Writing Loops and Conditional Statements in IDL

1
Writing Loops and Conditional Statements in IDL

• Matt Jolly
• NTSG, Univ. of MT.

2
Outline

• Review of topics from last time
• Definition of loops and conditionals
• Introduction to the IF statement
• Introduction to the FOR loop

3
Review

• You can create your own IDL programs in four
  ways Batch files, Main-level programs, Functions
  and Procedures
• The two most often used are Functions and
  Procedures
• Functions return a single value, Procedures
  return as many values as desired

4
Todays Quick Tip

• You can save a IDL variable or entire session for
  later work using the SAVE command
• SAVE, Filenamemywork.sav
• Later, you can restore all the IDL variables you
  were working on using the restore command
• RESTORE, Filenamemywork.sav

5
Program Control Statements

• Two main categories
• Decision Does a value meet a certain criteria?
• Repetition Perform certain operations or
  calculations sequentially

6
Decisions

• True or False expressions
• Use the IF statement
• Multiple decisions use the CASE statement
• General form
• IF lttestgt THEN ltstatementgt
• lttestgt is any boolean comparison that returns
  true or false
• ltstatementgt is any valid IDL syntax or block of
  syntax

7
Conditional Testing

• IDL does not use traditional programming style
  comparisons such as gt , lt or
• It uses the equivalent in text, similar to Perl
• For example if you wanted to print the variable
  X if variable X is greater than or equal to Y
  then you would type
• IF X GE Y THEN print, X

8
Comparison Operators

• EQ Equal To ()
• LT Less Than (lt)
• LE Less Than or Equal To (lt)
• GT Greater Than (gt)
• GE Greater Than or Equal To (gt)
• NE Not Equal To (ltgt or !)

9
Multiple Line Statements

• Statements can be blocked in IDL using the BEGIN
  END commands
• This tells IDL to execute them as a block, rather
  than individually
• The syntax for IDL blocked IF statements is then
  an extension of the simpler case
• IF lttestgt THEN BEGIN
• ..
• ENDIF

10
Extending IF with ELSE

• You can extend the IF statement by specifying an
  alternative set of statements to perform if the
  condition is NOT met
• IF lttestgt THEN BEGIN
• Print, X
• ENDIF ELSE BEGIN
• Print, Y
• ENDELSE

11
The CASE statement

• The case statement is an extension of the IF
  statement for cases where the test is not boolean
• For a CASE statement there is one input and based
  on the value of that input, there could be any
  number of program responses
• In fact, all case statements could be written
  with multiple IFs

12
Example CASE Statement

• CASE myVAR OF
• 0 x 5
• 1 BEGIN
• X 5
• Y X 2
• END
• ELSE x 0
• ENDCASE
• Example from Fanning text
• In this case, if myVar was equal to 0, then the
  program would set X 5

13
Repetition (Loops)

• Two kinds of repetition fixed count and variable
  count
• Fixed loops are ALWAYS performed in a program and
  variable loops are similar to IF statements and
  are only done while certain conditions are met
• Fixed count repetition is usually done with a FOR
  loop
• Example Loop through all the values in an array
  of a known or determinable size
• Variable count is done with a WHILE or REPEAT
  control statement

14
FOR Loops

• FOR loops repeat an operation one or more times
• This is one of the most useful tools in IDL and
  one you should master quickly
• You can use FOR loops to cycle through all the
  elements in a data array and perform some
  operations
• Many loops in IDL are IMPLICIT, such as when we
  multiply an array by a scalar value
• myArr fltarr(10)
• myArr myArr 10
• This is essentially an implicit for loop

15
FOR Loop syntax

• You must define an index variable and the limits
  of the loop
• FOR i0,9 DO myArri I

16
Loops can be nested

• Nested loops are loops INSIDE other loops
• At NTSG, this a fairly common because it is
  easier to loop through rows and columns of a
  raster dataset
• They are written the same way
• FOR row0,10 DO BEGIN
• FOR col0,10 DO BEGIN
• Print,myRasterrow,col
• ENDFOR
• ENDFOR

17
Multiple Line For Loops

• FOR loops are extended to multiple lines the same
  way IF statements were with BEGIN and END
  commands
• FOR i0,9 DO BEGIN
• myArri I
• Print,myArri
• ENDFOR

18
Variable Length Loops

• Main decision do you want to try looping through
  the data at least once no matter what?
• If so, use REPEAT. If not, use WHILE
• For REPEAT, the program statements are executed
  at least once and then tested at the end of the
  list of program statements
• For WHILE, the condition must be met before the
  loop is started
• Both cases, the statements are executed WHILE or
  UNTIL a requirement is met

19
WHILE Loops

• Only starts if the initial condition is met and
  then continues until the condition is not met
• WHILE (x lt 100) DO BEGIN
• X X 0.35
• ENDWHILE

20
REPEAT Loops

• Always execute the code AT LEAST once
• REPEAT BEGIN
• X X 0.35
• ENDREP UNTIL X GE 100

21
Differences Between WHILE and REPEAT

• WHILE loop may NEVER function if the condition is
  not met and REPEAT always does it at least once
• WHILE executes when a condition is met
• REPEAT executes until a condition is not met

22
The Exercise

• Use the program we wrote last time to analyze the
  Montana DEM and only display the values greater
  than 2000 meters