ENGG 1801 Engineering Computing

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ENGG 1801Engineering Computing

• MATLAB Lecture 2 Background for Week 4 Tutorial
• Simple Programming in MATLAB

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Outline of lecture

• MATLAB as a calculator revisited
• Concept of M-files
• Decision making in MATLAB
• Use of IF and ELSEIF commands
• Example Real roots of a quadratic

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MATLAB as a calculator

• MATLAB can be used as a clever calculator
• This has very limited value in engineering
• Real value of MATLAB is in programming
• Want to store a set of instructions
• Want to run these instructions sequentially
• Want the ability to input data and output results
• Want to be able to plot results
• Want to be able to make decisions

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Example revisited

• Can do using MATLAB as a calculator
• gtgt x 110
• gtgt term 1./sqrt(x)
• gtgt y sum(term)
• Far easier to write as an M-file

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How to write an M-file

• File ? New ? M-file
• Takes you into the file editor
• Enter lines of code (nothing happens)
• Save file (we will call ours L2Demo.m)
• Exit file
• Run file
• Edit (ie modify) file if necessary

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L2Demo Version 1

• n input(Enter the upper limit )
• x 1n Matlab is case sensitive
• term sqrt(x)
• y sum(term)
• What happens if n lt 1 ?

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L2Demo Version 2

• n input(Enter the upper limit )
• if n lt 1
• disp (Your answer is meaningless!)
• end
• x 1n
• term sqrt(x)
• y sum(term)

Jump to here if TRUE
Jump to here if FALSE
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Decision making in MATLAB

• For simple decisions?
• IF END (as in last example)
• More complex decisions?
• IF ELSEIF ELSE ... END
• Example Real roots of a quadratic equation

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Roots of ax2bxc0

• Roots set by discriminant
• ? lt 0 (no real roots)
• ? 0 (one real root)
• ? gt 0 (two real roots)
• MATLAB needs to make decisions (based on ?)

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One possible M-file

• Read in values of a, b, c
• Calculate ?
• IF ? lt 0
• Print message No real roots? Go END
• ELSEIF ? 0
• Print message One real root? Go END
• ELSE
• Print message Two real roots
• END

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My M-file

• 
  
• Demonstration of an m-file
• Calculate the real roots of a quadratic
  equation
• 
  
• clear all clear all variables
• clc clear screen
• coeffts input('Enter values for a,b,c (as a
  vector) ') Read in equation coefficients
• a coeffts(1)
• b coeffts(2)
• c coeffts(3)
• delta b2 - 4ac Calculate discriminant
• Calculate number (and value) of real roots
• if delta lt 0
• fprintf('\nEquation has no real roots\n\n')

Header
Initialisation
Calculate ?
Make decisions based on value of ?
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Conclusions

• MATLAB is more than a calculator
• its a powerful programming environment
• Have reviewed
• Concept of an M-file
• Decision making in MATLAB
• IF END and IF ELSEIF ELSE END
• Example of real roots for quadratic equation
• Same structure as Week 4 tutorial problem