slang.tm

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   The language supports several different types of statements such as
   assignment statements, conditional statements, and so forth.  These
   are described in detail in the following sections.

\sect{Variable Declaration Statements}
   Variable declarations were already discussed in chapter ???.  For
   the sake of completeness, a variable declaration is a statement of
   the form
\begin{tscreen}
     variable \em{variable-declaration-list} ;
\end{tscreen}
   where the \em{variable-declaration-list} is a comma separated list
   of one or more variable names with optional initializations, e.g.,
#v+
     variable x, y = 2, z;
#v-
\sect{Assignment Statements} #%{{{

   Perhaps the most well known form of statement is the \em{assignment
   statement}.  Statements of this type consist of a left-hand side,
   an assignment operator, and a right-hand side.  The left-hand side
   must be something to which an assignment can be performed.  Such
   an object is called an \em{lvalue}.
   
   The most common assignment operator is the simple assignment
   operator \var{=}.  Simple of its use include
#v+
      x = 3;
      x = some_function (10);
      x = 34 + 27/y + some_function (z);
      x = x + 3;
#v-
   In addition to the simple assignment operator, \slang
   also supports the assignment operators \var{+=} and \var{-=}.
   Internally, \slang transforms
#v+
       a += b;
#v-
   to
#v+
       a = a + b;
#v-
   Similarly, \exmp{a -= b} is transformed to \exmp{a = a - b}.  It is
   extremely important to realize that, in general, \exmp{a+b} is not
   equal to \exmp{b+a}.  This means that \exmp{a+=b} is not the same
   as \exmp{a=b+a}.  As an example consider
#v+
      a = "hello"; a += "world";
#v-
   After execution of these two statements, \var{a} will have the
   value \exmp{"helloworld"} and not \exmp{"worldhello"}.

   Since adding or subtracting \exmp{1} from a variable is quite
   common, \slang also supports the unary increment and decrement
   operators \exmp{++}, and \exmp{--}, respectively.  That is, for
   numeric data types, 
#v+
       x = x + 1;
       x += 1;
       x++;
#v-
   are all equivalent.  Similarly,
#v+
       x = x - 1;
       x -= 1;
       x--;
#v-
   are also equivalent.

   Strictly speaking, \var{++} and \var{--} are unary operators.  When
   used as \var{x++}, the \var{++} operator is said to be a
   \em{postfix-unary} operator.  However, when used as \var{++x} it is
   said to be a \em{prefix-unary} operator.  The current
   implementation does not distinguish between the two forms, thus
   \var{x++} and \var{++x} are equivalent.  The reason for this
   equivalence is \em{that assignment expressions do not return a value in
   the \slang language} as they do in C.  Thus one should exercise care
   and not try to write C-like code such as
#v+
      x = 10;
      while (--x) do_something (x);     % Ok in C, but not in S-Lang
#v-
   The closest valid \slang form involves a \em{comma-expression}:
#v+
      x = 10;
      while (x--, x) do_something (x);  % Ok in S-Lang and in C
#v-

   \slang also supports a \em{multiple-assignment} statement.  It is
   discussed in detail in section ???.

#%}}}

\sect{Conditional and Looping Statements} #%{{{

  \slang supports a wide variety of conditional and looping
  statements.  These constructs operate on statements grouped together
  in \em{blocks}.  A block is a sequence of \slang statements enclosed
  in braces and may contain other blocks. However, a block cannot
  include function declarations.  In the following,
  \em{statement-or-block} refers to either a single
  \slang statement or to a block of statements, and
  \em{integer-expression} is an integer-valued expression.
  \em{next-statement} represents the statement following the form
   under discussion.

\sect1{Conditional Forms} #%{{{
\sect2{if}
   The simplest condition statement is the \kw{if} statement.  It
   follows the syntax
\begin{tscreen}
        if (\em{integer-expression}) \em{statement-or-block}
        \em{next-statement}
\end{tscreen}
   If \em{integer-expression} evaluates to a non-zero result, then the
   statement or group of statements implied \em{statement-or-block}
   will get executed.  Otherwise, control will proceed to
   \em{next-statement}.

   An example of the use of this type of conditional statement is
#v+
       if (x != 0) 
         {
            y = 1.0 / x;
            if (x > 0) z = log (x);
         }
#v-
   This example illustrates two \var{if} statements where the second
   \var{if} statement is part of the block of statements that belong to
   the first.

\sect2{if-else}
   Another form of \kw{if} statement is the \em{if-else} statement.
   It follows the syntax:
\begin{tscreen}
      if (\em{integer-expression}) \em{statement-or-block-1}
      else \em{statement-or-block-2}
      \em{next-statement}
\end{tscreen}
   Here, if \em{expression} returns non-zero,
   \em{statement-or-block-1} will get executed and control will pass
   on to \em{next-statement}. However, if \em{expression} returns zero,
   \em{statement-or-block-2} will get executed before continuing with
   \em{next-statement}.  A simple example of this form is
#v+
     if (x > 0) z = log (x); else error ("x must be positive");
#v-
   Consider the more complex example:
#v+
     if (city == "Boston")
       if (street == "Beacon") found = 1;
     else if (city == "Madrid") 
       if (street == "Calle Mayor") found = 1;
     else found = 0;
#v-
   This example illustrates a problem that beginners have with
   \em{if-else} statements.  The grammar presented above shows that
   the this example is equivalent to
#v+
     if (city == "Boston")
       {
         if (street == "Beacon") found = 1;
         else if (city == "Madrid")
           {
             if (street == "Calle Mayor") found = 1;
             else found = 0;
           }
       }
#v-
   It is important to understand the grammar and not be seduced by the
   indentation!

