| .IF...THEN...ELSE Statement Details. |
|
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| Block IF...THEN...ELSE Details |
|
| IF booleanexpression1 THEN |
| [statementblock-1] |
| [ELSEIF booleanexpression2 THEN |
| [statementblock-2] |
| . . . |
| [ELSE |
| [statementblock-n]] |
| END IF |
| |
| Part Description |
| |
| booleanexpression1, booleanexpression2 Any expression that |
| evaluates to true (nonzero) |
| or false (zero) |
| |
| statementblock-1, statementblock-2, One or more BASIC |
| statementblock-n statements on one or more |
| lines |
| |
| 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: |
| |
| ž The block form provides more structure and flexibility than the |
| single-line form by allowing conditional branches across several |
| lines. |
| ž With the block form, more complex conditions can be tested. |
| ž The block form lets you use longer statements and structures |
| ž The block form allows your program's structure to be guided by |
| logic rather than by how many statements fit on a line. |
| ž Programs that use block-form IF...THEN...ELSE are usually easier |
| to read, maintain, and debug. |
| |
| |
| Single-line IF...THEN...ELSE |
| |
| Syntax |
| IF booleanexpression THEN thenpart [ELSE elsepart] |
| |
| 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: |
| |
| Part Description |
| booleanexpression Any expression that evaluates to true (nonzero) |
| or false (zero). |
| thenpart, elsepart The statements or branches performed when |
| booleanexpression is true (thenpart) or false |
| (elsepart). Both parts have the same syntax, |
| which is described below. |
| |
| 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: |
| |
| Part Description |
| statements One or more BASIC statements, separated by colons |
| |
| linenumber A valid BASIC program line number |
| |
| linelabel A valid BASIC line label |
| |
| 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 |
