Chapter 3: Introduction to MATLAB Programming (4 th ed.)

Transcription

1 Chapter 3: Introduction to MATLAB Programming (4 th ed.) Algorithms MATLAB scripts Input / Output o disp versus fprintf Graphs Read and write variables (.mat files) User-defined Functions o Definition o Calling a function o Passing arguments o Local variables 1

2 3.1 Algorithms An algorithm is the sequence of steps needed to solve a problem Top-down design: the problem solution is broken down into separate steps, and then each step is further refined until the resulting steps are small enough to be manageable tasks Example. calculate the area of a circle Algorithm: Input: radius (r) Calculate the area Show the output 2 ( π r ) Questions: Where does the input come from? (external file/keyboard) and in what units? Constant π is needed Where does the output go: extern file/screen 2

3 3.2. MATLAB scripts (programs) Computer program: a sequence of instructions, in a given language, that accomplishes a task Execute (run): the computer actually follows the instructions Compiler: translates (in advance) a high-level language program (the source code) into machine language (object code) that can be executed by the computer Interpreter: translates and executes each command line-by-line Script- of M-files: sequence of MATLAB instructions Program: a set of scripts and functions (that belong to each other) The content of a script can be displayed using the type command and executed by simply entering the name of the file (without the.m extension) at the MATLAB prompt (>>) Before you create or modify a script or function, please note that the right working directory is selected, i.e. Current Folder refers to the correct subdirectory! 3

4 3.2.1 MATLAB scripts-documentation Use File New Script to create a new script The words after the % are comments and are ignored by MATLAB Script1b.m % This script calculates the area of a circle % First the radius is assigned radius = 5 % The area is calculated based on ¼ pi * (radius^2) The 1 st line describes what the script does >>help script1b This script calculates the area of a circle The very first comment line is called the "Hl line"; it is what the function lookfor searches Through Several lines can be commented out/in at once: ctrl-r (ctrl-t to remove) 4

5 3.3.1 Input Function Input statements read in values from the default or standard input device (usually the keyboard) >> rad =input ('Enter the radius: ') Enter the radius: 5 rad = 5 This method only works well if the user actually enters a number (or the name of a variable) >> mystr=input('enter a string: ','s') Enter a string: " Test " (2 spaces before and after Test) mystr = Test >> length(mystr) ans = 8 Use str2num() and str2double() to convert strings to numbers 5

6 3.3.2 Output Statements: disp and fprintf >> disp(-3^2) -9 >> fprintf('the value is %d!\n',-3^2) The value is -9! Place holders (character in the place holder is the so-called conversion character): %d integer (decimal integer) %f float (real number) %c character (one character) %s string or characters Field width: specifies how many characters in total are to be used in printing Ex. %5d, %-5d, %10s, %6.2f and %.3f (minus-sign can be used to left-justified output) >> fprintf('2x five: x%5dx, x%-5dx and 2x 4.9: x%6.2fx, x%.2fx\n',5,5,4.9,4.9) 2x five: x 5x, x5 x and 2x 4.9: x 4.90x, x4.90x 6

7 3.3.2 Output Statements: disp and fprintf You can also use fprintf to show vectors and matrices >> mat=[5 9 8;4 1 10] mat = >> fprintf('%d',mat); fprintf('\n') >> fprintf('%d ',mat); fprintf('\n') >> fprintf('%d\n',mat) >> fprintf('%d %d %d\n',mat') For vectors and matrices, the disp function may be easier than fprintf 7

8 3.5.1 Plot Function % Create coordinate variables and plot a red '*' x = 11; y = 48; plot (x,y, 'r*' ) axis ( [ ]) %Change the axes and label them xlabel ('Time') ylabel('temperature') title ('Time and Temp') % Put a title on the plot 55 Time and Temp 50 Temperature Time 8

9 Color, Line types and Markers Colors: Lines: b blue c cyan -- dashed line g green k black -. dashed dotted line m magenta (purple) r red : dotted line y yellow - solid line Markers: o circle d diamond h hexagram p pentagram + plus. point s square * star v down triangle < left triangle > right triangle ^ up triangle x x-mark 9

10 Color, Line types and markers - example 12 clf x = 1:5; % unnecessary y1 = [ ]; y2 = [ ]; figure(1) bar(x,y1) figure(2) plot( x,y1,'g*--','linewidth',1.5) hold on plot( x,y2,'r^:','linewidth',1.5) grid on legend('y1','y2') y1 10 y

11 3.6 Introduction to File Input (load) and Output (save) save filename variablenaam ascii Ex. >>mymat=rand(2,3); >> save testfile.dat mymat -ascii >>type testfile.dat e e e e e e-002 If you want to append data to a file, use the qualifier append Reading from a file is accomplished using load filename.ext: will read data into a matrix variable with the same name as filename columns/rows of matrices can be assigned later load tijdtemp.dat time=tijdtemp(1,:); temp=tijdtemp(2,:); this only works if there are the same number of values in each line 11

12 3.7 User-defined Functions that Returns a Single Value A function in MATLAB consists of the following: the function header (the first line) comprised of: o the reserved word function o the name of the output argument followed by the assignment operator = o the name of the function (IMPORTANT: this should be the same as the name of the file in which this function is stored) o the input arguments in parentheses, which correspond to the arguments that are passed to the function in the function call a comment that describes what the function does (this is printed when help is used) the body of the function, which includes all statements and eventually must put a value in the output argument end at the end of the function (note that this is not necessary) 12

13 3.7.1 User-defined Function: example calcarea.m function area= calcarea(rad) % calcarea calculates the area of a circle % Format of call: calcarea (radius) % Returns the area area= pi* rad * rad; end Call: >>calcarea(4) The result returned from this function can also be stored in a variable or showed: >>area=calcarea(4) >>disp(calcarea(4)) >> fprintf('de area is %.1f\n',calcarea(4)) Or called from another script 13

14 3.7.4 Passing Multiple Arguments When a function needs multiple arguments, you can pass more than one argument separated by commas Ex. the volume of a cone (radius r and height h) is given by: V= π rh( = base area h) 3 3 function outarg = conevol(radius, height) % conevol calculates the volume of a cone % Format of call : conevol (radius, height) % Returns the volume outarg = (pi/3) *radius.^2.*height; end Important: the function must be called with exact the number of arguments as in the code the order makes a difference: conevol(4,6) conevol(6,4) 14

15 3.7.5 Functions with Local Variables When a variable is defined / created within a function, then is becomes a local variable: the scope of a local variable is only the function itself, i.e. it does not exist outside the function and vice versa, so variables that are defined / created outside a function do not exist within the function o this means that a variable x in a script and a variable x in a function refer to two different memory places M-script x=5; disp(x); y=maaltwee(x); disp(x); M-function function x = maaltwee(y) x=2*y; disp(x); end 15