IDL 5220 Week 1 - Programming Basics

1. Definition of a Program
   A series of commands that manipulate the values of variables to solve a problem.
2. IDL
   IDL, Interactive Data Language, is a powerful array-based language. Although procedures can be compiled, it is generally an interpreted language. Interpreted languages are slightly slower than compiled ones such as C++, but much easier to debug. They are also far less structured as they do not require stuff like variable declarations. IDL programs can be written using any text editor such as vi or emacs. Programs should be given a filename ending in .pro. To enter IDL, just type idl at a command prompt. Since it is interpreted, you can enter a command or a series of commands at an IDL prompt instead of writing an entire program in some cases. To run a program, type .run program_name at an IDL prompt. You do not need to include the .pro from the program's name. The same command is used for compiling a procedure. The procedure is then run simply by typing its name at an IDL prompt, along with any required arguments.
3. IDL Program Structure
   Programs must have "end" as their last line. There is no need to declare variables. IDL is dynamically typed: an operation on a variable can change that variable's type.
4. Primitives
   
   • Data Types:
     There are 8 basic, atomic data types used in IDL. The more important ones include Integer (16-bit so values can only go up to 32,767), Long (same as integer but 32-bit so values can range to +/- 2 billion), Floating-point (32-bit, +/- 10^38), Double-precision (64-bit, +/- 10^308), and Strings (sequences of characters).
   • Conversion Functions:
     STRING: convert to string
     FIX: convert to integer
     LONG: convert to long
     FLOAT: convert to float
     DOUBLE: convert to double precision
     
   • Other ways of conversion:
     5 / 2 = 2 because 5 and 2 are both integers, but if you want to convert the result to a floating point, try 5 / 2., which = 2.5. The decimal point added onto the 2 makes it a floating point, and the result of expressions in IDL is always the type with the highest precision. Examples: 5 / 2 + 1. = 3. while 5 / 2. + 1 = 3.5
     
5. Variables
   
   • Definition:
     Variables are references to memory locations where data is stored. This data can be stored, recalled, and manipulated.
     
   • ASSIGNMENT STATEMENT
     You can assign a value to a variable with the assignment statement. Examples:
     X = 5
     valueOfPi = 3.14
     text2 = "Go Gators!"
     text3 = '"quote"'
     Strings can be delimited by either " or '.
     A variable can have a name up to 128 characters long. It must start with a letter but it can contain numbers, _, and $. Spaces and @ are not allowed. Names are not case sensitive and also can not be reserved words such as PRINT, FOR, DO, AND, etc.
     When the program executes an assignment statement X = 5, it stores the value 5 in memory. It then creates a reference for this memory location named X. X can now be substituted for 5 anywhere in the rest of the program.
     
6. Maths and Expressions
   • Order of Operations:
     First: ^ (exponentiation)
     Second: * (multiplication), / (division), MOD (modulus), # and ## (matrix multiplication)
     Third: + (addition), - (subtraction and negation),  < (minimum),  > (maximum), NOT (boolean negation)
     Fourth: EQ (equality), NE (not equal), GT, GE, LT, LE
     Fifth: AND, OR, XOR
     
   • Expressions:
     Expressions are read right to left. For instance the command B=B+1 is actually executed in IDL by pusing 1 onto the program stack, then the value of B. Then it adds the two values and then stores the result back in B.
   • Examples:
     A = 5 + 3^2 : stores 14 in A
     bool = 5 gt 6 : stores FALSE (represented by 0) in bool
     s = "String"
     x = s eq "String": stores TRUE (represented by 1) in x
     (5 + 3) * 2  > 5 * ( 3 + 2): returns the value 25
     
7. Input/Output
   • Print:
     Output is accomplished by the PRINT command. For instance: print,5*3 will print 15 to the screen. print,y will print the contents of the variable y to the screen.
     Examples:
     

     IDL > print,5*3
           15
     IDL > y="test"
     IDL > print,y
     test
     IDL > print,5*3," This is a "+y
           15 This is a test
     

   • Read
     Sometimes, it is necessary to prompt the user for input. This is done via the READ command: read, [prompt], variable. The prompt is optional. The READ command will pause until the user enters something from the keyboard and then it will store that value in the specified variable. If the user is to input a string, the variable must be declared to be a string before the read statement to avoid a conversion error. For example: S = ''
     Examples:
     read,x : prompts the user to enter a number to be stored in x.
     name = '' : sets the variable name to be a string
     read, "Enter your name: ",name : prompts the user to enter their name, which will be stored in the variable name.
     