\sect2{!if}

   One often encounters \kw{if} statements similar to
\begin{tscreen}
     if (\em{integer-expression} == 0) \em{statement-or-block}
\end{tscreen}
   or equivalently,
\begin{tscreen}
     if (not(\em{integer-expression})) \em{statement-or-block}
\end{tscreen}
   The \kw{!if} statement was added to the language to simplify the
   handling of such statements.  It obeys the syntax
\begin{tscreen}
     !if (\em{integer-expression}) \em{statement-or-block}
\end{tscreen}
   and is functionally equivalent to
\begin{tscreen}
     if (not (\em{expression})) \em{statement-or-block}
\end{tscreen}

\sect2{orelse, andelse}

  These constructs were discussed earlier.  The syntax for the
  \var{orelse} statement is:
\begin{tscreen}
     orelse {\em{integer-expression-1}} ... {\em{integer-expression-n}}
\end{tscreen}
  This causes each of the blocks to be executed in turn until one of
  them returns a non-zero integer value.  The result of this statement
  is the integer value returned by the last block executed.  For
  example,
#v+
     orelse { 0 } { 6 } { 2 } { 3 }
#v-
  returns \var{6} since the second block is the first to return a
  non-zero result.  The last two block will not get executed.

  The syntax for the \var{andelse} statement is:
\begin{tscreen}
     andelse {\em{integer-expression-1}} ... {\em{integer-expression-n}}
\end{tscreen}
  Each of the blocks will be executed in turn until one of
  them returns a zero value.  The result of this statement is the
  integer value returned by the last block executed.  For example,
#v+
     andelse { 6 } { 2 } { 0 } { 4 }
#v-
  returns \var{0} since the third block will be the last to execute.

\sect2{switch}
  The switch statement deviates the most from its C counterpart.  The
  syntax is:
#v+
          switch (x)
            { ...  :  ...}
              .
              .
            { ...  :  ...}
#v-
   The `\var{:}' operator is a special symbol which means to test
   the top item on the stack, and if it is non-zero, the rest of the block
   will get executed and control will pass out of the switch statement.
   Otherwise, the execution of the block will be terminated and the process
   will be repeated for the next block.  If a block contains no
   \var{:} operator, the entire block is executed and control will
   pass onto the next statement following the \kw{switch} statement.
   Such a block is known as the \em{default} case.
   
   As a simple example, consider the following:
#v+
      switch (x)
        { x == 1 : message("Number is one.");}
        { x == 2 : message("Number is two.");}
        { x == 3 : message("Number is three.");}
        { x == 4 : message("Number is four.");}
        { x == 5 : message("Number is five.");}
        { message ("Number is greater than five.");}
#v-
   Suppose \var{x} has an integer value of \exmp{3}.  The first two
   blocks will terminate at the `\var{:}' character because each of the
   comparisons with \var{x} will produce zero.  However, the third
   block will execute to completion.  Similarly, if \var{x} is
   \exmp{7}, only the last block will execute in full.

   A more familiar way to write the previous example used the
   \kw{case} keyword:
#v+
      switch (x)
        { case 1 : print("Number is one.");}
        { case 2 : print("Number is two.");}
        { case 3 : print("Number is three.");}
        { case 4 : print("Number is four.");}
        { case 5 : print("Number is five.");}
        { print ("Number is greater than five.");}
#v-
   The \var{case} keyword is a more useful comparison operator because
   it can perform a comparison between different data types while
   using \var{==} may result in a type-mismatch error.  For example,
#v+
      switch (x)
        { (x == 1) or (x == "one") : print("Number is one.");}
        { (x == 2) or (x == "two") : print("Number is two.");}
        { (x == 3) or (x == "three") : print("Number is three.");}
        { (x == 4) or (x == "four") : print("Number is four.");}
        { (x == 5) or (x == "five") : print("Number is five.");}
        { print ("Number is greater than five.");}
#v-
  will fail because the \var{==} operation is not defined between
  strings and integers.  The correct way to write this to use the
  \var{case} keyword:
#v+
      switch (x)
        { case 1 or case "one" : print("Number is one.");}
        { case 2 or case "two" : print("Number is two.");}
        { case 3 or case "three" : print("Number is three.");}
        { case 4 or case "four" : print("Number is four.");}
        { case 5 or case "five" : print("Number is five.");}
        { print ("Number is greater than five.");}
#v-

#%}}}

\sect1{Looping Forms} #%{{{

\sect2{while}
   The \kw{while} statement follows the syntax
\begin{tscreen}
      while (\em{integer-expression}) \em{statement-or-block}
      \em{next-statement}
\end{tscreen}
   It simply causes \em{statement-or-block} to get executed as long as
   \em{integer-expression} evaluates to a non-zero result.  For
   example, 
#v+
      i = 10; 
      while (i) 
        {
          i--;
          newline ();
        }
#v-
   will cause the \var{newline} function to get called 10 times.
   However, 
#v+
      i = -10;
      while (i) 
        {
          i--;
          newline ();
        }
#v-
   would loop forever (or until \var{i} wraps from the most negative
   integer value to the most positive and then decrements to zero).

   
   If you are a C programmer, do not let the syntax of the language
   seduce you into writing this example as you would in C:
#v+
      i = 10;
      while (i--) newline ();
#v-
   The fact is that expressions such as \var{i--} do not return a
   value in \slang as they do in C.  If you must write this way, use
   the comma operator as in
#v+
      i = 10;
      while (i, i--) newline ();
#v-

\sect2{do...while}
   The \kw{do...while} statement follows the syntax
\begin{tscreen}
      do 
         \em{statement-or-block}
      while (\em{integer-expression});
\end{tscreen}
   The main difference between this statement and the \var{while}