Mars Data Analysis Using IDL GG7101 Lecture 6

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Mars Data Analysis Using IDLGG710-1Lecture 6

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

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Review of Monday

• Procedures and Functions
• Main program follows subroutines
• Data passed to a
• Pro in a comma delimited list pro_name, var1,
  var2
• Fun in a comma delimited list surrounded by
  parens
• xfun_name(var1, var2)
• Data can be passed back from a
• Pro or Fun Changing parameters passed to it
• Fun Using return statement
• Control Statements
• Most commonly used if, for, while
• Also can use case, switch
• ASH Example of Function/Procedure

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Today

• Questions from last time
• Quiz
• HW returned
• Finish Control Statements
• Global Variables efficient programming
• I/O
• Standard I/O
• ASCII I/O
• Binary I/O
• HW Predict the pressure for the surface of Mars
• Review of today's class

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Questions from 9/28

• 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

• Show the command to resize an array containing
  THEMIS floating point data called them_data that
  is 800 samples by 1000 lines to a new array
  called new_data that is 700 samples by 800 lines
• Assume there is a function called
  seismic_scale_depth that is handed density and
  returns a distance. Show how to call this
  function with a density of 2.7 and store the
  returned data in a variable zdepth
• Show a for loop that prints the numbers 5.5 to
  10.5 in steps of 0.5
• Show an if statement that prints the words 'Got
  ya!' if a variable named status1 equals 4.3

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Homework1 Scores

• Overall, good!
• Answers last time
• Scores
• 80 85 93 100 100 100 100

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Repeat Form

• Similar to while, but executes statement before
  checking condition
• repeat statement until condition
• repeat begin
• statement
• statement
• endrep until condition

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Repeat

• Example
• x0.4
• repeat begin
• print, x
• xx0.3
• endrep until (x le 10)

0.400000
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Goto

• Goto is used to jump from the current position in
  the code to a new position located by a variable
  label
• Considered poor programming technique, as it
  difficult to follow logical jumps
• x0.4
• goto new_code1
• print, x
• xx0.3
• new_code1
• print, Jumped!

Goto Form goto, label skipped statement skipped
statement label
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Break

• Break is used to immediately exit from a for,
  while, repeat, case, or switch statement
• Execution jumps to the next statement after the
  loop
• x0.4
• repeat begin
• print, x
• xx0.3
• if (x eq 1.0) then break
• endrep until (x ge 10)
• print, x

0.400000 0.700000 1.000000
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Continue

• Continue is used to jump to the next iteration of
  a for loop without executing intervening code
• for ctr0,10 do begin
• if (ctr eq 5) then continue
• print, ctr
• endfor

0 1 2 3 4
6 7 8 9 10
No 5!
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Global Variables

• Variables are available only to the
  routine/subroutine they are created in, unless
  specifically passed to another routine
• Variables that can be seen by all
  routines/subroutines without being explicitly
  passed are called Global Variables
• Considered poor programming, because it is hard
  to tell where they are being changed when
  debugging or reading a program
• There are two types of global variables in IDL
• System defined global variables (some not
  changeable, start with !)
• User defined global variables (changeable)

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Unchangeable Global Variables
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Changeable Global Variables

• Two ways to generate a user defined global
  variable
• defsysv create a changeable system variable
• defsysv, '!my_var1', 33.3
• print, !my_var1
• common block creates a user global variable
  accessible to any routine with a common block
  definition

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Common Blocks

• Form
• common block_name, var1, var2, var_n
• Any routine with
• common block_name, var1, var2, var_n
• inside of it can access var1-var_n

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Example of Common Blocks

• pro subroutine1
• common rockin, var1, var2, var_n
• var1'Yippee!'
• end
• pro main_routine1
• common rockin, var1, var2, var_n
• subroutine1
• print, var1
• end

Note var1 not handed to subroutine1, but still
changed, because it is global as a result of
common block!
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Efficient Code

• IDL is optimized for array processing
• If you can avoid control statements that create
  loops, and instead use arrays, IDL can run
  significantly faster
• pro test
• adist(500,1000)
• tvscl, dist(50,100)
• sum0.0
• t0 systime(1)
• for i0L, (n_elements(a)-1L) do
• if (ai gt 100.0) then sum sum ai
• t1 systime(1)
• print, t1 - t0
• end

Slow! 1.0 Sec
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Efficient Code

• IDL is optimized for array processing
• No loops, using IDL array functions
• pro test
• adist(500,1000)
• sum0.0
• t0 systime(1)
• idx where(a gt 100.0, cnt)
• if (cnt gt 0) then sumtotal(aidx)
• t1 systime(1)
• print, t1 - t0
• end

Faster! 0.088 Sec
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Efficient Code