   • Hello Example (command prompt):
     
     • Type idl at a command prompt
     • name = ''
     • read, 'Enter Your Name? ',name
     • print,"Hello " + name + '.'
   • Hello Example (program):
     
     • create a text file called hello.pro (vi hello.pro if you use vi)
     • name = ''
     • read, 'Enter Your Name? ',name
     • print,"Hello " + name + '.'
     • end
     • Save and close the file
     • Type idl at a command prompt
     • .run hello
8. Blocks:
   Blocks can be used in place of a single command. Blocks consist of a BEGIN, a series of commands, and an END.
9. Conditionals
   Conditionals and Loops are the two fundamental building blocks of programs. Conditionals execute a command based on the result of an expression.

            /-\        |-| 
   P--------|B|---B----|S|-----Q
            \_/        |_|  |
             |___!B_________|
   

   If B then S
   
   • If-Then
     If expression then command
     If expression then begin
     ...commands...
     endif
     Example: If a gt 5 then print,"A is higher than 5"
     
   • If-Then-Else
     If B then S0 else S1
     If expression then begin
     ...commands...
     endif else begin
     ...commands...
     endelse
     Example:

     If name eq 'Your_Name' then begin
       print,'Hi '+name 
       print,"Go Gators!"
     endif else begin
       if name ne '' then print,"Wrong name." else print,"You didn't enter anything"
       print,"Goodbye."
     endelse
     

   • Case
     Case offers you multiple options based on the value of a variable. It is preferable to multiple layers of if a eq 1 then ... else if a eq 2 ... else if a eq 3 ... else...
     CASE expression OF
     Expression: Command
     ...
     Expression: Command
     Else: Command
     endcase
     Example: The variable name contains the name of a b-ball player

     CASE name OF
        'Matt Bonner': print,"senior forward"
        'Brett Nelson': print,"senior shooting guard"
        'Justin Hamilton': print,"senior point guard"
        else: print,"this player is not a senior"
     endcase
     

     The else clause is optional but must come at the end if included.
10. Loops
    Loops repeat a series of commands while a certain condition is true. Loops have the structure {I} while B do S {I and !B}. I is the loop invariant that must be true before, during, and after the loop, and B is the condition. While B is true, the loop continues. When B is made false, the program exits the loop.

            ____________________
            |                  |
           /-\        |-|      |
    ---I---|B|---B----|S|----I-|
           \_/        |_|
            |
            |_____!B______--- >
    

    • For Loops
      For loops are a special case of loops that allow you to do a specified number of iterations of the loop. FOR variable = expression, expression DO command. The variable is called an index variable and is only defined during the loop. The first expression is the starting value for the index, and the second expression is the limit expression. The variable is incremented by 1 until the limit expression is reached.
      Example: for J = 1, 4 do print, j^2 will result in:
      1
      4
      9
      16
      A second type of FOR loop lets you specify the increment. Example:

      x=0
      for j = 4, 1, -1 do begin
         x=x+j^2
         print,x
      endfor
      Results in:
      16
      25
      29
      30

    • While Loops
      While loops, as seen above execute the loop until the expression becomes false. WHILE expression DO command. In WHILE statements, the loop is never executed if the expression is initially false.
      Example:

      x=1
      while x lt 10 do begin
        x=2*x+1  
        print,"X="+string(x)
      endwhile
      Results in:
      X=  3
      X=  7
      X=  15

    • Repeat-Until Loops
      Repeat loops are the opposite of While loops. The condition is checked after the command is executed, so the loop is always executed at least once. Repeat loops go until the condition is true. REPEAT command UNTIL expression.
      Example:

      x=1
      repeat begin
         x=2*x+1
         print,"X="+string(x)
      endrep until x ge 10
      Does the same thing as the while example above.
      

11. Random #s
    A random number between 0 and 1 with a uniform distribution can be generated by executing the following statement:
    RANDOMU(seed)
    So in order to set X to a random number between 1 and 100, you would type:
    X = fix(randomu(seed)*100)+1
    randomu(seed)*100 is done first since it is in parenthesis and results in a floating point random number between 0 and 100. fix converts this to an integer by truncation, meaning it is now between 0 and 99, inclusive. Adding 1 makes the number between 1 and 100, inclusive.
    
12. Help and ?
    • help:
      typing help at an IDL prompt will display information about all variables, procedures, and functions currently defined. help,x for instance, would just print out information about the variable x.
    • ?
      IDL's online help menu is accessible by typing ? at an IDL prompt. You can search the manual for information on any command or keyword.