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Loops and branches

The programs in the earlier chapters consist of a number of instructions which are executed one by one, from start to finish.

However, there are a number of other ways a program can proceed:

Repeating a set of instructions (called loops)

Doing one set of instructions or another (called IF statements)

Jumping from one line of your program to another

Repeating instructions (loops)

The DO...UNTIL and WHILE...ENDWH commands are structures they don't actually do anything to your data, but control the order in which other commands are executed:

There is a test condition at the end of the DO...UNTIL loop, and at the beginning of the WHILE...ENDWH loop.


The instruction "DO" says to OPL:

"Execute all the following instructions until an UNTIL is reached. If the condition following UNTIL is not met, repeat the same set of instructions until it is."

The first time through the loop, "a%"=10. 1 is subtracted from "a%", so that "a%" is 9 when the UNTIL statement is reached. Since "a%" isn't zero yet, the program returns to DO and the loop is repeated.

"a%" goes down to 8, and again it fails the UNTIL condition. The loop therefore repeats 10 times until "a%" does equal zero.

When "a%" equals zero, the program continues with the instructions after UNTIL.

The statements in a DO...UNTIL loop are always executed at least once.


The instructions between the WHILE and ENDWH statements are executed only if the condition following the WHILE is true in this case if "a%" is greater than 0.

Initially, "a%"=10 and so "A=10" is displayed on the screen. "a%" is then reduced to 9. "a%" is still greater than zero, so "A=9" is displayed. This continues until "A=1" is displayed. "a%" is then reduced to zero, and so "Finished" is displayed.

Unlike DO...UNTIL, it's possible for the instructions between WHILE and ENDWH not to be executed at all.

Example using WHILE...ENDWH

This procedure ignores any keys which may already have been typed, then waits for a new keypress.

KEY returns the value of a key that was pressed, or "0" if no key has been pressed. "WHILE KEY :ENDWH" reads any keys previously pressed, one by one, until they have all been read and "KEY" returns zero.

Choosing between instructions

In a program, you might have several possible cases ("x%" may be 1, or it may be 2, or 3...) and want to do something different for each one (if it's 1, do this, but if it's 2, do that...). You can do this with the IF...ENDIF structure:

These lines would do either



and then continue with the statements after the ENDIF.

You can cater for as many cases as you like with ELSEIF statements. You don't have to have any ELSEIFs. There may be either one ELSE statement or none; you do not specify conditions for the ELSE statement.

Every IF in your program must be matched by an ENDIF otherwise you'll see an error message when you try to translate the module. The structure must start with an IF and end with an ENDIF.

"Nesting" loops the `Too complex' message

You can have up to 8 DO...UNTIL, WHILE...ENDWH and/or IF...ENDIF structures nested within each other. If you nest them any deeper, a `Too complex' error message will be displayed.

Example using IF

% operator
The program checks character codes with the % operator. "%a" returns the code of "a", "%Z" the code of "Z" and so on. Using "%A" is entirely equivalent to using 65, the actual code for "A", but it saves you having to look it up, and it makes your program easier to follow.

Be careful not to confuse character codes like these with integer variables.

OR operator
OR lets you check for either of two conditions. OR is an example of a logical operator. There is more about logical operators later in this chapter.

Example using DO...UNTIL and IF

This procedure checks for a `Y' or `N' keypress. You'd put your own code in the IF statement, where "..." has been used.

Arguments to functions

Some functions, as with commands like PRINT and PAUSE, require you to give a value or values. These values are called arguments. The UPPER$ function needs you to specify a string argument, and returns the same string but with all letters in upper case. For example, "UPPER("12.+aBcDeF")" returns ""12.+ABCDEF"".

Functions as arguments to other functions
Since GET$ returns a string, you can use this as the argument for UPPER$. "UPPER$(GET$)" waits for you to press a key, because of the "GET$"; the "UPPER$" takes the string returned and, if it's a letter, returns it in upper case. This means that you can check for ""Y"" without having to check for ""y"" as well.

`True' and `False'

The test condition used with DO...UNTIL, WHILE...ENDWH and IF...ENDIF can be any expression, and may include any valid combination of operators and functions. Examples:

Condition		Meaning
"x=21"			does the value of "x" equal 21? (Note  as 
this is a test
			condition, it does not assign "x" the value 21)
"a%<>b%	"	is the value of "a%" not equal to the value of "b%"?
"x%=(y%+z%)"		is the value of "x%" equal to the value of "y%+z%"?
			(does not asssign the value "y%+z%" to "x%").

