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Matlab Programming
Gerald W. Recktenwald Department of Mechanical Engineering Portland State University [email protected]
These slides are a supplement to the book Numerical Methods with Matlab: Implementations and Applications, by Gerald W. Recktenwald, ©c 2001, Prentice-Hall, Upper Saddle River, NJ. These slides are ©c 2001 Gerald W. Recktenwald. The PDF version of these slides may be downloaded or stored or printed only for noncommercial, educational use. The repackaging or sale of these slides in any form, without written consent of the author, is prohibited.
The latest version of this PDF file, along with other supplemental material for the book, can be found at www.prenhall.com/recktenwald.
Version 0.97 August 28, 2001
Overview
Creating Side effects
- Function m-files Syntax of I/O parameters Text output Primary and secondary functions
- Flow control
Relational operators Conditional execution of blocks Loops
Using vector operations instead of loops Preallocation of vectors and matrices Logical and array indexing
- Programming tricks Variable number of I/O parameters Indirect function evaluation Inline function objects Global variables
Script Files
- Not really programs No input/output parameters Script variables are part of workspace
- Useful for tasks that never change
- Useful as a tool for documenting homework: Write a function that solves the problem for arbitrary parameters Use a script to run function for specific parameters required by the assignment
Free Advice: Scripts offer no advantage over functions. Functions have many advantages over scripts. Always use functions instead of scripts.
Script to Plot tan(θ) ( 1 )
Enter statements in file called tanplot.m
- Choose New... from File menu
- Enter lines listed below
Contents of tanplot.m:
theta = linspace(1.6,4.6); tandata = tan(theta); plot(theta,tandata); xlabel(’\theta (radians)’); ylabel(’tan(\theta)’); grid on; axis([min(theta) max(theta) -5 5]);
- Choose Save... from File menu Save as tanplot.m
- Run it
tanplot
Script Side-Effects ( 1 )
All variables created in a script file are added to the workplace. This may have undesirable effects because
- Variables already existing in the workspace may be overwritten
- The execution of the script can be affected by the state variables in the workspace.
Example: The easyplot script
% easyplot: Script to plot data in file xy.dat % Load the data D = load(’xy.dat’); % D is a matrix with two columns x = D(:,1); y = D(:,2); % x in 1st column, y in 2nd column plot(x,y) % Generate the plot and label it xlabel(’x axis, unknown units’) ylabel(’y axis, unknown units’) title(’Plot of generic x-y data set’)
Script Side-Effects ( 2 )
The easyplot script affects the workspace by creating three variables:
clear who (no variables show) easyplot who
Your variables are:
D x y
The D, x, and y variables are left in the workspace. These generic variable names might be used in another sequence of calculations in the same Matlab session. See Exercise 10 in Chapter 4.
Function m-files ( 1 )
- Functions are subprograms:
Functions use input and output parameters to communicate with other functions and the command window Functions use local variables that exist only while the function is executing. Local variables are distinct from variables of the same name in the workspace or in other functions.
- Input parameters allow the same calculation procedure (same algorithm) to be applied to different data. Thus, function m-files are reusable.
- Functions can call other functions.
- Specific tasks can be encapsulated into functions. This modular approach enables development of structured solutions to complex problems.
Function m-files ( 2 )
Syntax:
The first line of a function m-file has the form:
function [outArgs] = funName(inArgs)
outArgs are enclosed in [ ]
- outArgs is a comma-separated list of variable names
- [ ] is optional if there is only one parameter
- functions with no outArgs are legal
inArgs are enclosed in ( )
- inArgs is a comma-separated list of variable names
- functions with no inArgs are legal
Function Input and Output ( 2 )
twosum.m
function twosum(x,y) % twosum Add two matrices % and print the result x+y
threesum.m
function s = threesum(x,y,z) % threesum Add three variables % and return the result s = x+y+z;
addmult.m
function [s,p] = addmult(x,y) % addmult Compute sum and product % of two matrices s = x+y; p = x*y;
Function Input and Output Examples ( 3 )
Example: Experiments with twosum:
twosum(2,2) ans = 4
x = [1 2]; y = [3 4]; twosum(x,y) ans = 4 6
A = [1 2; 3 4]; B = [5 6; 7 8]; twosum(A,B); ans = 6 8 10 12
twosum(’one’,’two’) ans = 227 229 212
Notes: 1. The result of the addition inside twosum is exposed because the x+y expression does not end in a semicolon. (What if it did?)
- The strange results produced by twosum(’one’,’two’) are obtained by adding the numbers associated with the ASCII character codes for each of the letters in ‘one’ and ‘two’. Try double(’one’) and double(’one’) + double(’two’).
Function Input and Output Examples ( 5 )
Example: Experiments with threesum:
a = threesum(1,2,3) a = 6
threesum(4,5,6) ans = 15
b= threesum(7,8,9);
Note: The last statement produces no output because the assignment expression ends with a semicolon. The value of 24 is stored in b.
Function Input and Output Examples ( 6 )
Example: Experiments with addmult:
[a,b] = addmult(3,2) a = 5 b = 6
addmult(3,2) ans = 5
v = addmult(3,2) v = 5
Note: addmult requires two return variables. Calling addmult with no return variables or with one return variable causes undesired behavior.
Text Input and Output
It is usually desirable to print results to the screen or to a file. On rare occasions it may be helpful to prompt the user for information not already provided by the input parameters to a function.
Inputs to functions:
- input function can be used (and abused!).
- Input parameters to functions are preferred.
Text output from functions:
- disp function for simple output
- fprintf function for formatted output.
Prompting for User Input
The input function can be used to prompt the user for numeric or string input.
x = input(’Enter a value for x’);
yourName = input(’Enter your name’,’s’);
Prompting for input betrays the Matlab novice. It is a nuisance to competent users, and makes automation of computing tasks impossible.
Free Advice: Avoid using the input function. Rarely is it necessary. All inputs to a function should be provided via the input parameter list. Refer to the demonstration of the inputAbuse function in § 3.3.1.
