Variables, data types

As with most languages, IDL has several types of variables. I list some of them here:

type	range	bytes	to define	to convert
byte	0 to 255	1	b=15B	b=byte(x)
integer	-32768 to +32767	2	i=15	i=fix(x)
long	-2147483648 to +2147483647	4	j=long(15)	j=long(x)
			j=147483647
floating pt	$\pm 10^{38}$, 7 sig figs	4	y=1.7	y=float(x)
double prec.	$\pm 10^{308}$, 14 sig figs	8	y=1.7d0	d=double(x)
complex	two floating point no.s	8	z=complex(1.2,0.3)	z=complex(x)
string	(used for text)	0-32767	s='blah'	s=string(x)

Variable names can have letters, numbers and underscores in them. They are NOT case-sensitive: foo, Foo and FOO are all the same variable.

Note that the default type of integer is only a 2-byte integer. If you want a long integer you have to remember to ask for it. Remember if you want a big loop index variable! BUT remember loops are BAD in IDL.

You can convert one variable from one form to another using fix. Eg

str_var=fix(153.15,type=7) ;sets str_var to a string
help,str_var 
;STR_VAR         STRING    = '      153.150'; shows that it is a string, but it has much white space
;remove this with
str_var=strtrim(str_var,2); 2 removes white space from the front and back

Again I do this so often that I have a routine called trmfix that combines fix,type=7 and strtrim.

IDL is a dynamically typed language. That means that you don't have to define your variables or say which variable is which type at the start of your program. You can say gak=37.5d0 at any point in your program and a double precision variable called gak will spring into existence and take on the value 37.5 . If you had a variable of a different type called gak at some earlier point in your program, it will vanish when you create the double precision variable with the same name.