idlwave: Lesson I---Development Cycle
3.1 Lesson I: Development Cycle
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The purpose of this tutorial is to guide you through a very basic
development cycle using IDLWAVE. We will paste a simple program into a
buffer and use the shell to compile, debug and run it. On the way we
will use many of the important IDLWAVE commands. Note, however, that
IDLWAVE has many more capabilities than covered here, which can be
discovered by reading the entire manual, or hovering over the shoulder
of your nearest IDLWAVE guru for a few days.
It is assumed that you have access to Emacs or XEmacs with the full
IDLWAVE package including online help. We also assume that you are
familiar with Emacs and can read the nomenclature of key presses in
Emacs (in particular, ‘C’ stands for <CONTROL> and ‘M’ for <META> (often
the <ALT> key carries this functionality)).
Open a new source file by typing:
C-x C-f tutorial.pro <RET>
A buffer for this file will pop up, and it should be in IDLWAVE mode,
indicated in the mode line just below the editing window. Also, the
menu bar should contain ‘IDLWAVE’.
Now cut-and-paste the following code, also available as
‘tutorial.pro’ in the IDLWAVE distribution.
function daynr,d,m,y
;; compute a sequence number for a date
;; works 1901-2099.
if y lt 100 then y = y+1900
if m le 2 then delta = 1 else delta = 0
m1 = m + delta*12 + 1
y1 = y * delta
return, d + floor(m1*30.6)+floor(y1*365.25)+5
end
function weekday,day,month,year
;; compute weekday number for date
nr = daynr(day,month,year)
return, nr mod 7
end
pro plot_wday,day,month
;; Plot the weekday of a date in the first 10 years of this century.
years = 2000,+indgen(10)
wdays = intarr(10)
for i=0,n_elements(wdays)-1 do begin
wdays[i] = weekday(day,month,years[i])
end
plot,years,wdays,YS=2,YT="Wday (0=Sunday)"
end
The indentation probably looks funny, since it’s different from the
settings you use, so use the <TAB> key in each line to automatically
line it up (or, more quickly, _select_ the entire buffer with ‘C-x h’,
and indent the whole region with ‘C-M-\’). Notice how different
syntactical elements are highlighted in different colors, if you have
set up support for font-lock.
Let’s check out two particular editing features of IDLWAVE. Place
the cursor after the ‘end’ statement of the ‘for’ loop and press <SPC>.
IDLWAVE blinks back to the beginning of the block and changes the
generic ‘end’ to the specific ‘endfor’ automatically (as long as the
variable ‘idlwave-expand-generic-end’ is turned on; 
Lesson
II---Customization). Now place the cursor in any line you would like
to split and press ‘M-<RET>’. The line is split at the cursor position,
with the continuation ‘$’ and indentation all taken care of. Use ‘C-/’
to undo the last change.
The procedure ‘plot_wday’ is supposed to plot the day of the week of
a given date for the first 10 years of the 21st century. As in most
code, there are a few bugs, which we are going to use IDLWAVE to help us
fix.
First, let’s launch the IDLWAVE shell. You do this with the command
‘C-c C-s’. The Emacs window will split or another window will popup to
display IDL running in a shell interaction buffer. Type a few commands
like ‘print,!PI’ to convince yourself that you can work there just as
well as in a terminal, or the IDLDE. Use the arrow keys to cycle
through your command history. Are we having fun now?
Now go back to the source window and type ‘C-c C-d C-c’ to compile
the program. If you watch the shell buffer, you see that IDLWAVE types
‘.run "tutorial.pro"’ for you. But the compilation fails because there
is a comma in the line ‘years=...’. The line with the error is
highlighted and the cursor positioned at the error, so remove the comma
(you should only need to hit ‘Delete’!). Compile again, using the same
keystrokes as before. Notice that the file is automatically saved for
you. This time everything should work fine, and you should see the
three routines compile.
Now we want to use the command to plot the day of the week on January
1st. We could type the full command ourselves, but why do that? Go
back to the shell window, type ‘plot_’ and hit <TAB>. After a bit of a
delay (while IDLWAVE initializes its routine info database, if
necessary), the window will split to show all procedures it knows
starting with that string, and ‘plot_wday’ should be one of them.
Saving the buffer alerted IDLWAVE about this new routine. Click with
the middle mouse button on ‘plot_wday’ and it will be copied to the
shell buffer, or if you prefer, add ‘w’ to ‘plot_’ to make it
unambiguous (depending on what other routines starting with ‘plot_’ you
have installed on your system), hit <TAB> again, and the full routine
name will be completed. Now provide the two arguments:
plot_wday,1,1
and press <RET>. This fails with an error message telling you the ‘YT’
keyword to plot is ambiguous. What are the allowed keywords again? Go
back to the source window and put the cursor into the “plot” line and
press ‘C-c ?’. This shows the routine info window for the plot routine,
which contains a list of keywords, along with the argument list. Oh, we
wanted ‘YTITLE’. Fix that up. Recompile with ‘C-c C-d C-c’. Jump back
into the shell with ‘C-c C-s’, press the <UP> arrow to recall the
previous command and execute again.
This time we get a plot, but it is pretty ugly: the points are all
connected with a line. Hmm, isn’t there a way for ‘plot’ to use symbols
instead? What was that keyword? Position the cursor on the plot line
after a comma (where you’d normally type a keyword), and hit ‘M-<Tab>’.
A long list of plot’s keywords appears. Aha, there it is, ‘PSYM’.
Middle click to insert it. An ‘=’ sign is included for you too. Now
what were the values of ‘PSYM’ supposed to be? With the cursor on or
after the keyword, press ‘M-?’ for online help (alternatively, you could
have right clicked on the colored keyword itself in the completion
list). A browser will pop up showing the HTML documentation for the
‘PYSM’ keyword. OK, let’s use diamonds=4. Fix this, recompile (you
know the command by now: ‘C-c C-d C-c’), go back to the shell (if it’s
vanished, you know what to do: ‘C-c C-s’) and execute again. Now things
look pretty good.
Let’s try a different day. How about April fool’s day?
plot_wday,1,4
Oops, this looks very wrong. All April Fool’s days cannot be
Fridays! We’ve got a bug in the program, perhaps in the ‘daynr’
function. Let’s put a breakpoint on the last line there. Position the
cursor on the ‘return, d+...’ line and press ‘C-c C-d C-b’. IDL sets a
breakpoint (as you see in the shell window), and the break line is
indicated. Back to the shell buffer, re-execute the previous command.
IDL stops at the line with the breakpoint. Now hold down the SHIFT key
and click with the middle mouse button on a few variables there: ‘d’,
‘y’, ‘m’, ‘y1’, etc. Maybe ‘d’ isn’t the correct type. CONTROL-SHIFT
middle-click on it for help. Well, it’s an integer, so that’s not the
problem. Aha, ‘y1’ is zero, but it should be the year, depending on
delta. Shift click ‘delta’ to see that it’s 0. Below, we see the
offending line: ‘y1=y*delta...’ the multiplication should have been a
minus sign! Hit ‘q’ to exit the debugging mode, and fix the line to
read:
y1 = y - delta
Now remove all breakpoints: ‘C-c C-d C-a’. Recompile and rerun the
command. Everything should now work fine. How about those leap years?
Change the code to plot 100 years and see that every 28 years, the
sequence of weekdays repeats.
Info Catalog
idlwave: Getting Started
idlwave: Lesson II---Customization