1. Functions and Procedures

2. Introduction IDL programs fall into two classes: functions and procedures Functions: can encapsulate only one operation, and return one result Procedures: can encapsulate several operations, and return several results Therefore, procedures are preferred than functions Functions and procedures need to be compiled first before they are called in the main driver program. You may put all your functions and procedures in a separate file, and compile it before running your main program

3. Functions I: Intrinsic functions in IDL IDL provides over 750 intrinsic math and other functions. Please see IDL reference guide for a list of all intrinsic functions: http://idlastro.gsfc.nasa.gov/idl_html_help/Functional_List _of_IDL_Routines.html Often-used math functions: abs, sqrt, exp, alog, alog10, sin, cos, tan, asin, acos, atan, randomn, randomu print, sin(!pi/3.0) y=randomu(seed, 100) plot, y

4. Functions I: Intrinsic functions in IDL (cont.) Often-used data-processing functions: interpol, smooth, histogram y2=smooth(y, 11, /edge_truncate) Linear analysis: a_correlate, c_correlate, regress, linfit, curvefit lag=indgen(10) a=a_correlate(y,lag) plot,lag,a c=c_correlate(y,y2,lag) plot,lag,c Spectral analysis: FFT, wavelet f=fft(y) plot, f EOF/SVD analysis: Eigenvec, SVDC Statistical distributions: binomial, gauss_pdf, gauss_cvf, TM_test, CTI_test

5. Functions II: User-defined Functions A function begins with a “Function” statement containing function name, a list of input arguments, and/or a list of keywords. A function also includes a “return” statement to return the result to the caller, and ends with an “end” statement Function f_to_c, deg_f return, (deg_f - 32.0)*(5.0/9.0) end Calling sequence: Result = Function_name(argument1, argument2, …) deg_f = 80.0 deg_c = f_to_c(deg_f)

6. Procedures A procedure begins with a “Pro” statement containing procedure name, an argument list and/or a keyword list. A procedure ends with an “end” statement Pro f_to_c2, deg_f, deg_c deg_c = (deg_f - 32.0)*(5.0/9.0) end Calling sequence: Procedure_name, argument1, argument2, …) deg_f = 80.0 f_to_c2, deg_f, deg_c print, deg_c Example: A subroutine for overplotting a square box

7. Line plots: Logarithm, histogram and bar plots Logarithm: Plot, x, y, /xlog, /ylog Calculate histogram: density=histogram(data, max=max, min=min, $ nbins=nbins, locations=loc) binsize=(max-min)/(nbins-1) locations gives the locations of the starting point of each bin Plot histogram: plot, loc, density, psym=10 Bar plot: bar_plot, data, barnames=bn, $ title=tit, xtitle=xtit, ytitle=ytit

8. In-class assignment III Calculate y = x 0.5 + e x/100.0 + ln(x) + sin(  x) + cos(2  x) for x=3.0 Do the above calculation using a function for x=1.0, 3.0, 5.0 Do the above calculation using a procedure for x=2.2, 3.0, 6.0 X=findgen(200)/50.0+1.0, calculate y using one of the above methods. Assume lag=indgen(10), (1) apply an 11-point smoothing to y and get y2 (2) calculate and plot auto-correlation of y (3) calculate and plot cross-correlation of y and y2 (4) calculate and plot the histogram of y Write a subroutine for overplotting a triangle