Q(uick)BASIC Statement: IF...THEN...ELSE

IF...THEN...ELSE Details

In executing a block-form IF, QuickBASIC tests the first Boolean expression (booleanexpression1). If the Boolean expression is true (nonzero), the statements following THEN are executed. If the first Boolean expression is false (zero), QuickBASIC begins evaluating each ELSEIF condition in turn. When QuickBASIC finds a true condition, the statements following the associated THEN are executed. If none of the ELSEIF conditions are true, the statements following the ELSE are executed. After the statements following a THEN or ELSE are executed, the program continues with the statement following the END IF.

The ELSE and ELSEIF blocks are both optional. You can have as many ELSEIF clauses as you want in a block IF. Any of the statement blocks can contain nested block IF statements.

QuickBASIC looks at what appears after the THEN keyword to determine whether or not an IF statement is a block IF. If anything other than a comment appears after THEN, the statement is treated as a single- line IF statement.

A block IF statement must be the first statement on a line. The ELSE, ELSEIF, and END IF parts of the statement can only have a line number or line label in front of them. The block must end with an END IF statement.

The block form of the IF...THEN command provides several advantages over the single-line form:

• The block form provides more structure and flexibility than the single-line form by allowing conditional branches across several lines.
• With the block form, more complex conditions can be tested.
• The block form lets you use longer statements and structures
• The block form allows your program's structure to be guided by logic rather than by how many statements fit on a line.
• Programs that use block-form IF...THEN...ELSE are usually easier to read, maintain, and debug.

Single-line IF...THEN...ELSE

The single-line form of the statement is best used for short, straightforward tests where only one action is taken.

The following list describes the parts of the single-line form:

The thenpart and the elsepart both have the following syntax:

• {statements | [GOTO]linenumber | GOTO linelabel }

The following list describes the parts of the thenpart and elsepart syntax:

Note that GOTO is optional with a line number but is required with a line label.

The thenpart is executed if the booleanexpression is true; if the booleanexpression is false, the elsepart is executed. If the ELSE clause is not present, control passes to the next statement in the program.

You can have multiple statements with a condition, but they must be on the same line and separated by colons:

• IF A > 10 THEN A = A + 1: B = B + A: LOCATE 10,22: PRINT B,A

• The argument condition is an expression that BASIC evaluates as true (nonzero) or false (zero).
• The argument statementblock includes any number of statements on one or more lines.
• The argument thenpart includes the statements or branches performed when condition is true. The syntax of thenpart is:

• {statements | [GOTO] linenumber | GOTO linelabel}

• The argument elsepart includes the statements or branches performed when condition is false. The syntax is the same as thenpart. If the ELSE clause is not present, control passes to the next statement in the program.

Usage Notes

• The advantages of the single-line and block forms of the syntax are:

• In executing a block-form IF, BASIC tests the first Boolean expression (condition1). If the Boolean expression is true (nonzero), the statements following THEN are executed. If the first Boolean expression is false (zero), BASIC begins evaluating each ELSEIF condition in turn. When BASIC finds a true condition, the statements following the associated THEN are executed. If none of the ELSEIF conditions is true, the statements following the ELSE are executed. After the statements following a THEN or ELSE are executed, the program continues with the statement following the END IF.
• The ELSE and ELSEIF blocks are both optional. You can have as many ELSEIF clauses as you want in a block IF. Any of the statement blocks can contain nested block IF statements.
• BASIC looks at what appears after the THEN keyword to determine whether an IF statement is a block IF. If anything other than a comment appears after THEN, the statement is treated as a single- line IF statement.
• A block IF statement must be the first statement on a line. The ELSE, ELSEIF, and END IF parts of the statement can have only a line number or line label in front of them. The block must end with an END IF statement.
• Note that GOTO is optional with a line number but is required with a line label.
• The single-line form is never required. Any program using single-line IF...THEN...ELSE statements can be written using block form.
• You can have multiple statements with a condition, but they must be on the same line and separated by colons:

• IF A > 10 THEN A = A + 1: B = B + A: LOCATE 10, 22: PRINT B, A