IF ... END IF

General Form:


     ...
     [ELSE]
     ...
     END IF
    
Where:
    
    

The IF ... END IF statement is used to execute the statements surrounded by the IF and END IF pair if the overall result of all conditions is true. Each IF statement must have a corresponding END IF statement otherwise a run time error will result. There is no limit to the number of statements that can appear between the IF ... END IF pair.

Each pair of operands within the condition should be of the same type (alpha, numeric or field). When evaluating numeric expressions each side of the condition is calculated before the evaluation takes place. The following is a list of valid numeric conditions:

Statements can also contain any number of parentheses to allow conditions to be grouped before they are operated on:

IF ((x==1 AND y==1) OR (a==1 AND b==1))
...
END IF

True if the values of x and y are both equal to 1 or the values of a and b are equal to one, e.g. x=1, y=1, a=2, b=3, or x=1, y=2, a=1, b=1.

The evaluation of alphanumeric values is performed on a byte by byte basis working from left to right. The comparison of any two characters is based upon their binary values. For example, the binary value of `A' is HEX(41), and the binary value of `B' is HEX(42), therefore the condition "B" > "A" will be true. The comparison continues until unequal characters are found (if any). The first pair of unequal characters determines the relationship between the values. If alphanumeric values of differing lengths are compared, then the shorter value will be padded out with trailing spaces before the comparison takes place. Alpha expressions can be compared in the same way as with numeric expressions.

Both alpha and numeric field variables can be tested with IF, although only the `==' or `<>' may be used, e.g.

IF (.field1== .field2 or .name$<>.re$)
...
END IF

Logical operators

Multiple conditions may be separated with the logical operators AND, OR and XOR. The conditions are processed from left to right.

If the AND operator separates two conditions, the result of both conditions must be true before the overall result can be true. If the first condition is false then the second condition will not be evaluated. E.g.

IF (old$==new$ AND was==1)
...
END IF

If OR separates two conditions, the result of either one condition or both conditions must be true before the overall result can be true. If the first condition is true then the second condition will not be evaluated. E.g.

IF (old$==new$ OR was==1)
...
END IF

If XOR separates two conditions, the result of exactly one condition must be true before the overall condition is true. Both conditions are evaluated in all cases. E.g.

IF (old$==new$ XOR was==1)
...
END IF

The END condition

The END keyword may be used in place of any condition within an IF statement. END is used to test whether the global end-of-file marker was reached by the previous READ # statement. The result is true if the end-of-file marker was reached. E.g.

IF (END)

Preceding END with NOT means that the test will be true if the previous READ # statement has not reached the end-of-file marker. Testing the END condition resets it to FALSE.

Because it was a global flag and because it is reset when used, the END condition is now deprecated.

IF (NOT END)

The ELSE clause

The ELSE clause can be used within the IF ... END IF pair. The ELSE forces the statements between the ELSE clause and the corresponding END IF to be executed if result of the preceding IF condition is false. If the preceding IF condition is true then the statement in the ELSE clause will be ignored. Complex cascaded IF's are possible using ELSE IF.