August 2013

SCALARS, VECTORS, PLOTTING

NOTE: commands intended to be typed into your terminal during your IDL session are given in CAPITAL letters below so they will stand out. However, IDL itself is case-insensitive (except for file names), so you needn't follow this convention. 0) From a UNIX shell, start X-windows if it is not already running. From a shell in the X-window, "cd" into your "IDL" directory. Normally that would be named "~/idl". Start IDL. [On most UNIX systems, just type "idl". This will start IDL running in the command-line mode using the window from which you called it. In IDL V.5+ you can use GUI-based "Development Environment" or "Workbench" versions of IDL. These offer convenience features to the experienced user but are correspondingly more complicated and not recommended for learning the basics.] Before starting the exercises, give the IDL command: ON_ERROR,1 This will return control to the main program level if an error occurs in a called routine. 1) PRACTICE WITH SCALARS Define X and Y to be two different integers Print their sum and difference to the terminal screen Repeat, all on one IDL command line, using the "&" character Repeat all of the above, using the recall buffer ("up" arrow) Print the result of adding 3 times X to 2 times Y Print the product of (X-Y) and (X+Y) Print the common logarithm of the absolute value of X+Y (use the ABS and ALOG10 functions) Confirm this result by raising 10.0 to the power you just obtained What is the difference between X = 5/2 and Y = 5./2 ? Between X = FLOAT(5/2) and Y = FLOAT(5)/2 ? Set C equal to the numerical value of the speed of light in cgs units Successively print powers of C until you locate the point at which the result becomes "Inf". What power yields this result? [The answer is non-integer.] Now define C to be the speed of light in double precision. Repeat the exercise above. [Hint: you should accelerate all such exercises by making liberal use of the recall buffer and line editing.] String example: define a string variable for each word in the sentence: "This is a concatenation"; then concatenate them and print the result. 2) PRACTICE WITH VECTORS Create X = INDGEN(100). [NOTE on IDL syntax: INDGEN is a function. In IDL the arguments of a function are enclosed in parentheses. In IDL versions 1 through 4, the indices of vector or array variables were also enclosed in parentheses, as in X(10). Obviously, this introduced possible ambiguities between function and array notation. Therefore, starting with IDL version 5, array indices are expected to be enclosed by brackets, as in X[10]. You should use this syntax when referring to array elements. This notation for array elements is not compatible with IDL V.4 and earlier. However, IDL versions 5 and later do accept the parenthesis syntax for array elements, so are compatible with earlier IDL code in this regard.] Use built-in IDL functions N_ELEMENTS, MIN, MAX, and TOTAL to answer the following: How many elements does X contain? What is the minimum value in X? What is the maximum value in X? What is the sum of all the elements in X? Use the IDL HyperHelp facility to obtain information on the built-in TOTAL routine you just used: ?TOTAL. Experiment on X or other vectors you create with some of the special keywords available for TOTAL. Print X to your terminal window How are values in X related to the corresponding subscripts? Is X a floating-point array? Compare the output you just got with the result of PRINT,FINDGEN(100). Print the fifteenth entry in X to your terminal Then print the entry containing the number 15 Is X a row-vector or a column-vector? The default configuration of the printed data on your screen will tell you. To confirm this, try the following: PRINT,TRANSPOSE(X) Try the following one-line command and inspect its output: FOR I=0,99 DO PRINT,I,X[I] Now try: FOR I=1,100 DO PRINT,I,X[I] Using the WHERE function: Define Q=2*X, then type FIND = WHERE(Q LE 40, COUNT) FIND will be a vector. Examine the contents of FIND and COUNT so that you understand how the WHERE function operates. Predict and confirm the response if you type PRINT,Q(FIND) Print to your terminal the values of the vector X/10 Then print the values of X/10.0 and compare the results. Do the same for FLOAT(X)/10 Do the same for FIX(X/10.0) What is the difference between Z = X*0.0 and Z = 0? Print the 11 elements centered on X = 10 First do this using a FOR loop (on a single line) Then do this using the standard IDL subscript range notation, e.g. X[2:6]. No FOR loop is needed. Compare to the following: K=5 & PRINT,X[10-K:10+K] Define Y to be the subarray consisting of those 11 elements, using subscript range notation; print Y to your window as a check. Using information utilities: Verify lengths of X,Y using the N_ELEMENTS utility Use the SIZE utility to find the sizes of X and Y; what other info does it supply? What information does the command HELP,X,Y provide? Define Q = Y+3 -- Note the values that Q contains Define Q = Y*3 -- Note the values that Q contains Define Q = Y^3 -- Note the values that Q contains Define Q = Y^4 -- Note the values that Q contains; why are they not monotonic? Define Q = FLOAT(Y)^4 -- Note the change. Print the vector which results from Q = X*Y What did IDL actually do to arrive at this result? Using the built-in functions TOTAL and N_ELEMENTS: Find and print the mean values of X and Y; Find and print the variances of the two arrays Do the same using the built-in functions MEAN and VARIANCE. Do the same using the built-in function MOMENT Determine the nature of the vector which results when you write Q = X & Q[2] = Y. What will happen if you write Q[98] = Y? Try it. Predict, then verify, the outcome of the following operations: Q = [Y,Y] Q = [Q,Y] Create a 16-element vector [1.01, 1.02, 1.03...] using a simple one-line command employing FINDGEN. Verify. Create a 16-element vector: [1,2,4,8,16....] using a simple one-line command employing FINDGEN. Verify. Create the 100-element vector Z = 10*X - 0.1*X^2, where X contains the integers between 0 and 99. Use MAX to determine the maximum value of Z. Use WHERE to locate the X value for which this maximum occurs. X was an integer vector but Z is not. Why? Using the SHIFT function, shift the elements of Z three entries to the left. Verify that the maximum is now in the expected location. Create, using a simple one-line command employing FINDGEN, a 1001-element vector containing the base-10 logarithms of the integers between 0 and 1000. Name this "ILOG". Verify its contents. Now create a five element vector, Y, containing the integers 2, 100, 500, 20, and 999. Explain the vector resulting when you type NEW = ILOG[Y]. Why did we include an entry for 0 in defining ILOG? Optional problem: ILOG in the previous exercise is called a "lookup table." It can be used to accelerate computations in problems where a large number of time-consuming transformations such as logarithms are needed. You can estimate the time savings for this example as follows: Write a simple one-line IDL command script using "&" as a link between individual commands. Use the SYSTIME(1) function to determine the start time. Then, use a FOR loop to compute the logarithm of an integer 40000 times using the standard ALOG10 function. Then use SYSTIME(1) again to determine the end time. Print the elapsed time. Repeat, now using the "ILOG" lookup table instead of ALOG10. 3) JOURNAL FILES AND SCRIPT FILES Use the JOURNAL function to start a journal file & record your session for posterity. Test starting & stopping a journal file. Inspect the file to see what kinds of communications between you & the IDL session are actually recorded. [Note that journal files are actually written to disk by IDL only at long intervals or after another JOURNAL command or EXIT.] Practice cut-and-pasting commands from a saved journal file listing into an active IDL window. Practice generating a "script" file from edited parts of an IDL journal file. Save it, rename it, and use it to re-create the original session by using the "@" command. Note that comments can be added at any time to the journal by prefacing remarks with a semicolon. 4) BASIC PLOTTING Type "WINDOW,0" to bring up a plotting window. Move the cursor into the window. Troubleshooting: If the other parts of your terminal screen blink out or change color, then you are using an 8-bit (rather than 24-bit) color monitor for which the X-windows system is not properly configured. To ameliorate the problem, try this: Exit IDL Restart IDL As the very first two commands, type: DEVICE,PSEUDO_COLOR=8 WINDOW,0,COLORS=200 If this does not remedy the blinking, then other X-windows applications have "reserved" too many display colors. You may be able to reduce the color hogging using various "Preference" or other settings on your applications. Exit other applications (e.g. browsers) before starting IDL. If this doesn't work, for the moment the best approach is just to live with the blinking and try later to reconfigure. For reference, you may want to call up the on-line help documentation for the plotting keywords by typing: ?PLOT Create the 100-element