• IDL is optimized for array processing
• Using only arrays and IDL functions
• pro test
• adist(500,1000)
• sum0.0
• t0 systime(1)
• sum total(a (a gt 100.0))
• t1 systime(1)
• print, t1 - t0
• end

Fastest! 0.083 Sec
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Input and Output

• Three primary types of I/O
• Reading and writing from the keyboard and screen
• Called standard I/O or
• stdio
• Reading and writing text characters to a file
• ASCII I/O
• Reading and writing binary data to a file
• Binary I/O

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STDIO Writing

• Print used to write numbers and text to screen
• Can control the precision and style of numbers
  presented on screen using format commands
• print, !pi, format(f7.2)
• print, findgen(5), format(5f7.2)
• print, 2, !pi, format(i4, 2x, f7.2)
• Details of the format codes can be found in Table
  4.2, and Chapter 4.1

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STDIO Writing
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STDIO Reading

• Can read data from the user at the command line
  using the read command
• read, var1, var2, var_n, prompt'your text here'
• read, sin_var, prompt'Enter value to take sine
  of '
• If not previously defined, the default type of
  variables in read is float.
• To use another type, such as int or string,
  define the variable first with a bogus value
• ivar0
• svar' '
• read, ivar, svar, prompt'Enter integer and
  string '

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Working with files (ASCII or binary)

• Difference between ASCII and binary
• Can write data into a file as human readable
  (ASCII) or the actual bits/bytes required for
  variable type (binary)
• Example In the file looks like
• An ASCII Integer 10 gt _ _ _ _ _ _10 (eof)
• 8 bytes1 for eof, each 8 bits long
• A binary integer 10 gt _at_
• 2 bytes long (16 bits matches type)
• Quiz Which is longer, an ASCII or binary file
  with 10 floating point numbers in it?

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Working with files (ASCII or binary)

• To write or read data from a file, several steps
  must be taken
• The file needs to be opened for access
• Data needs to be written or read
• The file needs to be closed

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Working with files (ASCII or binary)

• IDL can open a file for reading, writing, or both
• openr read
• openw write
• openu both
• Opening a file LUN's
• IDL uses a string containing the name of a file
  to create a variable associated with the file
• Variable associated with file called a "Look Up
  Number" or LUN
• There are a limited number of files that can be
  open simultaneously (128)
• IDL can automatically provide us with an
  available LUN when opening a file

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Examples of opening a file

• Form of command
• openr, lun_var, filename_str, /get_lun
• openw, lun_var, filename_str, /get_lun
• openu, lun_var, filename_str, /get_lun
• Example
• openr, iln, 'scott_data.txt', /get_lun

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Closing a file

• When done reading or writing a file, close it
  with free_lun
• Gives the LUN back into the limited pool of files
  that can be open simultaneously
• Form of command
• free_lun, lun_var
• Example
• openr, iln, 'scott_data.txt', /get_lun
• statements to read the file go here
• free_lun, iln

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Writing ASCII files

• To write data to an ASCII file, use printf
• Form
• printf, lun_var, var_1, var_2, var_n,
  formatfmt_string
• Example
• openw, olun, 'test.txt', /get_lun
• printf, olun, 5, 6, 7, 9.0
• datafindgen(5,5)
• printf, olun, data
• datafindgen(100)
• printf, olun, data, format'(f7.2)'
• free_lun, olun

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Reading ASCII files

• To read data from an ASCII file, use readf
• Form
• readf, lun_var, var_1, var_2, var_n,
  formatfmt_string
• Example
• openr, ilun, 'test.txt', /get_lun
• readf, ilun, var1, var2, var3, var4
• datafltarr(5,5)
• readf, ilun, data
• datafltarr(100)
• readf, ilun, data, format'(f7.2)'
• free_lun, ilun

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Writing Binary files

• To write data to an binary file, use writeu
• Form
• writeu, lun_var, var_1, var_2, var_n (note no
  format!)
• Example
• openw, olun, 'test.txt', /get_lun
• writeu, olun, 5, 6, 7, 9.0
• datafindgen(5,5)
• writeu, olun, data
• free_lun, olun

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Reading Binary files

• To read data from an binary file, use readu
• Form
• readu, lun_var, var_1, var_2, var_n
• Example
• openr, ilun, 'test.txt', /get_lun
• readu, ilun, var1, var2, var3, var4
• datafltarr(5,5)
• readu, ilun, data
• free_lun, ilun

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Mixed ASCII/Binary in a file

• Can mix ASCII and binary in a file
• Typically used to create a human readable header,
  containing information on file size, lines,
  samples, data range etc.
• This is followed by the data in a binary form
• Example
• nx 5
• ny 7
• data findgen(nx,ny)
• openw, olun, 'test.dat', /get_lun
• printf, olun, nx, ny write a header
• writeu, olun, data
• free_lun, olun
• The header also sometimes comes in a separate
  file, called a detached header

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Endian Ewoks?

