## Using And

Discussion of Scheme

### Using And

Lets say we have:

((and (something1)(something2)(something3)(something4)) #t)

How does it compute it, does it test if all four are true or does it in pairs like:

((something1)and(something2)=true and((something3)and(something4)=true)) Then the whole thing is true???

Thanks.
dsjoka

Posts: 14
Joined: Thu Feb 10, 2011 1:30 pm

### Re: Using And

From the R5RS standard, and is defined as

Code: Select all
`-- library syntax: and <test1> ...,     The <test> expressions are evaluated from left to right, and the     value of the first expression that evaluates to a false value (see     section *note Booleans::) is returned.  Any remaining expressions     are not evaluated.  If all the expressions evaluate to true     values, the value of the last expression is returned.  If there     are no expressions then #t is returned.     (and (= 2 2) (> 2 1))                  ==>  #t     (and (= 2 2) (< 2 1))                  ==>  #f     (and 1 2 'c '(f g))                    ==>  (f g)     (and)                                  ==>  #t`

nuntius

Posts: 449
Joined: Sat Aug 09, 2008 10:44 am
Location: Burlington, MA

### Re: Using And

dsjoka wrote:Lets say we have:

((and (something1)(something2)(something3)(something4)) #t)

Your expression appears malformed and would generate an error (unless quoted); I am guessing you want something such as:
(and (something1)(something2)(something3)(something4) #t)

dsjoka wrote:How does it compute it, does it test if all four are true or does it in pairs like:

In Scheme, 'and' is a special form (not a function). It evaluates each of its arguments one at a time, stopping when that evaluation is false (subsequent arguments will not be evaluated). If the end of all arguments is reached without any falses, the value of the last argument is returned.

(and w x y z)
is equivalent to
(if w (if x (if y z)))

Note that since 'and' is not a function, you can not use it with either 'map' or 'apply'.
saulgoode

Posts: 38
Joined: Tue Dec 14, 2010 1:39 am

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