Progn

I am still learning about "progn",but rather difficult to understand it,can
someone's inform to me,how to interpreting it with simple and clear,and give
me sample,here original from autocad help.
(if (= a b)
(progn
(princ "\nA = B ")
(setq a (+ a 10)
b (- b 10))
))
Thanks a lot for kind
Best regards
Ade Suharna

 

The progn function is used when more than one function is to be called when
only 1 function is allowed.
This is mainly used by the IF function to allow more than 1 expression to be
evaluated.

IF function in it's most basic form - only 1 expression evaluated when the
IF function evaluates as true and 1 for false
(if (= A B)
(princ "\nA = B") ;1 line for if True
(Princ "\nA /= B") ;1 line for if False
)

Complex Example -
(if (= A B)
(progn
(princ "\nA = B") ;1st line for true expression
(setq AB T) ;2nd line for true expression
..... ;more lines for true expression
)
(progn
(princ "\nA /=B") ;1st line for false expression
(setq AB nil) ;2nd line for false expression
..... ;more lines for true expression
)
)

I lined your program up to better show the multiple expressions
(if (= a b)
(progn ;start of true expression
(princ "\nA = B ") ;1st line for true expression
(setq a (+ a 10) ;2nd line for true expression
b (- b 10) ;continuation of 2nd line for true
expression
)
) ;end of true expression
)

Another approach could be -
(if (= a b)
(and
(princ "\nA = B")
(setq A (+ A 10)
B (- B 10)
)
)
)

 

Hi Alan,thanks a lot for your suggest