This purpose of this section is to give the reader a feel for the S-Lang language, its syntax, and its capabilities. The information and examples presented in this section should be sufficient to provide the reader with the necessary background to understand the rest of the document.
S-Lang is different from many other interpreted languages in the sense that all variables and functions must be declared before they can be used.
Variables are declared using the variable keyword, e.g.,
variable x, y, z;
x, y, and z. Note the
semicolon at the end of the statement. All S-Lang statements must
end in a semi-colon.
Unlike compiled languages such as C, it is not necessary to specify the data type of a S-Lang variable. The data type of a S-Lang variable is determined upon assignment. For example, after execution of the statements
x = 3;
y = sin (5.6);
z = "I think, therefore I am.";
x will be an integer, y will be a
double, and z will be a string. In fact, it is even possible
to re-assign x to a string:
x = "x was an integer, but now is a string";
variable x = 3, y = sin(5.6), z = "I think, therefore I am.";
Most functions are declared using the define keyword. A
simple example is
define compute_average (x, y)
{
variable s = x + y;
return s / 2.0;
}
The parameter list consists of a comma separated list of variable names. It is not necessary to declare variables within a parameter list; they are implicitly declared. However, all other local variables used in the function must be declared. If the function takes no parameters, then the parameter list must still be present, but empty:
define go_left_5 ()
{
go_left (5);
}
The language permits recursive functions, i.e., functions that call themselves. The way to do this in S-Lang is to first declare the function using the form:
define function-name ();
The most famous example of a recursive function is the factorial function. Here is how to implement it using S-Lang:
define factorial (); % declare it for recursion
define factorial (n)
{
if (n < 2) return 1;
return n * factorial (n - 1);
}
%' character to start a comment and all characters from
the comment character to the end of the line are ignored.
Perhaps the most appealing feature of any interpreted language is that it frees the user from the responsibility of memory management. This is particularly evident when contrasting how S-Lang handles string variables with a lower level language such as C. Consider a function that concatenates three strings. An example in S-Lang is:
define concat_3_strings (a, b, c)
{
return strcat (a, strcat (b, c));
}
strcat function for concatenating two strings. In C, the
simplest such function would look like:
char *concat_3_strings (char *a, char *b, char *c)
{
unsigned int len;
char *result;
len = strlen (a) + strlen (b) + strlen (c);
if (NULL == (result = (char *) malloc (len + 1)))
exit (1);
strcpy (result, a);
strcat (result, b);
strcat (result, c);
return result;
}
Binary operators have been defined to work with the string data
type. In particular the + operator may be used to perform
string concatenation. That is, one can use the
+ operator as an alternative to strcat:
define concat_3_strings (a, b, c)
{
return a + b + c;
}
The unary prefix operator, &, may be used to create a
reference to an object, which is similar to a pointer
in other languages. References are commonly used as a mechanism to
pass a function as an argument to another function as the following
example illustrates:
define compute_functional_sum (funct)
{
variable i, sum;
sum = 0;
for (i = 0; i < 10; i++)
{
sum += @funct (i);
}
return sum;
}
variable sin_sum = compute_functional_sum (&sin);
variable cos_sum = compute_functional_sum (&cos);
compute_functional_sum applies the
function specified by the parameter funct to the first
10 integers and returns the sum. The two statements
following the function definition show how the sin and
cos functions may be used.
Note the @ operator in the definition of
compute_functional_sum. It is known as the dereference
operator and is the inverse of the reference operator.
Another use of the reference operator is in the context of the
fgets function. For example,
define read_nth_line (file, n)
{
variable fp, line;
fp = fopen (file, "r");
while (n > 0)
{
if (-1 == fgets (&line, fp))
return NULL;
n--;
}
return line;
}
fgets function to read the nth line of a file.
In particular, a reference to the local variable line is
passed to fgets, and upon return line will be set to
the character string read by fgets.
Finally, references may be used as an alternative to multiple
return values by passing information back via the parameter list.