vector Z = 10*X - 0.1*X^2, where X is the vector containing the integers between 0 and 99. Type PLOT,X,Z. Examine the plot, noting the abscissa, ordinate, and default scaling adopted. Do the same for PLOT,X,2*Z. Note the change. Type OPLOT,X,Z and note what happens Do the same for PLOT,X,Z^2 Do the same for PLOT,Z^2,X Use the optional plotting parameters XRANGE and YRANGE with PLOT,X,Z^2 to enlarge various parts of the plot of the Z^2 function. Using the XLOG or YLOG keywords, plot log_10(X) vs. Z and X vs. log_10(Z). (Note: you'll have to limit the X,Y scales using the RANGE parameters to avoid infinities and plot compression. Keep the variable on the log axis greater than or equal to 1.0.) Label any of these plots, using the !P.TITLE, !X.TITLE, and !Y.TITLE system variable strings. All of the plots should have appeared in Window 0. Try alternating successive plots between Window 0 and Window 1. [NOTE: The intrinsic IDL routines for manipulating windows are as follows: To create a new window, use WINDOW,N. To expose an existing window, use WSHOW,N. To make a given window "active"--- i.e. ready for I/O---use WSET,N. The MOUSSE routine CHAN,N combines these three functions and is more convenient.] Create the 1000-element vector x, where x contains the FLOATING-POINT conversion of integer values between -500 and +499. Create Z = 10*X - 0.1*X^2 . Explore plotting features using PLOT,X,Z as in the previous exercise. Type PLOT,Z. How does the resulting plot differ from that for PLOT,X,Z? Create the 1000-element vector X, where x contains the INTEGER values between -500 and +499. Create Y = 10*X - 0.1*X^2. Type PLOT,X,Y. Why does the Y function differ from Z? Now define X to contain the integers in the range 0 to 100. Then compute Z = SIN(X)/X. Is Z a floating point variable? Print the value of Z at X = 0. If you weren't sure how IDL would respond there, how would you manually insert a value Z = 1.0 at X = 0? Do so. Confirm that the Z vector is now defined everywhere. Using the PSYM and LINESTYLE keywords, plot Z vs. X for integer values of X in the range 0 to 100: with a solid line then with plus signs then with open triangles then with a dashed line [Hint: for help with the keywords, open the online IDL HyperHelp system by typing "?" as the first element of the command line. Then enter "GRAPHICS KEYWORDS" in the search box.] Plot Z vs. X for the X range [0:10] with open triangles. Then, using OPLOT, add a solid line overplot. You can achieve the same result with a single PLOT command for your choice of plotting symbol, K, by using PSYM=-K rather than PSYM=K. Calculate Z = SIN(X)/X at intervals of 0.01 for X in the range 0 to 10. Plot Z vs X. Using the WHERE function, find all the locations where Z has an absolute value smaller than 0.05. How many are there? Print Z for all those locations (but only those locations) to your terminal. Print X for all those locations (but only those locations) to your terminal. Plot Z vs X using a solid line. Now overplot open triangles at those points you found where Z has an absolute value smaller than 0.05. You can verify graphically that these fall in the expected positions by plotting horizontal lines at Z = 0.05 and -0.05 (hint: define and plot two new vector functions of X). 5) EXPLORING PARAMETER SPACE Copy the program bio.pro from the IDLexercises/data directory to your local directory. This routine computes and displays a well-known simple function exhibiting "chaotic" behavior for certain choices of input parameters. To see the code and the header, type .RUN -T BIO . To see just the header section, type MAN,'BIO . [Note: MAN is a MOUSSE routine.] Run the program as suggested in its header to explore the behavior of this function as you change the two input parameters. Note the effects of small changes in these around the critical values. 6) ITERATED PLOTTING Graphically solve the transcendental equation X + 6.0*EXP(-X/2) = 5.0 using iterated plots on the screen. Use successively finer scale numerical grids to improve your estimate to 3 significant digits. Verify the solution numerically. Optional: It can be faster to solve the problem using the WHERE function. Do so. 7) MAKING PLOT HARDCOPIES Make a hardcopy of any one of your plots using a PostScript intermediary To send plots to a PostScript file instead of your terminal, type SET_PLOT,'PS. Then repeat the same plotting commands you gave to put the plot on your terminal (easiest to use the recall buffer). Close the file using the command DEVICE,/CLOSE. The name of the file will be "idl.ps". Print the file from within IDL by putting $ as a preface to the appropriate UNIX command as the first entry on the command line. Return to using your terminal as your output device by giving SET_PLOT,'X . Repeat, checking the status of your output device by using the HELP,/DEV command. 