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Interactive Data Language (IDL)PowerPoint Presentation

Interactive Data Language (IDL)

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### Interactive Data Language (IDL)

This PPT includes some materials downloaded from Internet, please don’t circulate it out of the LAB

What is IDL?

Interactive Data Language (IDL) is a programming language that is a popular data analysis language among scientists.

IDL is the trusted scientific programming language used across disciplines to create meaningful visualizations out of complex numerical data (ITT).

- 1970 – IDL’s predecessor, Laboratory for Atmospheric and Space Physics, CO
- Used by
- NASA
- Lockheed-Martin
- Medical Imaging Fields
- Remote sensing

What is IDL???

A More Powerful MATLAB, also more expensive.

A great tool for hyperspectral imaging!

A fast tool for processing large images!!

A great GREAT tool for ENVI programming!!!

Overview of IDL

- A high-level interpreted programming language with vector and array primitives
- Modern programming language
- Flow control
- Data structures
- Objects

- All operators and most functions work on scalar, vector or array data of any data type.
- Data visualization tool, advanced built-in graphics
- 2-D plots
- Contour plots
- Surface plots
- Shaded surfaces
- Gray scale/color images
- Volume rendering, …

- Multi-platform support
- Unix: Sun, Hewlett Packard, Silicon Graphics, IBM
- Linux, Microsoft Windows, Mac

How to Learn

- Online Help
- http://www.idlcoyote.com/
- Practice & Practice

Basic Variable Types

- Integers
- 1 byte – byte data type (0..256)

IDL> a=bindgen(1)

- 2 bytes – integers data type (0..2562-1)

IDL> b=indgen(1) & print,5/3 1

- 4 bytes – long integers (0..2564-1)

IDL> c=lindgen(1)

- 8 bytes – 64-bit long integers (0..2568-1)

- Floating data types
- 4 bytes – floats (exponent from-38 to 38)

IDL>print,5./3. & y=float(3)/float(2) 1.66667

IDL> a=1.23456789

IDL> print,a,format=‘(f20.10)’

- 8 bytes – double-precision

IDL> print,5d/3d 1.6666667

IDL> a=1.23456789d0 & print,a,format=‘(f20.10)’

IDL> xyouts,xloc,yloc,string(a,format=‘f(20.10)’)

- Strings (Text)
IDL>x=‘Hi there!!!’

Assignment Statements

A = B + 1

- A has the same structure as B, with a data type equal to that of the most precise operand in the expression on the right hand side. In this case it could be any type except string.
- If B is a vector or array then 1 is added to each element.
A = 0 ; A is a 16 bit integer

A = A * 0.5 ; A is now a 32 bit float

B = A(*,3) ; B is equal to the 4th row of A

A(*,3) = 0 ; Set all elements in 4th row of A equal to 0

- Examples:
image = fltarr(512, 512) ; zero filled array

b = image(0:127, 0:127) ; b is 128x128 array

image(*,100) = findgen(512) ; replace row 100

plot, image(*,100) ; plot row 121

; Display the power spectrum as an image

tvscl, alog(abs(fft(image, 1)))

Naming Variables

- Assign ‘readable’ variable names that make sense
- Variable names must start with a letter
- NO: 6a=“gamma” OK: a6=“gamma”

- Variable names may not have spaces in them
- NO: A 6=5 OK: A_6=5

- Some characters are illegal or have special meanings
- NO: a@=5, a.b=5 (used for structures), A+=5, A#=5

Organizational structures

- Scalars
IDL> x=3.

- Vectors
IDL> a=[0,1,2,3] & print,a[1] 1

- Arrays (see IDL help for matrices) (index from zero)
intarr(), fltarr(), dblarr(), strarr().indgen()

IDL> q=intarr(2,2) 0 0

0 0

IDL> m=indgen(2,2) 0 1

2 3

IDL> print,m(0,1) 2

IDL> a=findgen(100,100)

IDL> print,a[1,0] 1.00000

IDL> b=a[23:25,67:69]

IDL> indx=where(a gt 10.)

IDL> jndx=where(a[indx] le 100.)

IDL> b=a[indx[ jndx]]

Array operations

- Simple math operations (* by scalar, +, -); A 3-column by 2-row array:
IDL> A = [ [0, 1, 2],[3, 4, 5] ,[5,6,7]]

IDL>B=2*A 0 2 4

IDL>print, B 6 8 10

10 12 14

- More complex math
- #, Multiply an array by another IDL> C=A#B
- N_elements: Total number of elements in array
IDL> n=n_elements(A)

- Total(array): computes a sum of all elements
- Min(array): outputs the min value; also Max
- Minmax(array): outputs the min then max
- Rebin: Resize array IDL> C=Rebin(A,6,6)
- Eigenvec: finds eigenvectors

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

Basic Programming

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

- ENDCASE

- CASE name OF

Basic Programming

- DO-loop
- FOR n=0, number DO a[n] = fix(n)
- FOR n=0, number DO BEGIN
- a[n] = fix(n)
- B[n] = 2.0*a[n]^2 + 4.0*a[n] + 6.0 $
- + 8.0*a[n]^(-2.5)

- ENDFOR
- NOTE: $ is for continuation.

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

Plot and Processing

- Making a New Dataset
noisy = ((RANDOMU(SEED, 200) - .5) / 2)

original = SIN((FINDGEN(200)/35)^2.5)

- Plot
PLOT, noisy

- Plot them together
PLOT, original, THICK = 3, XTITLE = "Time", YTITLE ="Amplitude“

OPLOT, original+noisy

IDL Examples

IDL> a = sin(findgen(100)/99. * 2 * !pi)

IDL> help, a

IDL> plot, a

IDL> a = shift(alog(abs(fft(dist(256),1))),128,128)

IDL> isurface, a

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