Mars Data Analysis Using IDL GG7101 Lecture 2

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Mars Data Analysis Using IDLGG710-1Lecture 2

• F. Scott Anderson
• Office POST 526B
• Phone [phone]
• Email anderson_at_higp.hawaii.edu

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Today

• Questions from last time
• Help Can use ctrl-H (keyboard shortcuts)
• .compile and .run synonymous
• Quiz
• Review of last week
• Variables
• Arrays
• Operators
• Structures
• Final Project
• Review of today's class

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Quiz From 8/25 notes (5 min)

• Name a method to obtain help in IDL
• Write the commands to assign a value to a
  variable and display it on the screen
• Does capitalization matter for IDL commands
  (print versus Print)?
• Name 2 strengths of IDL
• Why use structured programming?

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Review

• Overview of the class
• What, why, grading, syllabus, accounts
• Strengths of IDL
• IDLDE versus Unix interface
• Starting IDL, examples, and help
• Runtime scripts versus compiled programs
• Structured programming

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Special Characters

• Comments are denoted by a semicolon
• pro test
• shello world
• Print the variable s
• print, s
• end
• To continue a command on the next line
• pro test
• shello world
• t Surfing Rocks
• print, s,
• t
• end

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Variables

• Variables are
• a symbol (like x or y) that is used in
  mathematical or logical expressions to represent
  a variable quantity
• They can be created at any time in IDL
• They are NOT case sensitive
• They can hold different types of information
• Strings my_varThe quick brown fox
• Numbers my_var3434557.23423
• The type can be redefined at any time
• Distinct from a constant
• x3 x is a variable, 3 is a constant

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Numeric Variable Data Types
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Numeric Variable Data Types

• Byte data can represent a number (0-255) or a
  single ASCII character, depending on how it is
  printed
• xbyte("aAbB")
• print, x
• print, string(97B, 65B, 98B, 66B)
• Note type conversion and array - will come back
  to this later!

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ASCII Character Codes
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Numeric Variable Data Types

• Integers are whole numbers
• The range of an integer is 2n, n is the of bits
• Can be signed or unsigned
• Example 00 0 01 1 10 2 11 3
• 2 bits range of 4
• 3rd bit could be used to indicate sign, or
  increase range

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Numeric Variable Data Types

• Floats/Doubles are used to represent fractions
• Use entirely different method to record a number
• For a floating point number (32 bits)
• 6 bits used to record the exponent
• Range for float 1038
• e.g. 25 bits 32, 26 bits 64
• 25 bits used for the mantissa
• 225 3.3 M, or 6 significant figures
• 1 bit for sign
• Floats guarantee 6.7 significant digits
• Doubles guarantee 13.2 significant digits
• All additional digits may be noise!
• Leads to "Round-off Error"

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String Variable Data Types

• String variables hold text
• Structures hold a group of other variables
• Others will be discussed later
• Help command/variable window can monitor variable
  types
• help, x

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Type Conversion

• Default type for an integer is int
• x3 gt 16 bit int
• Default type for fractional number is float
• x3.1415 gt 32 bit float
• In an expression, type follows precedence type
  from Table 2.1

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In Precedence Order
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Precedence Example

• To manually convert (and guarantee type) use type
  conversion operators
• Can write a constant to indicate type
• 190B
• 1.0D
• Use the function to convert

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Type Conversion Examples

• Example
• ydouble(6.02e23)
• x(3.1415y)7 gt double
• y6.02D23
• x(3.1415y)7 gt double
• x34UL7 gt unsigned long
• Quiz
• x3/4 ?

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What About Down Conversion?

• If converting against precedence, data will be
  truncated (lost)
• print, long(4.72) 4
• To control the conversion of float to int
• floor Rounds down
• ceil Rounds up
• round Rounds to the nearest
• Conversion of text to numbers will provide any
  leading values
• Conversion of numbers to text provides the
  numbers as a string

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Wait! What about Variables!

• Variables can be off any data type
• Must begin with a letter
• 128 characters or less
• Case ignored
• OK a-z, 0-9, _
• Cannot use names already used by IDL
• Reserved words
• Existing IDL functions
• User created functions

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Reserved Words
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IDL/User Functions

• Use the help command to determine a conflict
• ? intarr
• Use .compile command to test for an existing user
  function
• Most variables names you create will be OK
• Especially if you use _ or numbers
• x_max or myintarr27

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Array Variable

• Any variable can also hold an array of values
• What is an array?
• Group of equally spaced items in rows and columns
• A list of numbers is an array
• Often called a vector (IDL does not distinguish)
• An image is a two dimensional array
• An array can be up to 8 dimensions
• Can even have an array with a single value
• Each dimension has a direction

