Mars Data Analysis Using IDL GG7101 Lecture 5

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Mars Data Analysis Using IDLGG710-1Lecture 5

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

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Today

• Questions from last time
• Quizzes returned/Answers
• Review of yesterday
• Finish Procedures and Functions
• Control Statements
• In class exercise Atmospheric Scale Height
• Review of today's class

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Questions from 9/22

• To reset an IDL session
• .reset_session
• Local/user defined system/global variables
  destroyed
• Pointers, structures destroyed
• Compiled programs subroutines destroyed
• Graphics windows widgets destroyed

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Questions from 9/22

• Pointers Allocating the heap
• ptr_new() creates a pointer that always points to
  nothing
• Can be made to point at something with a 2nd
  ptr_new command
• Generally used as a marker of the end of an array
• ptr_new(/allocate_heap) creates a pointer to an
  undefined variable
• ptr_new(x) creates a copy of x, then makes a
  pointer to it
• ptr_new(x, /no_copy) makes a copy of x, creates a
  pointer to the copy, then deletes x

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Quiz Scores from 9/22

• Much improvement!
• Average went from 69 to 85
• Standard deviation from 20 to 9
• Quiz Scores
• 70 80 83 83 90 90 100

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9/22 Quiz Answers

• Create an anonymous structure containing a float,
  an int, and a double
• datax 0.0, y 0, z 0.0D
• Illustrate how to print the int from the
  anonymous structure
• print, data.y
• What does the function n_elements do?
• Counts the number of positions in an array

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9/22 Quiz Answers

• Create a pointer to a image of floating point
  numbers that is 1000 lines by 100 samples
• img_ptrptr_new(fltarr(100,1000))
• In the array x0, 7, 23, 9, 400 show the
  command that searches for and returns the index
  of the value 9
• print, where(x eq 9)
• partial credit for variations on
• for ctr0,4 do if (xctr eq 9) then print, ctr
• but as we havent covered this yet, it wasnt the
  main answer
• for ctr0,4 do if ((xctr mod 3) eq 0) then
  print, ctr
• wont work, as 0 mod 3 also is 0

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Review of 9/22

• Array resizing (congrid)
• Procedures and functions
• Definitions (functions always return a value)
• Order
• Naming
• Editing compiling
• Passing parameters
• Procedures use comma separated variables
• Functions use comma separated variables in
  parentheses
• Keywords Passing optional parameters
• Returning parameters

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Returning Data from Functions or PRO

• Three methods for returning values from functions
  or procedures

• By changing the parameters that are handed to
  them (applies to procedures and functions)
• By returning a specific value or structure from a
  function using return
• Both at the same time

IDLgt hello
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Functions Returning Parameters
filename hello.pro
Functions always return a value, even if there
is no return statement (in which case 0 is
returned)
IDLgt hello
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Both Return and Parameter Passing
filename hello.pro
FUNCTION manky, str1 print, str1 str1'Hot
Doggity!' return, 'Done!' END PRO hello var
"hello world" xmanky(var) print, var print, x end
IDLgt hello
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Notes on Return

• In a procedure subroutine, return can be used to
  send control back to the calling program
• However, a value cannot be returned in a
  procedure
• pro subroutine1, var
• print, Great subroutine
• if (var eq 1) then return
• print, Var ne 1
• end

pro main_routine subroutine1, 1 subroutine1,
2 end
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Notes on Return

• In a function subroutine, return is used to
  return control and a value to the calling routine
• For Functions and PRO, return can be called more
  than once from within conditional statements
• function subroutine2, var
• print, Really Great Subroutine!
• if (var eq 42) then return, 42
• print, var ne 42
• return, var10
• end

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Exception Parameter Passing

• Parameters may be passed to a PRO or FUNCTION by
  reference or by value
• Reference passing the location in memory
• Value passing the actual value
• Parameters passed by value to a subroutine will
  not be changed unless explicitly passed back to
  the calling routine with return

Changeable
Not Changed! Use return
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Exception Parameter Passing
not changeable by subroutine manky, 100 var9,
10, 11 manky, var01 dataarr2.3, 10.5,
'Bomber!' manky, data.arr0 use return to get
changed values back
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Control Statements

• Used to control the flow of your program
• Can be used to repeat a given step many times
• Can be used to check values and decide what to do
  next

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If

• If is used to check the value of a variable and
  select an action
• var10
• if (var eq 10) then print, var else print, 'Doh!'
• Operators in Table 2.6

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If

• Note if (var10) wrong
• Assigns 10 to var, and value in () is set to 10
• if (var 10) then print, var else print, 'Doh!'
• the same as
• if (10) then print, var else print, 'Doh!'
• If evaluates the value in () for true or false
• 0 or Null always evaluates to false!

