Flow-control structures
These are examples of control structures. You may also be interested in:
You are encouraged to solve this task according to the task description, using any language you may know.
In this task, we document common flow-control structures. One common example of a flow-control structure is the goto
construct. Note that Conditional Structures and Loop Structures have their own articles.
C++
goto
Compiler: GCC 3.3.4
#include <iostream> int main() { LOOP: std::cout << "Hello, World!\n"; goto LOOP; }
Note that "goto" may also be used in conjunction with other forms of branching.
Exceptions
Compiler: GCC 4.0.2
Exceptions are a way to give control back to a direct or indirect caller in case of an error.
#include <iostream> #include <ostream> void foo() { std::cout << "Going to throw an exception.\n"; throw 7; // almost any object can be thrown, including ints std::throw << "This output will never execute.\n"; } void bar() { std::cout << "Going to call foo().\n"; foo(); std::cout << "This will be skipped by the exception coming from foo.\n"; } void baz() { try // everything thrown from inside the following code block { // will be covered by the following catch clauses std::cout << "Going to call bar().\n"; bar(); std::cout << "This will be skipped by the exception coming from foo.\n"; } catch(...) // a simple catch-all, but doesn't give access to the thrown exception { std::cout << "An exception occured. I'll just throw it on.\n"; throw; // without an argument, the caught exception is re-thrown } std::cout << "This will not be executed due to the re-throw in the catch block\n"; } void foobar() { try { baz(); } catch(char const* s) { std::cout << "If foo had thrown a char const*, this code would be executed.\n"; std::cout << "In that case, the thrown char const* would read " << s << ".\n"; } catch(int i) { std::cout << "Caught an int, with value " << i << " (should be 7).\n"; std::cout << "Not rethrowing the int.\n"; } catch(...) { std::cout << "This catch-all doesn't get invoked because the catch(int) above\n" << "already took care of the exception (even if it had rethrown the\n" << "exception, this catch-all would not be invoked, because it's\n" << "only invoked for exceptions coming from the try block.\n"; } std::cout << "This will be executed, since the exception was handled above, and not rethrown.\n"; } int main() { try { foobar(); } catch(...) { std::cout << "The main function never sees the exception, because it's completely handled\n" << "inside foobar(). Thus this catch-all block never gets invoked.\n"; } }
IDL
goto
test: ..some code here goto, test
(This is almost never used)
on_error
on_error, test
(This resumes at the label test if an error is encountered)
on_ioerror
on_ioerror, test
(Same as on_error, but for EOFs and read-errors and such)
break
break
immediately terminates the innermost current loop (or if or case etc)
continue
continue
immediately starts the next iteration of the current innermost loop
Perl
Interpreter: Perl 5.x
Goto is typically looked down upon by most perl programmers
FORK: # some code goto FORK;
Tcl
exception
Tcl's catch command can be used to provide a basic exception-handling mechanism:
if {[catch { ... code that might give error ... } result]} { puts "Error was $result" } else { ... process $result ... }
custom control structures
A novel aspect of Tcl is that it's relatively easy to create new control structures (more detail at http://wiki.tcl.tk/685). Eg. defining a command to perform some operation for each line of an input file:
proc forfilelines {linevar filename code} { upvar $linevar line ; # connect local variable line to caller's variable set filechan [open $filename] while {[gets $filechan line] != -1} { uplevel 1 $code ; # Run supplied code in caller's scope } close $filechan }
Now use it to print the length of each line of file "mydata.txt":
forfilelines myline mydata.txt { puts [string length $myline] }