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Array Directionality
Array C 5,7
1,0
0,0
2,0
3,0
4,0
Array A7 or A7,1
0,1
1,1
2,1
3,1
4,1
0,2
1,2
2,2
3,2
4,2
Array B1,7

• IDL column major
• Position indices start at 0
• Trailing array dimensions of 1 can be dropped
• To get value in yellow square of Array C
• xc3,4

4,6
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Array Creation

• Horizontal vector array
• y0,1,2,3,4 gt INT Array5
• Vertical vector array
• x0,1,2,3,4 gt INT Array1,5
• xtranspose(y) gt INT Array1,5
• Matrix
• x0,1,2,3,4,0,1,2,3,4 gt INT Array5,2
• xy,y gt INT Array5,2

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Array Creation Functions
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More Array Creation Functions

• xreplicate(3.3, 3, 2)
• x 3.3, 3.3, 3.3
• 3.3, 3.3, 3.3
• xmake_array(3,2,/byte)
• x 0,0,0
• 0,0,0
• bytarr(3,2)
• make_array(3,2,/byte,/index) bindgen(3,2)
• make_array(3,2,/byte,/index, value5)

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Array Creation Examples

• xintarr(5) gt x0,0,0,0,0
• xindgen(5) gt x0,1,2,3,4
• xfltarr(3,2) gt x 0.0,0.0,0.0
• 0.0,0.0,0.0
• xfindgen(3,2) gt x 0.0,1.0,2.0
• 3.0,4.0,5.0
• xindgen(3)10 gt x 0,10,20

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Array Indexing

• 0 lt array index lt elements
• index cannot be fractional
• arrayindex1, index2, index3
• zfindgen(3,3)
• print, z1,2 gt 7.0
• arrayval1val2, val3val4
• print, z01, 12
• 3.0, 4.0
• 6.0, 7.0

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Array Indexing

• Can refer to all elements with
• xfltarr(4) 0.0, 0.0, 0.0, 0.0
• x1.0 1.0, 1.0, 1.0, 1.0
• x1 1.0, 1.0, 1.0, 1.0
• xfltarr(4,2)
• x,12.0 0.0, 0.0, 0.0, 0.0
• 2.0, 2.0, 2.0, 2.0
• print, x,0 0.0, 0.0, 0.0, 0.0
• print, x0, 0.0
• 2.0

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Array Indexing

• Can use variables as index
• p3
• print, xp,1 gt 2.0
• Can use arrays as indices
• x 2, 7, 4
• yindgen(10)5
• print, yx gt 10 35 20
• If array indices out of range will clip to min or
  max
• x1000, -1
• print, yx gt 45 0

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Array Indexing

• Rarely used form of indexing for applying an
  operation and then indexing
• (array expression) index
• n5
• xindgen(n)
• index0,2,4
• print, (x10)index gt 0 20 40
• Efficient way to apply multiple indexes in a
  single statement
• Example on page 33-34 of book

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Operators in IDL

• Many mathematical operators in IDL
• Follow typical rules for precedence in operations
• 5 10/5 7 not 3!
• (510)/5 3 not 7!
• 5102/5 25
• What are the rules of precedence?

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Operators Precedence
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Operator Notes

• Addition subtraction operate on arrays normally
• Operations on arrays of different sizes results
  in an array with the smallest dimension from each
  array
• Normal mathematical array multiplication uses
• Inverse of an array is NOT calculated using
  division (like in MATLAB)

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Min, max, mod operators

• Min and max operators
• print, 4lt5, 5lt5, 6lt5
• 4 5 5
• arr 3,4,5
• print, arr gt 4
• 4 4 5
• mod operator gives remainder
• print, 9 mod 5 gt 4

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Relational Operators

• Typically used for true false testing in
  conditional statements
• if
• while
• Can be used in commands however
• a10
• b20
• ca gt b c 0
• da lt b d 1

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Boolean Operators

• AND, OR, NOT
• Typically used in conditional statements
• Especially NOT Will come back to this later
• Can be used at the command line
• xfindgen(5) 0.5
• print, ((x gt 2) and (x lt 3))
• 0 0 1 0 0
• print, ((x lt 2) or (x gt 3))
• 1 0 1 1
• Little used XOR operator little used, see 49
• Byte masking also little used, see book 51-52

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Ternary Operator

• A short hand for conditional if statement
• if x is lt y then
• z a
• else
• zb
• Written
• z(x lt y) ? a b
• More on conditional statements shortly!

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Final Project

• Class Discussion of Ideas

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Today

• Questions from last time
• Help Can use ctrl-H (keyboard shortcuts)
• .compile and .run synonymous
• Quiz
• Review of last week
• Variables
• Operators
• Arrays
• Pointers
• Structures
• Final Project
• Review of this class