Mars Data Analysis Using IDL GG7101 Lecture 7

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Mars Data Analysis Using IDLGG710-1Lecture 7

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

2
Review of Monday

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

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Today

• Questions from last time
• Quiz returned
• Quiz
• Finish I/O
• Assoc, EOF, File pointers, File info, finding
  files, picking files
• In class exercise
• reading binary I/O with a structure
• Review of today's class

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Quiz Scores

• Tapped a deep vein of lack of understanding
• Average 75
• StdDev 27
• Scores
• 31 38 71 92 94 100 100

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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
• new_data congrid(them_data, 700, 800)
• 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
• zdepth seismic_scale_depth(2.7)

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Quiz

• Show a for loop that prints the numbers 5.5 to
  10.5 in steps of 0.5
• for ctr5.5, 10.5, 0.5 do print, ctr
• for ctr0, 10 do begin
• x5.5
• print, x
• xx0.5
• endfor
• Show an if statement that prints the words 'Got
  ya!' if a variable named status1 equals 4.3
• if (status1 eq 4.3) then print, 'Got Ya!'

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Questions from 9/28

• Goto with multiple labels Gives an error
• Common blocks
• Main routine definition must list all variables
• Use in subroutines does not require showing
  variables, but does require common block name
• Strings in read Remind me of the question?
• Other questions I missed last time

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Question strtrim

• Recall that IDL will convert a number that is
  written to the screen or a file as a string
• The default format for converting a number to a
  string includes many leading and trailing spaces
• Hence
• print, 'Value ', 10 Number auto converted
  to string
• results in
• Value 10 where the underline
  indicates spaces
• To remove spaces from a string, use strtrim
• Form
• result strtrim(variable, 0,1,2)
• print, 'Value ', strtrim(10) Trim
  trailing spaces
• print, 'Value ', strtrim(10, 0) Trim
  trailing spaces
• print, 'Value ', strtrim(10, 1) Trim
  leading spaces
• print, 'Value ', strtrim(10, 2) Trim both
  leading trailing spaces

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Question Filenames w/numbers

• You can create repeating numbered filenames
• This examples creates filenames that look like
• my_filename_no_1.dat
• my_filename_no_2.dat
• my_filename_no_3.dat
• my_filename_no_10.dat
• for ctr1,10 do begin
• filenamestring('my_filename_no_',
  strtrim(ctr,2), '.dat')
• openw, ilun, filename, /get_lun
• lets assume an array called joe with 1440,720
  data points
• printf, ilun, joe
• free_lun, ilun
• endfor

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Quiz

• What type of control statements result in
  inefficient IDL code?
• What are the three general steps required for
  interacting with any file?
• What is the difference between a floating point
  ASCII file and a floating point binary file?
• Is it possible to write a file with mixed ASCII
  and binary data?
• Show the IDL commands to write a 5 line, 10
  sample, indexed array to an ASCII file.

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More on I/O

• Three very useful functions for I/O

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EOF

• End of File denotes the actual end of data in a
  file
• The eof() function tests to see if you are at the
  end of a file
• Form
• valueeof(lun)
• value is either true or false
• Example
• Imagine you have a file with 5 samples x n lines.
  How to read?
• while(not eof(ilun)) do readf, ilun, d1, d2, d3,
  d4, d5

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fstat

• Provides a structure of information on an open
  file
• Form
• valuefstat(lun)
• Example
• openr, ilun, some_file.txt,
• /get_lun
• infofstat(ilun)
• help, info, /struct

Structure FSTAT, 17 tags, length64, data
length64 UNIT LONG
100 NAME STRING 'tmp' OPEN
BYTE 1 ISATTY BYTE
0 ISAGUI BYTE 0
INTERACTIVE BYTE 0 XDR
BYTE 0 COMPRESS BYTE 0
READ BYTE 1 WRITE
BYTE 1 ATIME LONG64
1097087305 CTIME LONG64
1097087344 MTIME LONG64
1097087344 TRANSFER_COUNT LONG
25 CUR_PTR LONG
330 SIZE LONG 330
REC_LEN LONG 0
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findfile

• Allows you to search for a file in a known
  directory and retrieve its exact filename
• Form
• valuefindfile(file_name, countvar1)
• Example
• iname findfile(/Users/home/my_data/.dat,
  countnn)
• openr, ilun, iname, /get_lun
• Should check nn to see if more than one file (or
  none), and use inamen to access each

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diaglog_pickfile

• Allows you to choose a file with a GUI interface
• Form
• valuedialog_pickfile(filtermy_string,
  many_other_options)
• Example
• filt.jpg, .tif, .gif
• iname dialog_pickfile(filterfilt)
• openr, ilun, iname, /get_lun
• Many options allowing you to select directories,
  multiple files, files to write, files to read,
  default file names, etc. Check idl_help for more
  details.

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point_lun

• Allows you to move to a given location in a file
• Can be used to skip headers, reread previously
  read data, or skip to the end of a file
• Form
• point_lun, lun, position_var
• if lun is negative, places the current position
  in position_var
• Example
• hdr_sz2048L Header of 2048 bytes
• openr, ilun, 'my_data.bin', /get_lun
• point_lun, ilun, hdr_sz
• point_lun, -ilun, pos
• print, pos
• start reading data here

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assoc

• Allows you to connect a variable (typically an
  array or structure) and a data file
• Allows you to read data repeatedly into this
  variable by making a region of memory and a
  region of the file equal
• Does not know how many sets of data you seek
• Hence must determine number of sets or records
  independently
• Form
• varfltarr(10) or perhaps varint0,
  dblarr(10,10)
• resultassoc(lun, var, offset, /packed)
• result is an array of type var, with a number of
  indices equal to the number of records
• offset is used to skip an initial header of
  offset bytes
• /packed is used to insure that structures are
  read correctly on a given computer
• must know number of records independently
• Does not actually read data until you ask for
  resultn, e.g.
• print, result3

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assoc example

• Assume a file with 2 integers and five floats,
  and 10 lines
• varv10, v20, v3fltarr(5)
• openr, ilun, 'my_file', /get_lun
• resultassoc(ilun, var, /packed)
• print, result0 Works
• print, result0.v2 Fails - IDL can't do
  structure
• members when using assoc
• tmp1 result0
• tmp2 result100
• print, tmp1.v2
• print, tmp2.v1
• Only works if there are at least 100 records,
  otherwise get EOF error

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In Class Exercise (10 minutes)

• Putting it all together
• Download the file my_data.bin from my website
• This file of mola data consists of an integer
  value for orbit track, and three floats, lat,
  lon, and topo
• Assume an unknown number of records
• Create a program to read this data and print the
  latitudes
• Use the fstat command to calculate the of
  records
• Create a structure and replicate it to read data
• Use readu to read this data

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In Class Exercise Answer
pro molat chunk orb0, lat0.0, lon0.0,
topo0.0 openr, ilun, 'my_data.bin',
/get_lun infofstat(ilun) nrec
info.size/(12. 34.) datareplicate(chunk,
nrec) readu, ilun, data free_lun, ilun
may need to byte swap data if on PC data
swap_endian(data) plot, data.lat,
data.topo stop end