What are Functions?
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Transcript What are Functions?
Today's Agenda
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1
Introduce Matlab User Functions
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What are Functions?
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Functions in MATLAB are similar to:
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functions in C
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subroutines in FORTRAN and BASIC
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procedures in Pascal
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Building blocks of larger programs
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Allows complex programs to be structured
and organized
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Defined by MATLAB or created by user
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Accept input values and return output values
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Matlab's Built-In Functions:
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Elementary
Functions
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Data analysis
functions
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Array Operations
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Graphing Functions
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Special Functions
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Matrix Operations
Polynomial
Functions
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Signal Processing
Functions
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Numerical Methods
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Using Elementary Functions
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Math Format
y = f(x)
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MATLAB Format
output_variable = function_name (input_variable)
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Example
squareroot_of_number = sqrt (16)
Any name you choose
(output value)
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number or variable
(input value)
MATLAB function
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Matlab runs two types of
programs:
1) SCRIPT files --- stand alone programs
like you've been writing
2) FUNCTION files --- sub-programs
intended to do a specific task and return
the results to the program that called it
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G:\common\eng1102\1102_200508\02matlab\m10.functions.sxi
Matlab user-defined FUNCTIONS:
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First line designates it as a function
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lists input values and returns output values
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Second line(s) are “prologue” to be printed in
response to a help command
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Following lines complete the calculations
NOTE: All the variables are local variables,
which means their values are only available
within the function
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Generic Function Example
function [output_variables] = function_name (input_variables);
% this is an example of a function
% input_variable and ouput_variable are both vectors
output_variables =input_variables .^ 2
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If only one output value the [ ] are optional
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First line tells MATLAB "this is a function"
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Prologue describes what the function does
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help function_name prints Prologue to screen
Calculations come after the Prologue
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Must SAVE function in EDITOR with .m extension
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Area of a Square Function
function [Asqr] = square(side)
% This function calculates the area of a square.
% Input variable (length of side) can be a scalar or vector
% Output variable will be a scalar or vector depending on input
[Asqr]=side.^2;
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output_variables = Asqr
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function_name = square.m
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input_variables = side
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Calling a Function
In Command Window or Parent program type:
[output_variable]=function_name(input_variable)
Examples:
>> side=2
>> [Asqr]=square(side)
OR
>> Area=square(2)
OR
>> Asqr=square([1,2,4])
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Area of a Square and Circle
Function
function [Asqr,Acir] = square_circle(side,radius)
% This function calculates the area of a square and circle.
% Input variables can be a scalar or vector
% Input variables are length of square and radius of circle
% Output variable will be a scalar or vector depending on input
% Output variables are area of square and area of circle
[Asqr]=side.^2;
[Acir]=pi*radius.^2;
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Calling this Function
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Variable names do NOT need to be the same as in
the function
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Variables must be in same location and order
Example:
>> length=2
>> r=2
>> [Area1,Area2] = square_circle(33,65)
>> [surf_a_sq,surf_a_ci]=square_circle(length,r)
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As a Team...
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Write a "statistics' function that calculates
mean, min, max, stdev for a set of data
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Name this function stat_pack.m
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Test this function in your command window
using a simple data set
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x=[1,2,3,4,5,6,7,8,9,10]
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Stepwise Refinement
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Design Strategy used to break a large task
into smaller tasks
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Continue to divide tasks until the tasks are
relatively simple and have an obvious
solution
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For example: plot a sine wave from 0 to 2*pi
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Generate a vector x with elements between 0
and 2pi
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Evaluate yi=sin(xi)
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Plot yi versus xi
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Program Modules
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Programs generally have a Main program
which calls individual modules (user functions)
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Modules should be dedicated to 1 task
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Each module can be individually developed
and tested
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Modules may be reused for other applications
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As a Team
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Read the Glen Canyon Dam problem (LM p.
44)
Plan your solution to the Glen Canyon Dam
Monitoring Analysis.
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Break tasks to modules
List Input and Output parameters for each
module
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Modules
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Main Program
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flow_stats
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daily_ave
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weekly_ave
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plot_data
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main
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Main routine
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Problem documentation
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Tasks:
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Load the data into MATLAB matrix
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Copy the data into a vector of times and a vector
of flow rates
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Call other functions to calculate statistics and
output results
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flow_stats
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Input:
hourly flow vector
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Output:
mean hourly flow, median hourly
flow, standard deviation of hourly
flow
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Tasks:
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Calculate mean, median, standard deviation hourly
flow for the entire year
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daily_ave
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Input:
hour vector, hourly flow vector
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Output:
vector
day vector, average daily flow
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Tasks:
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Determine number of days in data set
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Day vector = day of year
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Daily ave = mean (flow hours (a:b) )
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weekly_ave
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Input:
hour vector, hourly flow vector
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Output:
week vector, weekly flow vector
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Tasks:
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Determine number of weeks in data set
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Week vector = day of year, middle of the week
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Weekly ave = mean (hourly flow (a:b) )
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plot_data
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Input:
hour, hourly flow, day vector, daily
flow, week vector, weekly flow
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Output: plot of flow rates and histogram
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Tasks:
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Plot hourly data versus time
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Plot histogram of hourly flow, with 20 bins
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Plot daily data versus time as a line and weekly
data versus time as circle
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Testing Your Code
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Test each module with a simple version of
the problem, whose answers can be
checked by hand calculations
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Display intermediate calculations by
removing semicolons at the end of
statements or adding or removing print
statements
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Use MATLAB debugger
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Team Homework ...
Due: 13A
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Glen Canyon Dam Analysis (see handout)
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