Learn IDL

1
Learn IDL

• Wei Huang
• huangwei_at_ucar.edu
• Consulting Services Group
• Scientific Computing Division

2
What is IDL?

• IDL - Interactive Data Language
• The company
• ITT Visual Information Solutions
• http//rsinc.com/idl/

3
Start IDL

• Start IDL
• idl, or
• idlde
• Command Line
• PRINT, Hello, World!
• Quit IDL
• exit

4
Reference/Tutorial

• Tutorial over Internet
• http//www.msi.umn.edu/software/idl/tutorial/
• http//amath.colorado.edu/scico/tutorials/idl/
• An Introduction to Using IDL in Meteorology
• http//cgam.nerc.ac.uk/visualisation/idl/guide/idl
  .pdf

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IDL Variables

• Commonly Used Variables

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IDL Variables (Continued)

• User can define new type (structure)
• Dynamic (automatic) type definition
• Array index starts from 0 (like C)

7
Getting Started with IDL-Command Line Input

• Start with XY-Line Plot
• Making a Dataset
• original SIN((FINDGEN(200)/35)2.5)
• PLOT
• PLOT, original

8
Plot and Processing

• Making a New Dataset
• noisy ((RANDOMU(SEED, 200) - .5) / 2)
• Plot
• PLOT, noisy
• Plot them together
• PLOT, original, THICK 3,
  XTITLE Time, YTITLE Amplitude
• OPLOT, originalnoisy

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Files and Input/Output

• Write data to a file, formatted
• OPENW, 1, original.dat
• OPENW, 2, noisy.dat
• PRINTF, 1, original
• PRINTF, 2, noisy
• CLOSE, 1
• CLOSE, 2

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Files and Input/Output (Continue)

• Read data from a file, formatted
• base FLTARR(200)
• pert FLTARR(200)
• OPENR, 1, original.dat
• OPENR, 2, noisy.dat
• READF, 1, base
• READF, 2, pert
• CLOSE, 1
• CLOSE, 2

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Files and Input/Output (Continued)

• IDL can handle lots data format
• ASCII
• Formatted, free-format
• Binary
• C-binary, Fortran-binary, etc.
• NetCDF
• Network Common Data Form
• And More

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More on Plot and Processing

• Processing with SMOOTH
• full base pert
• smoothed SMOOTH(full, 5)
• PLOT, smoothed, TITLE Smoothed Data
• Filtering
• y FINDGEN(200)
• y101199 - REVERSE(y199)
• filter 1.0 / (1 (y/40)10)
• PLOT, filter
• lowpass FFT(FFT(full, 1) filter, -1)
• PLOT, lowpass
• highpass FFT(FFT(full, 1) ( 1.0 - filter),
  -1)
• PLOT, highpass

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More on Plot and Processing

• Put Together
• !P.MULTI 0, 2, 3
• PLOT, base, TITLE 'Original "Ideal" Data'
• PLOT, full, TITLE 'Noisy Data'
• PLOT, SHIFT(filter, 100), TITLE 'Filter
  Function'
• PLOT, lowpass, TITLE 'Lowpass Filtered'
• PLOT, highpass, TITLE 'Highpass Filtered'
• PLOT, smoothed, TITLE 'Smoothed with boxcar
  average'

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Basic Programming

• DO-loop
• FOR n0, number DO an fix(n)
• FOR n0, number DO BEGIN
• an fix(n)
• Bn 2.0an2 4.0an 6.0
• 8.0an(-2.5)
• ENDFOR
• NOTE is for continuation.

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Basic Programming

• IF-THEN BLOCK
• IF (logical-statement) THEN something
• IF (logical-statement) THEN BEGIN
• Something
• Other thing
• ENDIF
• IF (logical-statement) THEN BEGIN
• Something
• Other thing
• ENDIF ELSE some-other-thing

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Basic Programming

• CASE
• CASE name OF
• Match_1 do-something
• Match_2 do-other thing
• ELSE do-things not-matched
• ENDCASE

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IDL Programming-An Example

• PRO xplot
• sockeye 463, 459, 437, 433, 431, 433, 431,
  428, 430, 431, 430
• coho 468, 461, 431, 430, 427, 425, 424,
  420, 418, 421, 420
• chinook 514, 509, 495, 497, 497, 494, 493,
  492, 492, 493, 493
• humpback 467, 465, 449, 446, 445, 444, 443,
  443, 443, 443, 445
• year 1967, 1970, INDGEN(9) 1975
• allpts coho, sockeye, humpback,
  chinook
• names 'Coho', 'Sockeye', 'Humpback',
  'Chinook'
• n1 N_ELEMENTS(year) - 1

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IDL Programming (continued)

• PLOT, year, coho, yrange MIN(allpts),
  MAX(allpts), title
  'Salmon Population', xtitle 'Year',
  xrange 1975, 1990, /XSTYLE, ytitle 'Fish
  (thousands)
• FOR i 1, 3 DO OPLOT, year, allpts, i, LINE
  I
• FOR i 0, 3 DO XYOUTS, 1984, allptsn1, i,
  namesi
• END

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IDL Programming (Exercise)

• Run the above program
• Save it as xplot.pro
• All IDL programs have suffix .pro
• At IDL pommand line prompt
• idlgt.compile xplot
• idlgtxplot

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Contour Plot Example

• PRO contour_plot
• nx 400
• ny 300
• axfltarr(nx)
• ayfltarr(ny)
• xfindgen(nx)
• yfindgen(ny)
• ax2!PI/nxx
• ay2!PI/nyy
• z fltarr(nx, ny)
• FOR j0, ny-1 DO BEGIN
• FOR i0, nx-1 DO BEGIN
• zi,j 100.0 COS(axi) SIN(ayj)
• ENDFOR
• ENDFOR

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Contour Plot Example (continued)

• CONTOUR, z, NLEVELS 21, /FILL
• CONTOUR, /FOLLOW, z,
• LEVELS -80 findgen(5) 40,
• XSTYLE 1, YSTYLE 1, YMARGIN 5,
  MAX_VALUE 100,
• C_LINESTYLE 2, 1, 0,
• C_THICK 1, 1, 3, 1, 1,
• C_LABELS 1, 1, 3, 1, 1,
• C_CHARSIZE 1.25, FONT 0,
• TITLE 'COS x SIN',
• SUBTITLE 'Interval 10',
• XTITLE 'X - AXIS',
• YTITLE 'Y - AXIS',
• /NOERASE
• END

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Check Before You Write

• IDL has lots of functions
• SIN, COS, MIN, MAX, etc.
• Check if there is a function you can use, before
  writing your own
• Get help from IDL
• gtidlhelp on-site line user guide
• gtidldemo see what IDL has done

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Look Beyond the Basics

• We have covered xy-plot, contour
• IDL is not just this
• Advanced IDL Features
• Surface plot
• Iso-surface
• Volume
• Image
• And more

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Questions?

• Send email to us
• huangwei_at_ucar.edu
• consult1_at_ucar.edu
• Call us
• 303-497-1278 (consulting)
• 303-497-8924 (Wei Huang)