absolute.scm

(define (abs x)
    (cond ((= 0 x) 0)
          ((< 0 x) x)
          ((> 0 x)(- 0 x))))

(define abs
    (lambda (x)
    (cond ((= 0 x) 0)
          ((< 0 x) x)
          ((> 0 x)(- 0 x)))))

The genie appropriates the keyboard. “Let’s see, now...”

He types: (abs 14)
The machine responds: 14
He types: (abs -6)
The machine responds: 6
He types: (abs 0)
The machine responds: 0
“Very good.”

lists_and_lists.scm

[Welcome to the interpreter. Enter ":q" to exit, or ":m" for documentation, or ":?" for a list of other ":" commands.]
 >>(define cat '(100))
 (100)
 >>(define dog '(100))
 (100)
 >>:q
[Suspending interpreter. Press green button to reactivate.]

You lean back. The genie glances over, and asks, "Got it working yet?"

>yes
(to the genie)
The genie appropriates the keyboard. "Let's see, now..."

He types: cat
The machine responds: (100)
He types: dog
The machine responds: (100)
He types: (equal? cat dog)
The machine responds: t
He types: (eqv? cat dog)
The machine responds: nil
He types: (eqv? (cdr cat) (cdr dog))
The machine responds: t
"Perfect! There are actually two ways to solve this problem. You used the simpler one, using one-term lists. The trickier solution would be something like this:
(define tail '(end))
(define cat (cons 'head tail))
(define dog (cons 'head tail))
"The cdrs are eqv? because they are both the thing defined on the first line. See?"

recursive-sum-if.scm

“Define sum to be a function that adds up a list of integers. So (sum ‘(8 2 3)) should return 13. Make sure it works correctly for the
empty list; (sum nil) should return 0.”

> turn on computer
The computer returns to life...


[Welcome to the interpreter. Enter “:q” to exit, or “:m” for documentation, or “:?” for a list of other “:” commands.]
 >> (define sum  (lambda (ints)    (if (equal? ‘() ints)        0        (+ (car ints) (sum (cdr ints))))))
 [function]
 >> :q
[Suspending interpreter. Press green button to reactivate.]

You lean back. The genie glances over, and asks, “Got it working yet?”

> yes
(to the genie)
The genie appropriates the keyboard. “Let’s see, now...”

He types: (sum (quote (8 -5 16 25)))
[Error: undefined atom: if]
The machine responds: [ERROR]

sum-pt-1.scm

(define (print x) 
  (write x)
  (newline))

(define sum
  (lambda (ints)
    (if (equal? '() ints)
        0
        (car ints)
     )))

(print (sum '()))
(print (sum '(4)))
(print (sum '(2 7)))