Week 6 - Programming I

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Week 6 - Programming I

• So far, weve looked at simple programming via
  scripts programs of sequentially evaluated
  commands
• Today, extend features to
• additional operators
• branches to control operations
• loops to repeat operations
• Textbook chapter 7, pages 165-179, 182-187
  (sections 7.1 7.2.2, 7.4, 7.4.1)

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Relational Operators the Idea

• In formal English someone might ask you
  Is your age greater than or equal to 21?
• Answers include
• Yes, of course
• Heres my ID card
• Im 18
• I knew this would happen if I forgot my ID
• No

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• Using mathematical notation, we test or compute
  the relation
• age 21 or age gt 21
• And expect 1 of only 2 answers
• Yes or True
• No or False

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Relational Operators in MATLAB

• A operator B
• A and B can be
• Variables or constants or expressions to compute
• Scalars or arrays (match the sizes on arrays!)
• Numeric or string
• Operators gt gt lt lt
• Result is true (1) or false (0) perhaps an
  array

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Note value and class
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• More examples
• expression result
• 5 lt 7 1
• 3, 5, 2 gt 1, 0, 12 1 1 0
• max( 16 ) lt 7 1
• 3, pi, -12 gt 1 1 1 0
• 'Tom' 'Bob' 0 1 0
• 'Tom' 'm' 0 0 1

Note arrays and strings need to be the same
size
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• Notes
• Can compute using the result e.g.
• how many of a particular letter in a state
  name?

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• Dont confuse and
• Round off errors can impact
• sind(0) 0 1
• sind(180) 0 0
• instead, test for small values
• abs( sind(180) ) lt eps 1

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Matlab has Logical Operators as Well

• A operator B
• A and B can be
• Variables or constants or expressions to compute
• Scalars or arrays, numeric or string
• A and B are interpreted as logical (binary)
• Numeric 0 is interpreted as false
• All else is interpreted as true (equal to 1)
• Result is true (1) or false (0) perhaps an array

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• Basic operators and or
• xor not

A B AB AB xor(A,B) A
0 0 0 0 0 1
0 1 0 1 1 1
1 0 0 1 1 0
1 1 1 1 0 0
truth table
unary operator
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• Examples
• Are you between 25 and 30 years old?
• (agegt25) (agelt30)
• Is it winter?
• (month12 daygt22) (month1) (month2)
  (month3 daylt21)

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• Array example
• Score 70, 55, 88, 98, 80, 73, 90 C
  (Score gt 70) (Score lt 81)
• C 0 0 0 0 1 1 0
• Useful in counting how many entries satisfy a
  condition
• B_grades sum( Scorelt91 Scoregt80 )

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• Text examples
• 'Tom' 'm' 'Tom' 'o' 0 1 1
• name input('enter name','s')
• name 'Tom' name 'Bob'
• Rolling dice
• roll sum(ceil(6rand(1,2)))
• roll 7 roll 11

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• Other useful logical operators
• Extend and from binary to arrays
• any(X) all(X)
• To check array size, value, and data type
  isempty(A)
• isinf(A) isnan(A)
• ischar(A) isnumeric(A)
• To find the locations
• of events find( )

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Operator Precedence (left to right)

• Parentheses ( )
• Transpose(') and power(.)
• Negation (-) and logical negation ()
• Multiplication (.) and division (./),
• Addition () and subtraction (-)
• Colon operator ()
• Relational operators (lt, lt, gt, gt, , )
• Logical AND ()
• Logical OR ()

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Branches, Conditional Statements

• Commands to select and execute certain blocks of
  code, skipping other blocks.
• Three types in Matlab
• if/else
• switch
• try/catch

this week
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If/Else

• if expression
• commands if true
• else
• commands if false
• end

• Use relational and logical operators to determine
  what commands to execute

evaluate this
use of blue in editor also, auto indentation on
commands
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Example output whether a variable x is
positive or not x computed somehow
if x gt 0 disp('the value is
positive') else disp('the value is negative
or zero') end
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Example output a warning if the variable x
is negative (note that there is no else
portion in this example) x computed
somehow if x lt 0 disp('Warning negative
value') end
the else component is not required
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Example ask if a plot should be drawn x
input('Plot now? ', 's') if x 'yes' x
'YES' plot( .. ) end
more complicated expression to evaluate
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Example Write a script to put 2 numbers in
numerical order
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Loops

• Commands to repeatedly execute certain blocks of
  code
• Two types in Matlab
• for
• while

this week
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The for Loop

• Used for a specific number of repetitions of a
• group of commands
• for index array
• commands to be repeated go here
• end
• Rules
• One repetition per column of array
• index takes on the corresponding columns values

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• Example collect 7 digits of a telephone number
  and store in an array

7 repetitions since the array is 1 2 3 4 5 6 7

digit cycles through the 7 values to create the 1
by 7 array number
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• Example calculating interest for 10 years

command num2str converts numerical variables to
string variables for concatenating with other
strings
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Example implement a count down timer (in
seconds)
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• Example a general vector for array

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Example a matrix for array
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• Example even a string array

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Errors in Your Scripts

• Syntax errors

Note red text bad news
But tells you where
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• Run-time errors inf or NaN results

Note black text okay, just a warning
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• Logical errors in your program hard to find
• Example quadratic equation solver
• But x22x1 (x1)2 ? x 1
• Use the built-in debugger

Missing parentheses around 2a
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Matlab Data Files (not in the text)

• Types
• .asv auto save
• ascii regular text files
• .mat Matlabs proprietary format (multiple
  variables)