• The bytes of binary data are stored differently
  on different machines
• If a computer stores the most significant byte of
  a binary number first, they are called Big
  Endian, else Little Endian
• Suns, Mac OS, etc are Big Endian
• PC's are Little Endian
• Most binary PDS data, like the MOLA data, were
  saved on machines that are Big Endian.
• If you open and read this data on a Little Endian
  machine (like the PC's in the lab), most reverse
  the byte order
• openr, ilun, myfilename, /get_lun,
  /swap_if_little_endian
• or
• xswap_endian(y)

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Homework Mars Global Pressure as a function of Ls

• Plot the variation of pressure over a season at
  VL1 in units of 1 Ls.
• Create a program to calculate the theoretical
  atmospheric pressure for the entire surface of
  Mars using MOLA data, the known altitude of the
  VL-1 landing site, and the known variation in
  atmospheric pressure measured at VL-1 as a
  function of season.
• Do a screen capture of a contour map of pressure
  for Ls210
• What is the highest and lowest surface pressure
  at this season?
• Where are these places located in lat, lon?
• Create an image that shows MOLA topography
  everyplace with a pressure gt 4 mbar for Ls210

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Homework Mars Global Pressure as a function of Ls

• Assume
• I have provided a file of MOLA data and a
  detached header in
• z//bambam/Users/GG710-1
• T205 K everywhere all the time
• That there is no weather on Mars creating clouds,
  fronts, high or low pressure zones - all pressure
  change is due to changes in altitude
• That the MOLA elevation of VL-1 is -3.6 km

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Homework Mars Global Pressure as a function of Ls

• Pressure at VL-1 for given Ls

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Homework Mars Global Pressure as a function of Ls

• Plots
• Use the command plot to make x,y graphs on the
  screen
• plot, LS_array, P_array, xtitleLs,
  ytitlePressure
• see help for more details
• Use contour to make a contour plot
• this example assumes pres_array is 360, 181 in
  size
• lat_arrfindgen(181)-90
• lon_arrfindgen(360)
• contour, pres_array, lon_arr, lat_arr,
  xtitleLon, ytitleLat
• see help for more details

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Homework Mars Global Pressure as a function of Ls

• Plots
• Use the command tvscl to visualize your data
• TVSCL will make an image on screen that has the
  same number of pixels as the image has size, but
• The default window that IDL opens is 640 by 480
• To resize window, use the window command
• Can only make a window as big as the screen may
  need to reduce the size of the image to see it
  properly
• The image is displayed with the first index at
  the bottom left the last index at the top
  right this can lead to an upside down image that
  you may need to flip to see properly
• window, xsize1440, ysize720
• tvscl, reverse(an_image,2)

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Homework Mars Global Pressure as a function of Ls

• Think structured programming
• Want a program that we provide Ls, and it returns
  an array (or image) of pressure values
• Steps
• Use the equation provided to calculate pressure
  at VL-1 for given Ls
• Read the MOLA data
• Determine the difference in altitude between VL-1
  and rest of MOLA image
• Calculate the pressures using your ASH routine
• Create a contour plot of the results
• Use functions we learned about earlier to answer
  questions

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Today

• Finished Control Statements repeat, break,
  continue, goto
• Error Handling Next time
• Global variables
• Efficient Code
• I/O
• Homework 2

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Next Time

• More on Input and Output
• Direct Graphics

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Error Handling

• Default IDL behavior is to stop when it detects
  an error, at the location of the error
• Errors can be intercepted and dealt with by your
  code using catch
• Anytime an error occurs following a catch
  command, control is handed to the statement
  following catch
• Catch returns an error code if there is an error
• catch, var1
• To obtain the error message associated with the
  error, use the help procedure
• help, /last_message, output my_var
• To end error interception
• catch, /cancel

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Error Handling

• pro test
• error_stat0
• catch, error_stat
• if (error_stat ne 0) then begin
• help, /last_message, outputerrtext
• print, errtext
• print,'On Error, the ctr was ', ctr
• stop
• endif
• xfindgen(4)3.7
• for ctr0,4 do print, xctr
• for ctr5,-1,-1 do print, xctr
• end

Establish error handler that will tell us the
value of ctr if an error occurs, then stop
execution
Loop over a short array printing values. The
2nd loop is out of range, and will cause an error
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Handling Math Errors

• Default behavior on math error is continued
  operation
• Math errors x1.0/0.0 xsqrt(-1.0)
• IDL tells you the type of error at the end of
  routine/subroutine
• Can change IDL behavior by setting system
  variable !except
• !except2

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Handling Math Errors

• Can check for math errors in a variable using
  finite command which returns 0 if there has been
  an error, and 1 if OK
• x10.0, 1.0/0.0, sqrt(-1.0)
• print, finite(x)
• 1 0 0