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8. Statements

Loosely speaking, a statement is composed of expressions that are grouped according to the syntax or grammar of the language to express a complete computation. A semicolon is used to denote the end of a statement.

A statement that occurs within a function is executed only during execution of the function. However, statements that occur outside the context of a function are evaluated immediately.

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.

8.1 Variable Declaration Statements

Variable declarations were already discussed in the chapter on Variables. For the sake of completeness, a variable declaration is a statement of the form

variable variable-declaration-list ;
where the variable-declaration-list is a comma separated list of one or more variable names with optional initializations, e.g., 
     variable x, y = 2, z;

8.2 Assignment Statements

Perhaps the most well known form of statement is the 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 lvalue.

The most common assignment operator is the simple assignment operator =. Examples of its use include 

      x = 3;
      x = some_function (10);
      x = 34 + 27/y + some_function (z);
      x = x + 3;
In addition to the simple assignment operator, S-Lang also supports the binary assignment operators: 
     +=   -=   *=    /=   &=   |=
Internally, S-Lang transforms 
       a += b;
to 
       a = a + b;
Likewise a-=b is transformed to a=a-b, a*=b is transformed to a=a*b, and so on.

It is extremely important to realize that, in general, a+b is not equal to b+a. For example if a and b are strings, then a+b will be the string resulting from the concatenation of a and b, which generally is not he same as the concatenation of b with a. This means that a+=b may not be the same as a=b+a, as the following example illustrates: 

      a = "hello"; b = "world";
      a += b;                      % a will become "helloworld"
      c = b + a;                   % c will become "worldhelloworld"

Since adding or subtracting 1 from a variable is quite common, S-Lang also supports the unary increment and decrement operators ++, and --, respectively. That is, for numeric data types, 

       x = x + 1;
       x += 1;
       x++;
are all equivalent. Similarly, 
       x = x - 1;
       x -= 1;
       x--;
are also equivalent.

Strictly speaking, ++ and -- are unary operators. When used as x++, the ++ operator is said to be a postfix-unary operator. However, when used as ++x it is said to be a prefix-unary operator. The current implementation does not distinguish between the two forms, thus x++ and ++x are equivalent. The reason for this equivalence is that assignment expressions do not return a value in the S-Lang language as they do in C. Thus one should exercise care and not try to write C-like code such as 

      x = 10;
      while (--x) do_something (x);     % Ok in C, but not in S-Lang
The closest valid S-Lang form involves a comma-expression: 
      x = 10;
      while (x--, x) do_something (x);  % Ok in S-Lang and in C

S-Lang also supports a multiple-assignment statement. It is discussed in detail in the section on Multiple Assignment Statement.

8.3 Conditional and Looping Statements

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

Conditional Forms

if

The simplest condition statement is the if statement. It follows the syntax

if (integer-expression) statement-or-block next-statement
If integer-expression evaluates to a non-zero (boolean TRUE) result, then the statement or group of statements implied statement-or-block will get executed. Otherwise, control will proceed to next-statement.

An example of the use of this type of conditional statement is 

       if (x != 0)
         {
            y = 1.0 / x;
            if (x > 0) z = log (x);
         }
This example illustrates two if statements where the second if statement is part of the block of statements that belong to the first.

if-else

Another form of if statement is the if-else statement. It follows the syntax:

if (integer-expression) statement-or-block-1 else statement-or-block-2 next-statement
Here, if expression evaluates to a non-zero integer, statement-or-block-1 will get executed and control will pass on to next-statement. However, if expression evaluates to zero, statement-or-block-2 will get executed before continuing on to next-statement. A simple example of this form is 
     if (x > 0)
       z = log (x);
     else
       throw DomainError, "x must be positive";
Consider the more complex example: 
     if (city == "Boston")
       if (street == "Beacon") found = 1;
     else if (city == "Madrid")
       if (street == "Calle Mayor") found = 1;
     else found = 0;
This example illustrates a problem that beginners have with if-else statements. Syntactically, this example is equivalent to 
     if (city == "Boston")
       {
         if (street == "Beacon") found = 1;
         else if (city == "Madrid")
           {
             if (street == "Calle Mayor") found = 1;
             else found = 0;
           }
       }
although the indentation indicates otherwise. It is important to understand the grammar and not be seduced by the indentation!

ifnot

One often encounters if statements similar to

if (integer-expression == 0) statement-or-block
or equivalently,
if (not(integer-expression)) statement-or-block
The ifnot statement was added to the language to simplify the handling of such statements. It obeys the syntax
ifnot (integer-expression) statement-or-block
and is functionally equivalent to
if (not (expression)) statement-or-block

Note: The ifnot keyword was added in version 2.1 and is not supported by earlier versions. For compatibility with older code, the !if keyword can be used, although its use is deprecated in favor of ifnot.

orelse, andelse

As of S-Lang version 2.1, use of the andelse and orelse have been deprecated in favor of the && and || short-circuiting operators.

