CODEWITHSUNDEEP
lisp
Adam Lee
Adam Lee

Reputation: 576

Lisp: How to write an if statement with multiple conditions?

I understand how to write an if statement with an individual condition. For example:

(if (> a 20))

However, after researching online, I found no resources explaining how to write an if statement with multiple conditions (for example, checking if a > 20 or a < 10). How do I write an if statement with multiple conditions in Lisp?

Upvotes: 0

Views: 2131

Answers (2)

Sylwester
Sylwester

Reputation: 48745

The if special form is the same no matter what:

(if test-expression 
    true-expression
    false-expression)

If you want something to happen if both a is above 20 or below 10 you just do:

(if (not (<= 10 a 20)) ; same as (or (> 20 a) (< 10 a))
    true-expression
    false-expression)

But I get it. What if you have two expressions where one of them true should trigger, then you can use or:

(if (or (> 20 a) (< 10 a))
    true-expression
    false-expression)

Now and and or are macros for nexted ifs. Eg. The expression above can be done like this:

(if (> 20 a)
    true-expression
    (if (< 10 a)
        true-expression
        false-expression)))

Where true-expression is the same expression both places.

Upvotes: 2

Barmar
Barmar

Reputation: 781004

Use the OR and/or AND macros to combine conditions.

(if (or (> a 20) (< a 10))
 ;; do something
)

Upvotes: 1

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