COP 3223H meeting -*- Outline -*-

* Conditionals in Python

** conditional expressions
------------------------------------------
          CONDITIONAL EXPRESSIONS

Purpose:



Syntax:

  <expr> ::= <expr> if <expr> else <expr>

Examples:

  n / d if (d != 0.0) else 10.0
  sqrt(n) if n>0 else 0.0

Semantics:





------------------------------------------
   ... to give a value that depends on the program's state

   Show the equivalent syntax in C <expr>2 ? <expr>1 : <expr>3

   Examples in C:
       d!=0.0 ? n/d : 10.0
       n>0 ? sqrt(n) : 0.0

   The condition (following if) is evaluated first,
      if the condition is True, then the value is the value of the
      first expression (before if),
      otherwise the value is the value of the last expression (after else)

------------------------------------------
      AND, OR ARE SYNTACTIC SUGARS

e1 and e2  ==>  
e1 or e2   ==>  


So "and", "or" are short-curcuit operators,
the only evaluate their second argument
if necessary

 Examples:

     d!=0 and n/d > 3.0
     n<0 or sqrt(n) < 100.0
     
------------------------------------------
   ... e2 if e1 else e1 # e1 is False in the else case
   ... e2 if (not e1) else e1  # e1 is True in the else case

   However, these desugarings are a bit of a lie, since e1 is not
            evaluated twice as it would appear

   The term "syntactic sugar" means a translation within a language
   that defines the semantics of one thing (the sugar)
   in terms of another (the desugared thing)

** conditional statements
------------------------------------------
       IF-STATEMENTS

Purpose:



Syntax:

  <stmt> ::= if <expr> :
                <suite>
           | if <expr> :
                <suite>
             else:
                <suite>
           | if <expr> :
                <suite>
             <elif-list>
           | if <expr> :
                <suite>
             <elif-list>
             else:
                <suite>
           | ...
  <elif-list> ::= elif <expr> :
                    <suite>
                | <elif-list>
                  elif <expr> :
                    <suite>

Examples:

    if i < len:
       sum = sum + i

    if i < len:
       sum = sum + i
    else:
       pass

    if num == 1:
       return "one"
    elif num == 2:
       return "two"
    elif num == 3:
       return "three"
    else:
       return "more"

Semantics:
    valuates the first <expr>,



------------------------------------------
   ... to decide, based on the program's state,
       what actions to have the program take

   ... if that is True, then does the corresponding <suite>
       otherwise evaluates each <expr> in order (from top left),
       and runs the corresponding <suite> if it is True,
       if none are True, then runs the <suite> following else:, if any

   Show the syntax in C (but can simplify):
        <stmt> ::= if ( <expr> ) <stmt>
                 | if ( <expr> ) <stmt> else <stmt>
                 | { <block> }

        <block> ::= <decl-list-opt> <stmt-list>

        <decl-list-opt> ::=  | <decl> | <decl-list> <decl>

        <stmt-list> ::= <stmt> | <stmt-list> <stmt>

------------------------------------------
            FOR YOU TO DO

Write a function max(x,y) that returns the larger
of the numbers x and y passed as arguments,
so that if after
    res = max(x,y)
the following holds:
    res >= x and res >= y.







------------------------------------------

        ... def max(x,y):
               """Return the maximum of arguments x and y."""
               if x > y:
                   return x
               else:
                   return y