10. Flow Control Statements

Decision-making Statements

IF-THEN

Execute some code if condition is true.

IF (condition) THEN
  ...
END IF

IF-THEN-ELSE

Execute some code if condition is true else (i.e. condition is false) execute some other piece of code.

IF (condition) THEN
  ...
ELSE
   ...
END IF

IF-THEN-ELSIF-
THEN...

Simply extending the above but is really unnessary as can easily be created anyway.

IF (condition) THEN
  ...
ELSIF (condition) THEN
  ...
ELSIF (condition) THEN
  ...
ELSE
  ...
END IF

IF (condition) THEN
  ...
ELSE IF (condition) THEN
  ...
ELSE IF (condition) THEN
  ...
ELSE
  ...
END IF

SWITCH / CASE

'Most languages'

(Note on Perl below)

This type of conditional handling can easily be implemented using IF-THEN-ELSE statements but provide easier-to-read code.

SWITCH ( variable ) 
  CASE (variable matching 1) DO
     ...
     END CASE
  CASE (variable matching 2 DO
     ...
     END CASE
      ...
  DEFAULT:  (optional is most languages)
    ... (do if not matching any case)
    END DEFAULT CASE
END CASEd

action = prompt("action");

switch( action ) 
{
  case 1:   // "case A"
    document.writeln("numeric number one");
    break;
  case 2:   // "case B"
    document.writeln("numeric number two");
    break;
  case "1": // "case C"
    document.writeln("alphanumeric number one");
    break;
  case "2": // "case D"
    document.writeln("alphanumeric number two");
    break;
}

Note: some languages (e.g. C and C++) are very strict in what data types may be used for switch variables/cases and only allow integers. Other language - like our JavaScript example above - allow both integers and strings (and even mixed, as in the example).

Which of the four cases - "case A" through "case D" do you think will execute if you enter the number 1 in the prompt? Try running the code in Workbench 3L. Do you see the importance of data types?! And what type a certain function / method may be returning...

See also BREAK below.

Note on Perl: Perl does not natively include a switch construct.

- More at http://perldoc.perl.org/perlsyn.html#Switch-statements.

- As one example of discussions: Perl has no switch statements. How do I avoid a long if-else-if pattern? - http://perlmeme.org/howtos/syntax/switch_statements.html

Looping Statements

FOR ...

Loop or iteration statement as this type is called - repeated execution of some code for a defined number of times.

This form should be used when you have a specific and known number of times you want to repeat a task.

FOR (every month in year) DO
  print month
END FOR

FOR (initialization of counter; test condition; change of counter) DO
  ...
END FOR

for (n = 0; n < 10; n++)
{
  document.writeln(n);
}

WHILE

Another loop statement and this may or may not execute code depending on the condition.

Note that the condition is tested before the block and if condition is already false, nothing will be executed. This is a 'zero+ type', i.e. it can execute zero or more times.

WHILE (condition is true) DO
  ...
END WHILE

Warning/Note: this type of loops can create end-less loops if the condition never became false.

DO-WHILE

DO-UNTIL

REPEAT-UNTIL

Yes another loop statement but this flavor always execute the code at least once ('one+ type') as the test condition is at the end.

I.e. the condition is tested after the block.

DO 
  ..
WHILE (condition is true)

DO 
  ..
UNTIL (condition is true)

REPEAT 
  ..
UNTIL (condition is true)

And same as with the WHILE-loop - watch out for creating end-less loops that never exit the loop.

Branching Statements, Jump Statements

Called branching statements in (some) Java literature and jump statements in some C/C++ literature. We prefer the Jump nomenclature ourselves.

These statements unconditionally changes the flow control of your program.

CALL

BREAK

Typical use is in SWITCH-CASE statements to break (end execution) of a specific CASE but can also have use in other forms of flow control statements, depending on language. For C/C++:

"A break statement terminates the smallest enclosing while, do, for or switch statement. Execution resumes at the statement immediately following the terminated statement." [C++ Primer]

Note that a CASE without a BREAK may be an allowed syntax (in th specific language) and that flow then fall-through to the next CASE and this code is also executed. Which sometimes is the intended functionality (but often viewed as poor programming) but more often a difficult to find bug, simply missed including a BREAK.

CONTINUE

Another quote from C++ Primer, for C/C++:

"A continue statement causes the current iteration of the nearest enclosing while, for, or do loop to terminate.

 

In while and do loops, execution resumes with the evaluation of the control expression.

 

In for loops, continue causes execution to resume with the evaluation of expression-2.

 

Unlike the break statement, which terminates the loop, the continue statement terminates only the current iteration."

This statement is maybe mostly good for improving readability of code.

RETURN

This statement is used in functions (a.k.a. methods, subroutines, ...) to exit the function, return execution to the point where the function was called from. In general, the following is good to know about returning from functions, and return statement:

1. One variant of return statement is simply returning execution - return;
2. Another variant is returning with a value - (e.g.) - return(result);
3. A function without any explicit return statement returns execution at the end of function.
4. A program's main program, where execution starts, is also a function and if using a return statement here, the program exits (finishes).

Example (in JavaScript):

function calcCircumference( diameter )
{
  circumference = diameter * 3.14159; // pi
  return( circumference );
}

// main code
diameter = prompt("Enter diameter");
circumference = calcCircumference( diameter );
alert("The circumference of a circle with diameter " 
  + diameter + " is " +  circumference );

GOTO

'Most languages' and not 'All languages' as some languages are more restrictive to these forms of less obvious change of flow.

[C++ Primer] C++ Primer 2nd Edition, by Stanley Lippman (1991)

Exception-handling Statements

This type of statement is found in newer, more modern, languages (i.e. C - from the 1970s - does NOT have it, but C++ - from the 1980s - does have it).

TRY-CATCH

As this is an introductory course, for the time being we're only showing an example (JavaScript syntax):

try {
  undefinedfunction()
  alert('I guess you do exist')
}
catch(e) {
 alert('An error has occurred: '+e.message)
}
finally {
 alert('I am alerted regardless of the outcome above')
}

For the interested student, ee Exception Handling in C without C++ - http://www.on-time.com/ddj0011.htm.