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If Form

• Form
• if condition then statement else statement
• or
• if condition then begin
• statement
• statement
• endif else if condition then begin
• statement
• statement
• endif else begin
• statement
• statement
• endelse

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If Example

• pro test, var
• if ((var eq 7) or (var eq 9)) then begin
• print, 'Yowzer'
• var0
• endif else if (var eq 3) then begin
• print, 'Yahoo'
• var1
• endif else begin
• print, 'Oh Blah!'
• var2
• endelse
• print, var
• end

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Case

• Similar to if but easier to write for extensive
  lists of conditions
• pro test, var
• case var of
• 0 print, Var equals 0
• 1 print, 0ltvarlt1
• 2 begin
• print, This is getting old
• print, This is the last message
• end
• else print, Var not 0,1,2
• endcase
• end

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Case Form

• Must use scalar index variable for expression
• Else is optional, but will get error if no case
  selected
• case expression of
• exp1 statement
• exp2 statement
• exp_n begin
• statement
• statement
• end
• else statement
• endcase

• Once an expression is selected no more are
  executed

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Switch

• Similar to case
• Once a true expression is found, switch assumes
  all following expressions are true
• switch expression of
• exp1 statement
• exp2 statement
• exp_n begin
• statement
• statement
• end
• else statement
• endswitch

IDLgt test,1 Test 1 Test 2 Test 3 IDLgt test,2 Test
2 Test 3 IDLgt test,3 Test 3 IDLgt
test.pro
pro test, var switch var of 1 print,
'Test 1' 2 print, 'Test 2' 3
print, 'Test 3' endswitch end
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For

• Loop over a set of statements, incrementing an
  variable from a start value to an end value by a
  given increment
• xfindgen(10)3.7
• for ctr0,4 do print, xctr
• for ctr5,-1,-1 do print, xctr
• Default increment is 1

0.00000 3.70000 7.40000
11.1000 14.8000
18.5000 14.8000 11.1000
7.40000 3.70000 0.00000
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For Form

• for variablestart, finish, increment do
  statement
• for variablestart, finish, increment do begin
• statement
• statement
• endfor
• Note the loop variable is defined by the value
  contained in start
• I.e., if need to loop over floating point range,
  ensure that start is a float

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While

• Loop over a set of statements while the provided
  condition is true
• x0.4
• while (x le 10) do begin
• print, x
• xx0.3
• endwhile

0.40000 0.70000 1.00000
1.30000 1.60000
9.40000 9.70000
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While Form

• while condition do statement
• while condition do begin
• statement
• statement
• endwhile

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Atmospheric Scale Height

• Scale height
• Distance over which a parameter is reduced by a
  factor of 1/e
• Atmospheric Scale Height
• Distance over which the atmospheric pressure drop
  by 1/e
• Defined as
• On Earth, H7.5 km
• For mean T250K

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Mars Atmospheric Scale Height

• For Mars,
• Thus, atmospheric scale height is
• Write a program to return
• ASH as a function of Temp
• What is the scale height for 210 K?
• If the surface pressure is 7.4 mb, what is the
  pressure 21.6 km higher?

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ASH Example
As a function
filename mash.pro
IDLgt xmash(210) IDLgt print, x 10791.9 IDLgt
print, mash(210) 10791.9 IDLgt print,
7.4exp(-21600.0/x) 0.999970
function mash, tempr return, 191.17
float(tempr) / 3.72 end
Or as a procedure
filename mash.pro
IDLgt mash,210,x IDLgt print, x 10791.9 IDLgt
print, 7.4exp(-21600.0/x) 0.999970
pro mash, tempr, msh msh 191.17
float(tempr) / 3.72 end
Pressure at 21.6 km 1.0 mbar
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Today

• Procedures and functions
• Control Statements if, case, switch, for, while
• Atmospheric Scale Height Example

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Next Time

• Finish Control Statements
• Global variables, error handling, efficient
  coding
• Input and Output
• Standard I/O (to the screen/terminal)
• Formatting
• Files
• Formatted/ASCII
• Unformatted/Binary
• open, read, write, file pointers