if statement

The if statement tests the result of a logical expression. Depending on whether the expression evaluates to either true or false, the statements following the then or else clauses, respectively, are executed.

Syntax

Single-line format

if logical.exp then statement.block
if logical.exp else statement.block
if logical.exp then statement.block else statement.block

Multi-line format

Form 1

if logical.exp then statements else
statements
.
.
end

Form 2

if logical.exp then
statements
.
.
end else statements

Form 3

if logical.exp then
statements
.
.
end else
statements
.
.end

Form 4

if logical.exp else
statements
.
.
end

Form 5

if logical.exp then
statements
.
.
end

Parameter(s)

`logical.exp`	Expression to be tested. If it evaluates to nonzero, it is considered true. If it evaluates to 0, it is false.
then `statement.block`	Executes the specified statements if the logical expression evaluates to nonzero.
else `statement.block`	Executes the specified statements if the logical expression evaluates to 0.

Description

Commands can be placed on a single line (single-line format) or in a block structure (multi-line format). The two formats are functionally identical.

If more than one statement is contained in either the then or else clause, they must be separated by a semicolon.
The single- and multi-line formats can be combined.
If there is no clause for the current condition, the next sequential statement following the entire if statement is executed.
The then clause of an if statement is optional if the else clause is present. However, one or the both must be present.

Complex conditions can be tested by using the and and or connectives. Regardless of the complexity of the overall expression, each condition eventually evaluates to a simple true or false.

NULL statements are often included in then or else clauses as placeholders. A NULL statement performs no action. However, you can use NULL statements to make the logic of a conditional more clear and easier to read.

Note: This construction is not required. The then clause is not mandatory in an if statement, as long as an else clause exists.

If the then clause is omitted, the statement would make sense to the compiler, but it might not be clear to the user. The NULL statement is used to make clear to the user that if the match returns a value of true, no action is taken.

Many statements accommodate the then/else construct. More information is available at then/else statement blocks.

Example(s)

Example 1

if answer = ’exit’ then stop
if answer = ’y’ then print ’ok’; cnt=cnt+1; goto 20
if answer = ’y’ then
   print ’ok’
   cnt=cnt+1
end
if answer = ’exit’ then gosub 100 else gosub 200
if answer = ’y’ then print ’ok’;cnt=cnt+1 else print ’no good’;stop
if answer = ’y’ then
   print ’ok’
   cnt=cnt+1
end else
   print ’no good’
end

Example 2

The following is an example testing if a value is numeric:

IF NUM(PRICE) THEN
           NULL
END ELSE
     PRINT "ERROR: NON-NUMERIC PRICE. STOP"
STOP
END

if statement

Syntax

Parameter(s)

Description

Example(s)

See also