The IF statement

if condition then statement 1 else statement 2

IDL> i=2
IDL> if i eq 2 then print,'I is two' else print, 'i is not two'
I is two

or

IDL> if i eq 2 and j eq 3 then print,'I is two and J is three' else print, 'i and j are not two and three respectively'

The logical operators look like the FORTRAN ones (but without the dots), not like the C ones.

Purpose	IDL	C	FORTRAN
Relational Operators
Equal to	eq	==	.EQ.
Not equal to	ne	!=	.NE.
Less than or equal to	le	$<=$	.LE.
Less than	lt	$<$	.LT.
Greater than or equal to	ge	$>$=	.GE.
Greater than	gt	$>$	.GT.
Boolean Operators
And	and	&&	.AND.
Not	not	!	.NOT.
Or	or	||	.OR.
Exclusive OR	xor