The example involving fgets presented above provided an
illustration of this. Another example is
define set_xyz (x, y, z)
{
@x = 1;
@y = 2;
@z = 3;
}
variable X, Y, Z;
set_xyz (&X, &Y, &Z);
X set to 1, Y
set to 2, and Z set to 3. A C programmer will
note the similarity of set_xyz to the following C
implementation:
void set_xyz (int *x, int *y, int *z)
{
*x = 1;
*y = 2;
*z = 3;
}
The S-Lang language supports multi-dimensional arrays of all datatypes. For example, one can define arrays of references to functions as well as arrays of arrays. Here are a few examples of creating arrays:
variable A = Integer_Type [10];
variable B = Integer_Type [10, 3];
variable C = [1, 3, 5, 7, 9];
10 integers and assigns
it to the variable A. The second example creates a 2-d array
of 30 integers arranged in 10 rows and 3 columns
and assigns the result to B. In the last example, an array
of 5 integers is assigned to the variable C. However,
in this case the elements of the array are initialized to the
values specified. This is known as an inline-array.
S-Lang also supports something called an range-array. An example of such an array is
variable C = [1:9:2];
1
through 9 in increments of 2.
Arrays are passed by reference to functions and never by value. This permits one to write functions which can initialize arrays. For example,
define init_array (a)
{
variable i, imax;
imax = length (a);
for (i = 0; i < imax; i++)
{
a[i] = 7;
}
}
variable A = Integer_Type [10];
init_array (A);
10 integers and initializes all its
elements to 7.
There are more concise ways of accomplishing the result of the previous example. These include:
variable A = [7, 7, 7, 7, 7, 7, 7, 7, 7, 7];
variable A = Integer_Type [10]; A[[0:9]] = 7;
variable A = Integer_Type [10]; A[*] = 7;
A. In the second, the range of indices has been explicitly
specified, whereas the third example uses a wildcard form. See
section ??? for more information about array indexing.
Although the examples have pertained to integer arrays, the fact is that S-Lang arrays can be of any type, e.g.,
variable A = Double_Type [10];
variable B = Complex_Type [10];
variable C = String_Type [10];
variable D = Ref_Type [10];
10 element arrays of double, complex, string, and
reference types, respectively. The last example may be used to
create an array of functions, e.g.,
D[0] = &sin;
D[1] = &cos;
The language also defines unary, binary, and mathematical
operations on arrays. For example, if A and B are
integer arrays, then A + B is an array whose elements are
the sum of the elements of A and B. A trivial example
that illustrates the power of this capability is
variable X, Y;
X = [0:2*PI:0.01];
Y = 20 * sin (X);
double *X, *Y;
unsigned int i, n;
n = (2 * PI) / 0.01 + 1;
X = (double *) malloc (n * sizeof (double));
Y = (double *) malloc (n * sizeof (double));
for (i = 0; i < n; i++)
{
X[i] = i * 0.01;
Y[i] = 20 * sin (X[i]);
}
A structure is similar to an array in the sense that it is a
container object. However, the elements of an array must all be of
the same type (or of Any_Type), whereas a structure is
heterogeneous. As an example, consider
variable person = struct
{
first_name, last_name, age
};
variable bill = @person;
bill.first_name = "Bill";
bill.last_name = "Clinton";
bill.age = 51;
person. Then an
instance of this structure has been created using the
dereference operator and assigned to bill. Finally, the
individual fields of bill were initialized. This is an
example of an anonymous structure.
A named structure is really a new data type and may be created
using the typedef keyword:
typedef struct
{
first_name, last_name, age
}
Person_Type;
variable bill = @Person_Type;
bill.first_name = "Bill";
bill.last_name = "Clinton";
bill.age = 51;
variable People = Person_Type [100];
People[0].first_name = "Bill";
People[1].first_name = "Hillary";
The creation and initialization of a structure may be facilitated by a function such as
define create_person (first, last, age)
{
variable person = @Person_Type;
person.first_name = first;
person.last_name = last;
person.age = age;
return person;
}
variable Bill = create_person ("Bill", "Clinton", 51);
Other common uses of structures is the creation of linked lists, binary trees, etc. For more information about these and other features of structures, see section ???.
In addition to the global namespace, each compilation unit (e.g., a
file) is given a private namespace. A variable or function name
that is declared using the static keyword will be placed in
the private namespace associated with compilation unit. For
example,
variable i;
static variable i;
i. The first declaration
defines i in the global namespace, but the second declaration
defines i in the private namespace.
The -> operator may be used in conjunction with the name of
the namespace to access objects in the name space. In the above
example, to access the variable i in the global namespace,
one would use Global->i. Unless otherwise specified, a
private namespace has no name and its objects may not be accessed
from outside the compilation unit. However, the implements
function may be used give the private namespace a name, allowing
access to its objects. For example, if the file t.sl contains
implements ("A");
static variable i;
i via A->i.