Importing data

In most cases you will have far too much data to want to type it in.

E.g. to read data from am ascii file generated by MODTRAN (a radiative transfer code) You have a data file ( lecjm/public_html/Teaching/IDL_course/Data/pldat.dat) containing 14 columns of numbers:

 14000.   0.714  7.97E-31  1.56E-26  2.10E-07  4.12E-03  4.11E-08  1.18E-07  2.32E-03  2.94E-08  3.29E-07  6.44E-03  1.64E-05 0.3950
 14100.   0.709  4.95E-31  9.84E-27  2.13E-07  4.24E-03  4.15E-08  1.18E-07  2.35E-03  2.90E-08  3.31E-07  6.59E-03  4.96E-05 0.3940
 14200.   0.704  3.05E-31  6.15E-27  2.11E-07  4.25E-03  4.17E-08  1.15E-07  2.32E-03  2.81E-08  3.26E-07  6.58E-03  8.22E-05 0.3887
 14300.   0.699  1.84E-31  3.77E-27  1.90E-07  3.90E-03  4.06E-08  1.03E-07  2.11E-03  2.56E-08  2.93E-07  6.00E-03  1.12E-04 0.3734
 14400.   0.694  1.13E-31  2.35E-27  1.87E-07  3.88E-03  4.05E-08  9.90E-08  2.05E-03  2.46E-08  2.86E-07  5.93E-03  1.40E-04 0.3664
 14500.   0.690  6.53E-32  1.37E-27  1.60E-07  3.36E-03  3.74E-08  8.57E-08  1.80E-03  2.17E-08  2.46E-07  5.17E-03  1.65E-04 0.3446
.
.
.
 33800.   0.296  0.00E+00  0.00E+00  2.64E-10  3.01E-05  2.62E-10  2.91E-18  3.33E-13  1.99E-20  2.64E-10  3.01E-05  3.27E-03 0.0001
 33900.   0.295  0.00E+00  0.00E+00  1.68E-10  1.93E-05  1.67E-10  1.52E-19  1.75E-14  1.05E-21  1.68E-10  1.93E-05  3.27E-03 0.0000
 34000.   0.294  0.00E+00  0.00E+00  1.82E-10  2.10E-05  1.80E-10  1.83E-20  2.11E-15  1.26E-22  1.82E-10  2.10E-05  3.27E-03 0.0000

The columns are 201 rows long. We want to read this into a new variable called ``data''. Create a file (modtran.pro) containing the program below and run it in IDL using: modtran, data

pro modtran, data

;+
;Reads and plots modtran data from a particular file pldat.dat
;Hugh Pumphrey (a long time ago) modified by John Marsham (11/04/2005)
;-

;;;;;; Set up array to contain the  data ;;;;;;;;;;;;;;;
data=fltarr(14,201)

;;;;;; Open the file, attach it to unit no. 1 ;;;;;;;;;
openr,1,'~lecjm/public_html/Teaching/IDL_course/Data/pldat.dat'

;;;;;; read the data from the file attached to unit 1 ;;;;;;;
;;;;;; into the array modtrandata
readf,1,data
;;;;IF THE DATA WAS BINARY YOU'D USE READU;;;;;

;;;;;; Close the file ;;;;;;
close,1

;;;;;; Now make the plot ;;;;;;
plot,data(1,*),data(11,*), $
  xtitle='Wavelength (microns)',$
  ytitle='Radiance  (Watts/(cm!U2!N Steradian Micron))'

;;;;;; Don't forget the 'end' statement ;;;;;;

end

Then type: help,data.

Note that we have to know in advance how many rows and columns there are. We'll look at how to get around this later. Note also that you call this with: ``modtran, newvar'' or ``modtran, blah'', or whatever you like.

For an IDL array with n elements, the individual elements are numbered from 0 to n-1 , just as in C or Java. They are not numbered from 1 to n as in Fortran.

The data has 201 rows and 14 columns and is defined by an array using modtrandata=fltarr(14,201)=fltarr(ncolumns,nrows) .

(0,200) (1,200) (2,200) ... (13,200)
  .       .       .            .
  .       .       .            .
(0,2)   (1,2)   (2,2)   ... (13,2) 
(0,1)   (1,1)   (2,1)   ... (13,1)
(0,0)   (1,0)   (2,0)   ... (13,0)