The syntax for the orelse statement is:

orelse {integer-expression-1} ... {integer-expression-n}
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, 
     orelse { 0 } { 6 } { 2 } { 3 }
returns 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 andelse statement is:

andelse {integer-expression-1} ... {integer-expression-n}
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, 
     andelse { 6 } { 2 } { 0 } { 4 }
evaluates to 0 since the third block will be the last to execute.

switch

The switch statement deviates from its C counterpart. The syntax is: 

          switch (x)
            { ...  :  ...}
              .
              .
            { ...  :  ...}
The `:' operator is a special symbol that in the context of the switch statement, causes the top item on the stack to be tested, 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 : operator, the entire block is executed and control will pass onto the next statement following the switch statement. Such a block is known as the default case.

As a simple example, consider the following: 

      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.");}
Suppose x has an integer value of 3. The first two blocks will terminate at the `:' character because each of the comparisons with x will produce zero. However, the third block will execute to completion. Similarly, if x is 7, only the last block will execute in full.

A more familiar way to write the previous example is to make use of the case keyword: 

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

Looping Forms

In this section, the various looping statements are discussed. Each of these statements support an optional then clause, which is discussed in a separate section below.

while

The while statement follows the syntax

while (integer-expression) statement-or-block [ then statement-or-block ] next-statement
It simply causes statement-or-block to get executed as long as integer-expression evaluates to a non-zero result. For example, 
      i = 10;
      while (i)
        {
          i--;
          newline ();
        }
will cause the newline function to get called 10 times. However, 
      i = -10;
      while (i)
        {
          i--;
          newline ();
        }
would loop forever (or until 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: 

      i = 10;
      while (i--) newline ();
Keep in mind that expressions such as i-- do not return a value in S-Lang as they do in C. The same effect can be achieved to use a comma to separate the expressions as in 
      i = 10;
      while (i, i--) newline ();

do...while

The do...while statement follows the syntax

do statement-or-block while (integer-expression); [ then statement-or-block ]
The main difference between this statement and the while statement is that the do...while form performs the test involving integer-expression after each execution of statement-or-block rather than before. This guarantees that statement-or-block will get executed at least once.

A simple example from the jed editor follows: 

     bob ();      % Move to beginning of buffer
     do
       {
          indent_line ();
       }
     while (down (1));
This will cause all lines in the buffer to get indented via the jed intrinsic function indent_line.

for

Perhaps the most complex looping statement is the for statement; nevertheless, it is a favorite of many C programmers. This statement obeys the syntax

for (init-expression; integer-expression; end-expression) statement-or-block [ then statement-or-block ] next-statement
In addition to statement-or-block, its specification requires three other expressions. When executed, the for statement evaluates init-expression, then it tests integer-expression. If integer-expression evaluates to zero, control passes to next-statement. Otherwise, it executes statement-or-block as long as integer-expression evaluates to a non-zero result. After every execution of statement-or-block, end-expression will get evaluated.

This statement is almost equivalent to

init-expression; while (integer-expression) { statement-or-block end-expression; }
The reason that they are not fully equivalent involves what happens when statement-or-block contains a continue statement.

Despite the apparent complexity of the for statement, it is very easy to use. As an example, consider 

     s = 0;
     for (i = 1; i <= 10; i++) s += i;
which computes the sum of the first 10 integers.

loop

The loop statement simply executes a block of code a fixed number of times. It follows the syntax

loop (integer-expression) statement-or-block [ then statement-or-block ] next-statement
If the integer-expression evaluates to a positive integer, statement-or-block will get executed that many times. Otherwise, control will pass to next-statement.

For example, 

      loop (10) newline ();
will execute the newline function 10 times.

_for

Like loop, the _for statement simply executes a block of code a fixed number times. Unlike the loop statement, the _for loop is useful in situations where the loop index is needed. It obeys the syntax

_for loop-variable (first-value, last-value, increment) block [ then statement-or-block ] next-statement
Each time through the loop, the loop-variable will take on the successive values dictated by the other parameters. The first time through, the loop-variable will have the value of first-value. The second time its value will be first-value + increment, and so on. The loop will terminate when the value of the loop index exceeds last-value. The current implementation requires the control parameters first-value, last-value, and increment to be integer-valued expressions.

For example, the _for statement may be used to compute the sum of the first ten integers: 

     s = 0;
     _for i (1, 10, 1)
       s += i;

The execution speed of the _for loop is more than twice as fast as the more powerful for loop making it a better choice for many situations.

forever

The forever statement is similar to the loop statement except that it loops forever, or until a break or a return statement is executed. It obeys the syntax

forever statement-or-block [ then statement-or-block ]
A trivial example of this statement is 
     n = 10;
     forever
       {
          if (n == 0) break;
          newline ();
          n--;
       }

foreach

The foreach statement is used to loop over one or more statements for every element of an object. Most often the object will be a container object such as an array, structure, or associative arrays, but it need not be.

