;; CMPU101, Spring 2013 
;; Lecture #5

;; Read pages 42-47 of posted readings for discussion of if and
;; cond special forms.

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;; Making decisions--IF special form:
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;;
;; One of the most critical operations done by computer programs
;; is decision making. For example, a program may need to test
;; if a number has a particular value or is within some range of 
;; values.
;;
;; The most basic decision-maker is the IF special form. This 
;; special form is best suited to the situation where there are 
;; only 2 possible outcomes, #t or #f. 
;;
;; The general shape of an IF special form is:
;;
;;   (if  questionExpression  
;;        questionTrueReturn  
;;        questionFalseReturn)
;;
;; The keyword IF is followed by three arguments, separated by 
;; white space (that includes tabs, line breaks, etc). Naturally, 
;; the entire expression is enclosed in parentheses as are all
;; special forms.
;;
;; The if special form is evaluated as follows: 
;;
;;      - The questionExpression is always evaluated.
;;
;;      - If the questionExpression is true, the
;;        questionTrueResult is evaluated and the
;;        questionFalseResult is not evaluated.
;;
;;      - Else, if the questionExpression is false, 
;;        the questionTrueResult is not evaluated and 
;;        the questionFalseResult is evaluated.
;;
;;      - The questionTrueResult and questionFalseResult
;;        are never both evaluated.
;;
;;
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;;
;; Here is an example function that could use an if special form:
;;
;; Ex1: Define a function to return the inverse of its
;;      input for non-zero integers or 0 if input is 0.

;; CONTRACT: 

;; HEADER:

;; PURPOSE:  

;; PRE-FUNCTION TESTS:

;; FUNCTION DEFINITION:
;(define inverse 
;  (λ (x)
;    ...))
    

;;  In the event the result of the first expression is true, then 
;;  the second expression is evaluated (the true part); otherwise 
;;  the third expression is evaluated (the false part). Whichever 
;;  expression is evaluated, it is also the result (output) of the  
;;  entire if expression.

;;--------------

;; Ex:  Write a function called odd-or-even that consumes a natural number
;;      and returns the string "odd" if the number is odd and "even" if the
;;      number is even.

;;-------------- 

;; Ex:  Write a function called letter-grade that takes a natural number 
;;      less than or equal to 100 as input and produces the string
;;      "A" if the number is greater than or equal to 85, "B" if the number
;;      is less than 85 but greater than or equal to 72, "C" if the
;;      number is less than 72 but greater than or equal to 60, "D" if the
;;      number is less than 60 but greater than or equal to 50, and "F" if the 
;;      number is less than 50. Use "nested ifs".

;;-------------- 


; 
;  One of the most common errors beginner programmers make is to use an if 
;  statement when only a single boolean expression is needed.
;  


; Ex6: Write a function called may-drive? that consumes a number representing 
;      the age of a person and returning true if the age is >= 16 and false
;      otherwise.


; Nested Conditionals:
;   
;   You can write one decision statement inside another, e.g., an if inside 
;   a cond clause, a cond inside a cond clause, an if inside an if, or a cond 
;   inside an if. These are called nested ifs and nested conds.
;   


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;;
;; The letter-grade function requires the "cascading" of if expressions.
;; Cascading ifs can be hard to read and understand.
;;
;; For decisions that have more than 2 possibilities, it is more 
;; convenient to use another special form decision maker:  COND
;;
;; The general shape of a COND special form is:
;;
;;    (cond
;;      [question1 answer1]          
;;      [question2 answer2]          
;;      ...                          
;;      [else answerN])        
;; 
;; where question1...questionN are predicates (inside parens) 
;; that each evaluate to a boolean, and answer1...answerN evaluate 
;; to the return value of the cond statement. The [question answer] 
;; pairs are called CLAUSES.
;;
;; ELSE is a keyword that evaluates to #t and it can be replaced by 
;; #t in the final clause.
;;
;; The cond statement evaluates in the following way: if question1 
;; is true, evaluate answer1; otherwise if question1 is false, skip
;; answer1 to evaluate question2. If question2 is true, evaluate 
;; answer2; otherwise if question2 is false, skip answer2 to evaluate 
;; question3... 
;;
;; If none of the questions evaluates to true there is an error. That's
;; why the else is given at the end that is always evaluated when none
;; of the clauses that come before are evaluated.
;;
;; Only one answer is ever evaluated in a cond, just like the if.

;;----------------
;;
;; Ex: Rewrite the letter-grade function written above so that
;;     it uses a cond expression instead of a series of if state-
;;     ments.
;;
;;----------------
;; 
;; Ex: Develop a function classify that tells what type of data its 
;;     input is by returning one of the strings "image", "string",
;;     "number", "boolean", or "other", as appropriate.