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C/C++ Programming for Engineers:
Matlab Scripts and Functions
John T. Bell
Department of Computer Science
University of Illinois, Chicago
2
Reading Review
Which of the following would multiply the cosine of X times the common logarithm of X, when X is given in degrees? A. cos( X ) * log( X ) B. cos( X ) * log10( X ) C. cosd( X ) * log( X ) D. cosd( X ) * log10( X ) E. cos( X degrees ) * commonLog( X degrees )
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Matlab Script Files
- Any command that can be run in Matlab can be placed in an ordinary text file ( with a .m extension ) and run as a Matlab script.
- The easiest way to do this is to use Matlab’s built-in editor and drag commands from command history into the script, & then edit.
- Run the script by typing the script name, or by pressing the “run” button in the script editor. 4
Special Characters in Scripts
- The % percent sign is used to create comments, from the % to the end of the line. - %{ Large block comments in braces and % %}
- The ; semi-colon at the end of the line suppresses the display of the command.
- Three dots, … , at the end of a line … are used to continue … long lines onto the next line.
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Preview
What does the %g format specifier do in fprintf( )? A. It uses fixed-point format. B. It uses exponential notation. C. It uses engineering notation. ( Exponential in powers of 3, e.g. kilo, Mega, micro, etc. ) D. It uses either fixed-point or exponential notation, whichever “fits best” for the value being printed. E. It generates an error, since the % is for comments. 8
Simple Matlab Format Options
- format long / short – Adjusts # of digits shown.
- format compact / loose – single/double spacing.
- format bank – Dollars and cents
- format rat – Ratio of two integers
- help format – Shows more options.
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Formatted Output in Matlab
- The fprintf( ) command prints strings:
- fprintf( ‘This program plots sine waves.’ );
- New lines start (only) when \n is printed:
- fprintf( ‘This will print’ );
- fprintf( ‘ all on one line.\n’ );
- This will print all on one line.
- fprintf( ‘This will print\n on\n three lines.\n’ );
- This will print
- on
- three lines. 10
Format Specifiers in fprintf( )
- Whenever a % is encountered in the output string, fprintf( ) prints a data value: - fprintf( ‘The result is %f mpg.\n’, mileage );
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Floating Point Formats
Operator Description %f Fixed-point notation, 109. %e Scientific notation with lowercase e, as in 6.02e+23, 3.141593e+ %g Either %f or %e, whatever is shorter %E Scientific notation with capital E, as in 6.02E+23, 3.141593E+ %G Either %G or %E, whichever is shorter, 6.02E+23, 3. Table 3.6.1: Floating-point number formatting operators. 14
Preview
What happens if a Matlab workspace contains a variable X, and then a script is run that creates a new variable X and gives it a value? A. There will now be two variables named X. B. The workspace X will be wiped out, and replaced with a new X. C. The X created in the script will only exist while the script runs, and will then disappear. D. The script X will be automatically given a different name. E. An error will occur, since two variables can’t have the same name.
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A Problem with Scripts
- Commands executed from a script run in the same scope as the command window. - Script commands can clobber command-window variables with the same variable names. - Script variables can clutter up the workspace unnecessarily. - Solve this by writing functions instead of scripts. 16
Function Scope
- Functions operate in their own space.
- Variables defined within functions are totally independent of any variables defined in the command window, even if they happen to have the same name.
- Because of this, values have to be passed in to functions, and the results passed back.
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Matlab Function Outputs
- The [ out1, out2, … ] = format is for functions that return multiple results. - If a function returns a single result, the [ ] square brackets are not required. - If a function does not return any results, then the equals sign should also be omitted.
- The program that called the function gets its results through the output parameters. 20
Matlab Function Help Comments
- Every Matlab function should contain a “help” comment, which is the first comment block after the “function” line.
- The help comment is displayed whenever one types “help” for the function in question. ( The first line is displayed by lookfor. ) function [ finBalance, interest ] = BalanceWithInterestBoth( rate, initBalance ) % Calculates the final balance of an investment with compound interest. % Inputs: rate -- Interest rate, initBalance -- Initial balance. % Output: finBalance -- Final balance.
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Matlab Function Operation
- The code within a Matlab function is basically the same as a Matlab script.
- Sometime before the function returns, it must assign values to all the output parameters.
- The ‘return;’ statement ceases execution of the function, and returns control back to the calling program. No function code is executed after a return statement. 22
Calling Matlab Functions
- User-written Matlab functions are called by name, the same way that Matlab library functions are called.
- Values or variables must be passed in to the function input parameters.
- Variables must be provided to receive the output results.
