Importing data

You now know how to make a line plot of some data. In most cases you will have far too much data to want to type it in. Typically the data will be in a file and you will want IDL to read it in and then make a plot of it. As an example, we will use some numbers generated by the MODTRAN radiative transfer package. You have a file containing 14 columns of numbers, like this:

 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. Let us suppose that we want to plot the 12th column (which is total radiance) against the 2nd column (which is wavelength). Here is a short IDL program which will do this (as you can tell I didn't get the full data file off Hugh, so the plot is rather meaningless).

pro read_plot_modtran

;+
;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 ;;;;;;;;;;;;;;;
modtrandata=fltarr(14,201)

;;;;;; Open the file, attach it to unit no. 1 ;;;;;;;;;
openr,1,'/nfs/env-fs-04_u35/jmarsham/public_html/Teaching/IDL_course/pldat.dat'

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

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

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

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

end

Note that we have to know in advance how many rows and columns there are in the file. We'll look at how to get around this later in the course.

Note also that 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.

Also note that the data has 201 rows and 14 columns and we defined our array using modtrandata=fltarr(14,201) .