The simple type of foreach statement obeys the syntax

foreach var (object) statement-or-block [ then statement-or-block ]
Here object can be an expression that evaluates to a value. Each time through the loop the variable var will take on a value that depends upon the data type of the object being processed. For container objects, var will take on values of successive members of the object.

A simple example is 

     foreach fruit (["apple", "peach", "pear"])
       process_fruit (fruit);
This example shows that if the container object is an array, then successive elements of the array are assigned to fruit prior to each execution cycle. If the container object is a string, then successive characters of the string are assigned to the variable.

What actually gets assigned to the variable may be controlled via the using form of the foreach statement. This more complex type of foreach statement follows the syntax

foreach var ( container-object ) using ( control-list ) statement-or-block
The allowed values of control-list will depend upon the type of container object. For associative arrays ( Assoc_Type), control-list specifies whether keys, values, or both are used. For example, 
     foreach k (a) using ("keys")
       {
           .
           .
       }
results in the keys of the associative array a being successively assigned to k. Similarly, 
     foreach v (a) using ("values")
       {
           .
           .
       }
will cause the values to be used. The form 
     foreach k,v (a) using ("keys", "values")
       {
           .
           .
       }
may be used when both keys and values are desired.

Similarly, for linked-lists of structures, one may walk the list via code like 

     foreach s (linked_list) using ("next")
       {
            .
            .
       }
This foreach statement is equivalent 
     s = linked_list;
     while (s != NULL)
       {
          .
          .
         s = s.next;
       }
Consult the type-specific documentation for a discussion of the using control words, if any, appropriate for a given type.

break, return, and continue

S-Lang also includes the non-local transfer statements return, break, and continue. The return statement causes control to return to the calling function while the break and continue statements are used in the context of loop structures. Consider: 

       define fun ()
       {
          forever
            {
               s1;
               s2;
               ..
               if (condition_1) break;
               if (condition_2) return;
               if (condition_3) continue;
               ..
               s3;
            }
          s4;
          ..
       }
Here, a function fun has been defined that contains a forever loop consisting of statements s1, s2,...,s3, and three if statements. As long as the expressions condition_1, condition_2, and condition_3 evaluate to zero, the statements s1, s2,...,s3 will be repeatedly executed. However, if condition_1 returns a non-zero value, the break statement will get executed, and control will pass out of the forever loop to the statement immediately following the loop, which in this case is s4. Similarly, if condition_2 returns a non-zero number, the return statement will cause control to pass back to the caller of fun. Finally, the continue statement will cause control to pass back to the start of the loop, skipping the statement s3 altogether.

The looping then clause

As mentioned above, all the looping statements support an optional then clause. The statements that comprise this clause get executed only when the loop has run to completion and was not prematurely terminated via a break statement. As an example, consider the following: 

    count = 0;
    max_tries = 20;
    while (count < max_tries)
      {
         if (try_something ())
           break;

         count++;
         % Failed -- try again
      }
    if (count == 20)
      throw RunTimeError, "try_something failed 20 times";
Here, the code makes 20 attempts to perform some task (via the try_something function) and if not successful it will throw an exception. Compare the above to an equivalent form that makes use of a then-clause for the loop statement: 
    max_tries = 20;
    loop (max_tries)
      {
         if (try_something ())
           break;
         % Failed -- try again
      }
    then throw RunTimeError, "try_something failed 20 times";
Here, the then statement would get executed only if the loop statement has run to completion, i.e., loops 20 times in this case. This only happens if the try_something function fails each time through the loop. However, if the try_something function succeeds, then the break statement will get executed causing the loop to abort prematurely, which would result in the then clause not getting executed.

The use of such a construct can also simplify code such as: 

   if (some_condition)
     {
        foo_statements;
        if (another_condition)
          bar_statements;
        else
          fizzle_statements;
     }
   else fizzle_statements;
In this case the fizzle_statements are duplicated making the code ugly and less maintainable. Ideally one would wrap the fizzle_statements in a separate function and call it twice. However, this is not always possible or convenient. The duplication can be eliminated by using the then form of the loop statement: 
   loop (some_condition != 0)
     {
        foo_statements;
        if (another_condition)
          {
            bar_statements;
            break;
          }
     }
   then fizzle_statements;
Here, the expression some_condition != 0 is going to result in either 0 or 1, causing the code to execute 0 or 1 loops. Since the fizzle_statements are contained in the then clause, they will get executed only when the requested number of loops executes to completion. Executing 0 loops is regarded as successful completion of the loop statement. Hence, when some_condition is 0, the fizzle_statements will get executed. The fizzle_statements will not get executed only when the loop is prematurely terminated, and that will occur when both some_condition and another_condition are non-zero.

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