8) USING HISTOGRAMS AND "SAVE" FILES I recommend you use the IDL Astronomy Users Library procedure PLOTHIST for these exercises. You can accomplish the same thing with the IDL built-in HISTOGRAM routine and separate plotting commands, but this is awkward. PLOTHIST is simply an accelerator program which combines several built-in commands for convenience. To check that PLOTHIST is in your IDL path, simply type PLOTHIST. If present, the procedure responds by printing the syntax for the command to your terminal screen. If not present, you'll get an error message. [Note that this technique for checking syntax does not work for IDL built-in functions, but you can use the HELP,/ROU procedure or the HyperHelp facility (type ?) instead there.] The latest version of PLOTHIST requires software from the "Coyote" library of graphics programs, written by David Fanning. If your IDL installation does not include those, download them from this site. Put the downloaded tar file in a directory in your IDL path, then unzip and untar the file. The programs will be automatically compiled when you request them. Assuming PLOTHIST is in your path, then type: MAN,'PLOTHIST to see the header of the program; type .RUN -T PLOTHIST to see the whole program. Copy the file "grades.sav" from the IDLexercises/data directory to your local directory. This is a specially-formatted file that contains data from an earlier IDL session preserved by a "SAVE" command. Using the RESTORE command, "restore" the dataset to your IDL session. The save file contained the variable "GRADELIST". This is a set of actual final point scores for a UVa course. How many students were in the class? Determine the MEAN, MEDIAN, MAXIMUM, MINIMUM, and standard deviation (using VARIANCE) for the set of scores. Use PLOTHIST to produce a histogram of the scores The default bin size in PLOTHIST is always 1.0, regardless of the range of the variable being plotted. Using the BIN keyword, experiment with different bin sizes in the range 1 to 15. Graphically determine the mode of the scores and note how it changes with binsize. What is the mode for a binsize of 5? Plot the score histogram for a binsize of 5 with labels for the axes and the plot. Overplot a vertical line at the location of the median value. (Hint: use the PLOTS command) Make a hardcopy of the final plot via a PostScript file. 9) SORTING & PRINTING LISTS; WRITING AN ASCII FILE Continue using the "GRADELIST" vector of part (8) Using the SORT function, define a new list (call it "SCOREORDER") of scores in order from lowest to highest value. Verify. Now define and verify a new list (call it "RANKSCORE") of scores in order from highest to lowest value. (Hint: use REVERSE) Now define an auxiliary vector ("RANK") which gives the rank of each student in the class (assigning 1 to the student with the highest grade) Using a (one-line) FOR loop, an appropriately defined format string, and the PRINT command, print on your terminal the rank and score of the students in order from highest to lowest. Hint: the definition for an IDL format string might look like the following. (IDL format statements are similar, but not identical, to those in FORTRAN). FORMOUT='(A12,3X,I3,3X,F8.3,3X,E10.3)' and the command to print would look like this: FOR I=0,MAXI DO PRINT,FORMAT=FORMOUT,VAR1[I],VAR2[I].... Then, use the AstUseLib utility FORPRINT to do the same thing Use your rank-listing capability to determine the median of the grade distribution. Does it agree with the value returned by MEDIAN? Now assign letter grades to the students. Assume you are a tough grader and use a strict curve where the top 10% of the class gets an "A", the next 10% gets a "B" and so on. The lower 60% of the class will get an F in this scheme (appropriate, e.g., for a pre-Med class). Using STRARR, define a string array named "LETTER" to hold the letter grades for the students. Fill it with blanks initially. Consult the ranked list of scores from your terminal printouts and determine the letter grade break points. Then assign values to the "LETTER" array using those break points. [Hint: use the WHERE function.] "LETTER" should be in the order of the original "GRADELIST" Verify the assignments by using a FOR statement and a new format definition to print on your terminal a list of rankings, numerical scores, and letter grades in ranked order. WRITE AND VERIFY AN ASCII FILE Now create an output ASCII file on your disk and output to it, in the original (not ranked) order: the numerical grade, the class rank, and the final letter grade for each student. [Hint: You already have defined an appropriate format string. The other commands you will need are GET_LUN, OPENW, PRINTF, and CLOSE.] Verify your results (e.g. use $MORE). Now reverse the process and read the disk file back into your IDL session, using variables with different names. [Hint: You will need GET_LUN, OPENR, READF, and CLOSE.] Verify the results by comparing the variables. 