3.5.
Control Structures
GENESIS supports four kinds of conditional control structures:
1. if/elif/else/end
2. while/end
3. for/end
4. foreach/end
Each of these control structures is briefly described here.
Note that you can break out of any of these loops before their
expected termination by using the ``break'' or ``return'' statements.
3.5.1.
if/elif/else/end Structure
The GENESIS ``if ...'' conditional structure takes the following form:
if (expression)
statements
elif (expression)
statements
else
statements
end
The elif and else portions of the form are optional and multiple elif
portions may appear.
The ``if''/``elif'' expressions are evaluated in the order of
appearance. When an expression evaluates ``true'', all the statements
between the ``true'' expression and the following matching ``elif'',
``else'' or ``end'' will be executed; if all evaluated expressions are
``false'', then the statements between the ``else'' and the ``end''
are executed (or no statements are executed if there is no ``else''
clause).
Note that the GENESIS if construct does not recognize the keyword
``then''.
These are some examples of valid if statements:
if(1)
echo hello
end
if(1); echo hello; end
if( (GRAPHICS == 1) || (5+3 > 10) )
echo hello
end
The following function uses the if/elif/else/end construct and prints
``zero'', ``negative'' or ``positive'' depending on the first argument:
function iftest(arg)
int arg
if ( arg == 0 )
echo zero
elif ( arg < 0 )
echo negative
else
echo positive
end
end
3.5.2.
while/end Structure
The GENESIS ``while ...'' conditional looping statement has the
following syntax:
while (expression)
statements
end
The expression is evaluated. If it is ``true'' (i.e., not 0), then
all the statements will be executed, then the expression will be evaluated again; if the evaluated expression is ``false'', then all the
statements are skipped and control is turned to the first statement
after the ``end''.
An example of a while loop is the following:
str parent
while(getfield {{parent} object->name} != "xgraph")
parent = {el {parent}/..}
end
Note that the expression can be complicated and can involve some of
the actual work of the loop.
3.5.3.
for/end
The GENESIS ``for ...'' conditional structure uses an incrementable
variable in assessing the condition for continuing through the loop.
The syntax of the loop is as follows:
for (init_assignment; expression; incr_assignment)
statements
end
The init_assignment is an assignment statement which is evaluated once
prior to entering the loop. The incr_assignment is an assignment
statment which is executed following the statements each time through
the loop. Otherwise, the for/end statement operates in the same
manner as while/end.
To print the numbers one through ten:
int i
for (i = 1; i <= 10; i = i + 1)
echo {i}
end
3.5.4.
foreach/end
The GENESIS ``foreach'' statement loops through a set of statements
once for each item in a specified argument list, having assigned the
value of each item in the list to a looping variable for use by the
statements:
foreach loop-var ( arg1 [ arg ...] )
statements
end
For example:
str name
foreach name ({el {path}/##[OBJECT=xform]})
xshow {name}
end
The use of wildcards in the specification of pathnames is described in
the section `` Hierarchical Structure'' (Tree.doc). Again, you need
to have declared the variables used in the loop before their use in
the loop.