but not displayed If you need additional help with any MATLAB command then type help your_problem in the command window. For example if you want to know more about the zeros command (which creates a matrix of zeros) type help zeros. To repeat one of your previous commands use the and keys to retrieve that command. You can view a list of previous commands by showing the Command History window (use the View menu). When you have finished studying these help pages, you should know how to, Enter a matrix by typing it into the command window Sum rows, columns and leading diagonals of matrices Extract single elements of a matrix and store them as values Extract single rows and columns of matrices and store them as new matrices Add values together (and multiply, divide, etc). Create matrices full of ones, zeros or random numbers. Exercise 1 Create a matrix (of 12 rows and 5 columns) of uniformly distributed random numbers between 50 and 100. Use the command rand - do not enter the numbers manually! You will need to translate the random numbers from the 0-1 scale to the scale by using multiplication and addition operators. Call the matrix A. Lets say that your matrix contains synthesised monthly rainfall data for the years similar to that shown below. For example, below, the average rainfall for January, 1997 is 58.81mm

8 Sum each column to find the annual rainfall and store the total in a new matrix AS. Extract the second row of A to create another matrix which stores only the February data, called AF. Importing Data Usually, we do not want to enter data into the command window usually our data is provided in the form of text or binary files, for example from a digital datalogger or from another computer program. Return to the MATLAB help window and go to the page which is entitled The load function. Follow the instructions on this help page. Use Notepad to prepare the data file. NOTE When you use the load command, by default MATLAB will look in the working directory, which is displayed in the Current Directory Box. If you need to get data from another directory, then specify the full path name as part of the load command, or change the working directory as required. You can either save your data file as a text file with extension.txt or as a data file with extension.dat. Always include the extension when using the load command. TIPS To list the files in the working directory, type dir in the command window. To clear the Command Window, type clc When you have successfully used the load command, skip the next help page (we will cover M- files in the next Workbook) and go on to the Concatenation page. Continue working through the help files until you have covered the page Scalar Expansion Exercise 2 Down load from the Hydroinformatics website the files named Flows.txt and Concs.txt. These are text files which contain data from sources of pollution entering the Charles River, which is a large river in Massachusetts USA. Open these files using Notepad and have a look at their contents. For example, Flows.txt should look like - These data are flows from a number of sources entering the Charles River on three days in For example, Hea is the first of the sources. The data in the Concs.txt file are concentrations of Chemical Oxygen Demand (COD, a measure of organic pollution) in the same sources, on the same three days. The units of the data are mg/l for the COD, and m 3 /s for the flow.

9 How would you load the numeric data from these files (without including the text in the first rows)? The easiest option is to comment out the first two lines with a % sign (below). This means MATLAB will ignore these lines. Save the files then use the load command: >>load Flows.txt >>load Concs.txt These commands will save the imported numeric data in matrices called Flows and Concs. Do not delete or overwrite these matrices you will need them next week. TASK: calculate the total load of COD entering the river in units of kg/day on each of the three days. Store the result in a matrix called TDLS. The total daily load of COD to this stretch of the Charles River, to keep it reasonably clean, should not exceed 1500kg/day. In what months is the river clean? TIPS 1. In MATLAB, convert the units of Flows from m 3 /s to m 3 /day 2. Convert the units of Concs from mg/l to kg/m Daily load at each source in kg/day = Flows Concs (use the.* operator). 4. Total daily load in each month = the sum of all the sources in that month (use the sum function together with the transpose operator ) Exporting data and saving the MATLAB workspace Use the whos command to look at the size and type of all the variables you have generated (another way to view your stored matrices is in the Workspace window). You want to keep Flows, Concs, TDLS. Use the clear command to erase the other variables. For example, >>clear RG1 Export the Charles results to a data file using the command >>save TDLS.txt TDLS -ASCII This takes your array and stores it in the working directory as a text (ASCII) file named TDLS.txt. Open this file using Word. This is called a space-delimited file. You can save files in other formats by adding qualifiers to the save command, for example as a tab delimited file; >>save TDLS.txt TDLS ASCII -TABS

10 Often you want to save your whole workspace (all your variables and settings). To do this either use the File menu or use the command; >>save yourfilename This produces a file called yourfilename.mat which can only be opened in MATLAB - by using the File menu, or the load command. Save your workspace as Week1 >> save Week1 Summary Working in MATLAB involves using functions to manipulate your data within a MATLAB workspace, where you can save your data and results as matrices. You can think of MATLAB as a calculator, except that the data input, functions and memory are more advanced. Memory (workspace) Command window Functions Data input Below is a summary of the components used in MATLAB programming. M-files are used when you want to save a series of commands, instead of typing them into the command window one at a time. We will learn to use them in the next Workbook. We will also introduce MATLAB graphics. Inputs Memory Outputs Functions & commands: 1. From the command window 2. From M-files (.m files) Data sources: 1. From the command window 2. From text files (.dat or.txt files) Matlab workspace (saved as.mat files) Outputs: To graphs To text files To the command window