IF/THEN/ELSE/FI

Conditional Test

This directive tests a condition for true or false and executes the THEN clause if true, and the ELSE clause if false. If no ELSE clause is specified, a false condition causes execution to move to the next line-numbered statement or the FI statement terminator.

Syntax

IF condition [THEN] stmt [ELSE stmt] [FI]

condition

is a relational, Boolean, and/or logical expression used to establish a true or false condition.

stmt

(THEN) is the statement(s) to be executed if the condition tests true; (ELSE) is the statement(s) to be executed if the condition tests false.

Remarks

The FI terminator is available starting with release level 8.0.

The syntax for the condition is:

num/str-value relational-operator num/str-value
[[logical-operator num/str-value relational-operator num/str-value]…]
or
numeric-value

num/str-value

is any valid numeric or string expression.

relational-operator

is any of the valid comparison operators listed below.

logical-operator

is any of the valid logic operators listed below.

numeric-value

is any valid numeric expression. If numeric-value is zero, this condition is false; if other than zero, true.

relational-operator	meaning
=	equal to
<> or ><	not equal to
>	greater than
<	less than
>= or =>	greater than or equal to
<= or =<	less than or equal to
logical-operator	meaning
=ALL (string-value)	=ALL function
LIKE string-value	partial equality
OR	logical OR of multiple conditionals
AND	logical AND of multiple conditionals
( )	parentheses; to group multiple conditionals into specific priority order

The LIKE operator can specify a string value containing wildcards, which can match more than 1 character. LIKE automatically pads its values to the correct length.

LIKE wildcards	meaning
“*”	matches any string of characters (0 or more)
“?”	matches a single character
“[A-Z]”	matches a range for a single character
“[AGCF]”	matches a single character in a list
“[wildcard]”	matches the specified wildcard character
“[*]”	matches the asterisk
“[?]”	matches the question mark
“[[ ]”	matches the left bracket without a preceding,
unmatched left bracket
“]”	matches the right bracket

IF/THEN/ELSE/FI directives may be nested or strung together to form complex logical evaluations.

All IF/THEN/ELSE/FI directives are terminated by a new line-numbered statement.

Examples

IF X = 4 THEN LET Y = 7

sets Y to +7 if X contains a +4.

01000 IF TODAY < SATURDAY THEN
         LET GO_TO_SCHOOL$ = "YES";
         IF TODAY = TUESDAY OR TODAY = THURSDAY THEN
            LET DO_LAUNDRY$ = "YES"
         ELSE
            LET PUT_OUT_TRASH$ = "YES"
         FI
      ELSE
         IF TODAY = SATURDAY THEN
            LET MOW_LAWN$ = "YES"
         ELSE
            LET WATCH_FOOTBALL$ = "YES"
         FI
      FI;
      LET STUDY$ = "YES"

gives an example of a nested IF/THEN/ELSE/FI directive. Numeric variable TODAY is tested to establish what must be done on a given day. This logic results in GO_TO_SCHOOL$ for Monday through Friday; DO_LAUNDRY$ on Tuesday and Thursday; PUT_OUT_TRASH$ on Monday, Wednesday, and Friday; MOW_LAWN$ on Saturday; WATCH_FOOTBALL$ on Sunday; and STUDY$ every day of the week.

From the Thoroughbred Basic Language Reference, Volume II: F–Q, version 8.9.1. Open this page in the PDF