The expressions actually return a logical value that is, a value meaning either `True' or `False'. Any non-zero value is considered `True' (to return a `True' value, OPL uses -1), while zero means `False'. So if "a%" is 6 and "b%" is 7, the expression "a%>b%" will return a zero value, since "a%" is not greater than "b%".

These are the conditional operators:

"<"	less than		"<="	less than or equal to
""	greater than		"="	greater than or equal to
"="	equal to		"<>"	not equal to

Logical operators

The operators AND, OR and NOT allow you to combine or change test conditions. This table shows their effects. ("c1" and "c2" represent conditions.)

Example	Result						Integer returned
"c1 AND c2"	True if both "c1" and "c2" are true			-1
		False if either "c1" or "c2" are false		0
"c1 OR c2"	True if either "c1" or "c2" is true			-1
		False if both "c1" and "c2" are false		0
"NOT c1"	True if "c1" is false				-1
		False if "c1" is true				0

However, AND, OR and NOT become bitwise operators something very different from logical operators when used exclusively with integer or long integer values. If you use "IF A% AND B%", the AND acts as a bitwise operator, and you may not get the expected result. You would have to rewrite this as "IF A%<>0 AND B%<>0". (Operators, including bitwise operators, are discussed further in an Appendix.)

Jumping to a different line

Jumping out of a loop: BREAK

The BREAK command jumps out of a DO...UNTIL or WHILE...ENDWH structure. The line after the UNTIL or ENDWH statement is executed, and the lines following are then executed as normal. For example: (picture)

Jumping to the test condition: CONTINUE

The CONTINUE command jumps from the middle of a loop to its test condition. The test condition is either the UNTIL line of a DO...UNTIL loop or the WHILE line of a WHILE...ENDWH loop. For example: (picture)

Jumping to a `label': GOTO

The GOTO command jumps to a specified label. The label can be anywhere in the same procedure (after any LOCAL or GLOBAL variable declarations). In this example, when the program reaches the GOTO statement, it jumps to the label "exit::", and continues with the statement after it. (picture)

The two PRINT statements are missed out.

Labels themselves must end in a double colon. This is optional in the "GOTO" statement both "GOTO exit::" and "GOTO exit" are OK.

The jump to the label always happens it is not conditional.

Don't use GOTOs instead of DO...UNTIL or WHILE...ENDWH, as they make procedures difficult to understand.

Vectoring to a label: VECTOR/ENDV

VECTOR jumps to one of a list of labels, according to the value in an integer variable. The list is terminated by the ENDV statement. For example:

PRINT "P% was not 1/2/3" :GET :STOP
PRINT "P% was 1" :GET :STOP
PRINT "P% was 2" :GET :STOP
PRINT "P% was 3" :GET :STOP

Here, if "P%" is 1, VECTOR jumps to the label "FUNCA::". If it is 2, it jumps to "FUNCX::", and if 3, "FUNCR::". If "P%" is any other value, the program continues with the statement after the "ENDV" statement.

The list of labels may spread over several lines, as in this example, with a comma separating labels in any one line but no comma at the end of each line. Again, you can write each label in the list with a double colon, if you like.

VECTOR...ENDV can sometimes save you from having to write very long "IF...ENDIF" structures, with "ELSEIF" used many times.

Stopping a running program
This example introduces the STOP command. This stops a running program completely, just as if the end of the program had been reached. In a module with a single procedure, STOP has the same effect as using GOTO to jump to a label above the final ENDP.


Zero and non-zero are logical values meaning `False' and `True' respectively. "UNTIL 0" and "WHILE 1" therefore mean `do forever', since the condition 0 is never `True' and the condition 1 is always `True'. Use loops with these conditions when you need to check the real condition somewhere in the middle of the loop. When the real condition is met, you can BREAK out of the loop.

For example:

This example uses the KEY command. KEY returns 0 if no key has been pressed. When a key is pressed, KEY returns a non-zero value which counts as `True', and the BREAK is executed.


UNTIL condition

WHILE condition statements ENDWH

IF condition statements (ELSEIF condition statements) (ELSE statements) ENDIF

VECTOR int% label1, label2 label3... ENDV

GOTO label jumps to label::

BREAK goes to the first line after the end of the loop the line following the UNTIL or ENDWH line.

CONTINUE goes to the test condition of the loop the UNTIL or the WHILE line.

STOP stops a running program completely.

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