Here is my take on the subject:
The
if statement works like this
(if (this is true)
(then do this)
)(if (this is true)
(then do this)
(else do this when false)
)The
(cond) stmt on the other hand executes the all of code following
a true condition and then exits the condition stmt.
(cond
((= 1 0)
none of this will get executed
because the conditions is false
) ; end cond 1
((= 1 1)
do all of this because it is true
and this
and this
) ; end cond 2
((= 2 2)
none of this will get
executed even if it is true
because the prior stmt was true
) ; end cond 3
(T
This is often placed at the last position
in a condition statement and will be executed
if all the prior conditions are false
if any one of the above conditions are true
this will not be executed
) ; end cond 4
) ; end cond stmtThe
OR will process each line as long as they are
false, it will quit when
true is returned
(or (a. is true) ; stop here
(b. does not get this far)
)(or (a. is false) ; keep going
(b. do this & if false keep going)
(c. do this if b was false)
)So this will work as well with a variable with a
value or
nil(setq var "Some Text")
(or var ; Stop here if the var has a value, if nil then keep going
(setq var "Some Text")
)The
AND will process each line as long as they are
true, it will quit when
false is returned
(and (a. is true) ; keep going
(b. do this & if true keep going)
(c. do this if b was true)
)Note that
AND &
OR themselves only return
true or
nil,
whereas
IF and
COND will return the value of the expression.
(setq rtn
(or (+ 3 7) ; returns 10 which is not nil so it passes the test for true
(+ 2 4 6) ; return 12
)
)While the value in rtn is
t(setq rtn
(if (+ 3 7) ; returns 10 which is not nil so it passes the test for true
(+ 2 4 6) ; return 12
)
)The value in rtn is 12
(setq rtn
(if (> 3 7) ; 3 is not > 7 so this returns nil
(+ 2 4 6) ; return 12
)
)The value in rtn is
nil because there was no else line to consider
(setq rtn
(if (> 3 7) ; 3 is not > 7 so this returns nil
(+ 2 4 6)
(- 10 2)
)
)No supprise here, the rtn value is 8
The cond statement works simular to the if
Here is an example I like
(initget 0 "Yes No")
(setq ans
(cond
((getkword "\nGive your answer. [Yes/No] <Yes>: "))
("Yes")
)
)If the user hits the enter key
nil is returned and the
cond returns "Yes"
If the user enters text the text is returned
Some examples of
(prognNote that
progn returns the value of the LAST evaluated expression.
(if (this is true)
(progn
do all of this
and this
and this
)
(don't do this)
); endif
(if (this is false)
(don't do this)
(progn
do all of this
and this
and this
)
)So you see the progn is a way to group code together.
The
(if) stmt will execute the group of code when
true and
will execute the second group of code when
false,
if there is a second group of code. If you forget the
(progn) only the first
line of code will be executed if
true and the rest skipped.
There are other ways to group code together and you will learn them in time.
Outa Gas,
I know it's more that you wanted to know.
See ya.
The
(cond) stmt on the other hand executes all of the code following
a
true condition and then exits the condition stmt. Remember
true is anything
not nil.
(cond
((= 1 0)
none of this will get executed
because the conditions is false
) ; end cond 1
((= 1 1)
do all of this because it is true
and this
and this
) ; end cond 2
((= 2 2)
none of this will get
executed even if it is true
because the prior stmt was true
) ; end cond 3
(T
This is often placed at the last position
in a condition statement and will be executed
if all the prior conditions are false
if any one of the above conditions are true
this will not be executed
) ; end cond 4
) ; end cond stmtHere is a trick for the
cond statement when you want to conditionally execute the
cond.
Often we see it like this:
(if (something is true) ; reminder, 'true' here is really anything (not nil)
(cond
((= 1 1) ...)
((= 2 2) ...)
) ; end cond stmt
) ; endifIt can be written like this so that if something is
not true the first condition is true &
nothing is done but the
cond statement is satisfied that it found a
true condition.
The processing skips the remaining conditions.
(cond
((not (something is true))) ; if something is not true stop here
((= 1 1) ...)
((= 2 2) ...)
) ; end cond stmtThe
WHILE function as a conditional loop and continues the loop until the next
expression returns
nil. Here we test the value in cnt looking for it to become 0
so that we can exit the
while loop. The code within the loop is executed 5 times,
while the cnt value is 0 through 4.
(setq cnt 0)
(while (= cnt 5)
;; do some stuff
(setq cnt (1+ cnt))
)It can also be written like this and the cnt value is 0 through 4.
(setq cnt -1)
(while (= (setq cnt (1+ cnt)) 5)
;; do some stuff
)Sometimes the condition you want to test is set at the end of the code within
the loop. The code may look like this.
(setq continue_loop t)
(while continue_loop
;; do some stuff
(if (I want to leave the loop is true)
(setq continue_loop nil) ; flag to exit the loop
)
)This can also be written like this:
(setq continue_loop t)
(while
(progn
;; do some stuff
(not (I want to leave the loop is true)) ; flag to exit the loop
) ; progn
)So you see that when
(I want to leave the loop is true) returns
true, the
not will
return
nil and because it is the last expression in the progn, that is the value the
while sees. It then exits the loop.
Here is a real world example.
This is my solution for entering numbers like key words
The simulated key words would be "90 180 270 -90"
(while
(progn
(setq rang
(cond ((getint "\nEnter the rotation angle [90/180/270] <90>"))
(90)))
(if (not (vl-position rang '(90 180 270 -90)))
(not (prompt "\nError Angle must be 90 180 270, please re-enter.")))
)
)Added a little more.