10) ASTROPHYSICS PLOTTING PROBLEM #1 Make a plot of the energy distribution functions B_nu, B_lam, and N_lam, which are the flux per unit frequency, the flux per unit wavelength, and the number of photons per unit wavelength, respectively, for a black body (Planck law) at a temperature of 10000 K. All three functions should be displayed on a convenient scale on the same plot; each function should be normalized to its maximum. Plot against wavelength in Angstroms, covering the range 1000 to 10000 A. Label the plot properly. Make a hardcopy of the plot and save the resulting PostScript file. If you want to see a sample solution (J. Oishi, Fall 2001; PostScript file), click here. Optional: write an IDL "procedure" which will compute any of the three versions of the Planck function listed above for a given wavelength vector and temperature. Put it in your IDL directory (and give it a unique name, like "plot_a_planck.pro"). Use a "keyword" input parameter to select between the three functions. To verify performance and documentation, let someone else try to use your procedure. You might want to compare your program to "planck.pro" in the Astronomy User's Library. 11) ASTROPHYSICS PLOTTING PROBLEM #2 Explore the sensitivity of optical band "colors" to the temperature of a black body as follows. Compute the Planck function B_lam for selected temperatures in the range 1000 K to 100000 K for wavelengths of 1500, 3600, 4400, 5500, 10000, and 22000 Angstroms. (It's up to you to choose appropriate intervals for the T grid.) Compute "colors" with respect to 5500 A in the form ALOG10( B_LAM[I] / B_LAM[3] ) for each T. Plot these against T or LOG T on the same plot. Label the axes. Label each curve (see the XYOUTS procedure). Make a hardcopy of the plot and save the resulting PostScript file. Consider the usefulness of the various combinations. Which is most sensitive over the T range 3000-30000K appropriate for normal stars? 12) INTEGRATION IDL supports numerical integration of analytic or tabular functions in one, two and three dimensions. Intrinsic IDL routines performing integration of analytic functions of a single variable include QROMB and QSIMP. The latter uses Simpson's Rule. These routines all use the same, somewhat awkward, method for defining the integrand. The user must place the name (i.e. an IDL string) of an IDL function defining the integrand in the calling sequence of the integration routine. In this example, we illustrate integration of a simple power law. We use an updated version of the intrinsic QSIMP routine that is included in the Astronomy User's Library. Although its Library name is also QSIMP, to avoid confusion we have renamed it NEWSIMP and placed a copy in the IDL Exercises directory. NEWSIMP uses the same basic algorithm as QSIMP, but it is faster than intrinsic QSIMP because it takes a vector, rather than scalar, argument. Also, it takes advantage of "keyword inheritance" to provide additional arguments for the integrand function. This eliminates the need to make up a new integrand file for each change in the functional form of the integrand. Copy the following programs to your local directory: newsimp.pro xpwr.pro. To see the coding and instructions for use of NEWSIMP, type: .run -t newsimp To see the integrand function for a power law, f(x) = x^p, type: .run -t xpwr To perform an integration with power law index 3 between the bounds 0 and 10, type: newsimp,'xpwr',0,10,answer,pwr=3 & print,answer Experiment with other values for the range and power law index, and verify that the integrations are correct. [Note that since you are dealing with filenames here, you must give them exactly as they appear in the UNIX directories.]

END OF IDL EXERCISES PART I Part II covers 2D arrays and image displays. Part III covers image processing. IDL Tutorial Home Page

Copyright © 2000-2013 Robert W. O'Connell. All rights reserved. Last modified by RWO, August 2013