Difference between revisions of "Documentation/BASIC Guide/Branching"
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+ | {{DISPLAYTITLE:Branching}} | ||
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|NextPage=Documentation/BASIC Guide/Loops | |NextPage=Documentation/BASIC Guide/Loops | ||
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Use branching statements to restrict the execution of a code block until a particular condition is satisfied. | Use branching statements to restrict the execution of a code block until a particular condition is satisfied. | ||
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The most common branching statement is the <tt>If</tt> statement as shown in the following example: | The most common branching statement is the <tt>If</tt> statement as shown in the following example: | ||
− | + | <syntaxhighlight lang="oobas"> | |
− | + | If A > 3 Then | |
− | + | B = 2 | |
+ | End If | ||
+ | </syntaxhighlight> | ||
The <tt>B = 2</tt> assignment only occurs when value of variable <tt>A</tt> is greater than three. A variation of the <tt>If</tt> statement is the <tt>If/Else</tt> clause: | The <tt>B = 2</tt> assignment only occurs when value of variable <tt>A</tt> is greater than three. A variation of the <tt>If</tt> statement is the <tt>If/Else</tt> clause: | ||
− | + | <syntaxhighlight lang="oobas"> | |
− | + | If A > 3 Then | |
− | + | B = 2 | |
− | + | Else | |
− | + | B = 0 | |
+ | End If | ||
+ | </syntaxhighlight> | ||
In this example, the variable <tt>B</tt> is assigned the value of 2 when <tt>A</tt> is greater than 3, otherwise <tt>B</tt> is assigned the value of 0. | In this example, the variable <tt>B</tt> is assigned the value of 2 when <tt>A</tt> is greater than 3, otherwise <tt>B</tt> is assigned the value of 0. | ||
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For more complex statements, you can cascade the <tt>If</tt> statement, for example: | For more complex statements, you can cascade the <tt>If</tt> statement, for example: | ||
− | + | <syntaxhighlight lang="oobas"> | |
− | + | If A = 0 Then | |
− | + | B = 0 | |
− | + | ElseIf A < 3 Then | |
− | + | B = 1 | |
− | + | Else | |
− | + | B = 2 | |
+ | End If | ||
+ | </syntaxhighlight> | ||
If the value of variable <tt>A</tt> equals zero, <tt>B</tt> is assigned the value 0. If <tt>A</tt> is less than 3 (but not equal to zero), then <tt>B</tt> is assigned the value 1. In all other instances (that is, if <tt>A</tt> is greater than or equal to 3), <tt>B</tt> is assigned the value 2. | If the value of variable <tt>A</tt> equals zero, <tt>B</tt> is assigned the value 0. If <tt>A</tt> is less than 3 (but not equal to zero), then <tt>B</tt> is assigned the value 1. In all other instances (that is, if <tt>A</tt> is greater than or equal to 3), <tt>B</tt> is assigned the value 2. | ||
+ | |||
+ | |||
+ | A complete If statement may be written on a single line, with a simpler syntax. The first example of this page may be written as: | ||
+ | |||
+ | <syntaxhighlight lang="oobas"> | ||
+ | If A > 3 Then B = 2 | ||
+ | </syntaxhighlight> | ||
+ | The second example of this page may be written as: | ||
+ | <syntaxhighlight lang="oobas"> | ||
+ | If A > 3 Then B = 2 Else B = 0 | ||
+ | </syntaxhighlight> | ||
== <tt>Select...Case</tt> == | == <tt>Select...Case</tt> == | ||
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The <tt>Select...Case</tt> instruction is an alternative to the cascaded <tt>If</tt> statement and is used when you need to check a value against various conditions: | The <tt>Select...Case</tt> instruction is an alternative to the cascaded <tt>If</tt> statement and is used when you need to check a value against various conditions: | ||
− | + | <syntaxhighlight lang="oobas"> | |
− | + | Select Case DayOfWeek | |
− | + | Case 1: | |
− | + | NameOfWeekday = "Sunday" | |
− | + | Case 2: | |
− | + | NameOfWeekday = "Monday" | |
− | + | Case 3: | |
− | + | NameOfWeekday = "Tuesday" | |
− | + | Case 4: | |
− | + | NameOfWeekday = "Wednesday" | |
− | + | Case 5: | |
− | + | NameOfWeekday = "Thursday" | |
− | + | Case 6: | |
− | + | NameOfWeekday = "Friday" | |
− | + | Case 7: | |
− | + | NameOfWeekday = "Saturday" | |
+ | End Select | ||
+ | </syntaxhighlight> | ||
In this example, the name of a weekday corresponds to a number, so that the <tt>DayOfWeek</tt> variable is assigned the value of 1 for <tt>Sunday</tt>, 2 for <tt>Monday</tt> value, and so on. | In this example, the name of a weekday corresponds to a number, so that the <tt>DayOfWeek</tt> variable is assigned the value of 1 for <tt>Sunday</tt>, 2 for <tt>Monday</tt> value, and so on. | ||
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The <tt>Select</tt> command is not restricted to simple 1:1 assignments — you can also specify comparison operators or lists of expressions in a <tt>Case</tt> branch. The following example lists the most important syntax variants: | The <tt>Select</tt> command is not restricted to simple 1:1 assignments — you can also specify comparison operators or lists of expressions in a <tt>Case</tt> branch. The following example lists the most important syntax variants: | ||
− | + | <syntaxhighlight lang="oobas"> | |
− | + | Select Case Var | |
− | + | Case 1 To 5 | |
− | + | ' ... Var is between the numbers 1 and 5 (including the values 1 and 5). | |
− | + | Case > 100 | |
− | + | ' ... Var is greater than 100 | |
− | + | Case 6, 7, 8 | |
− | + | ' ... Var is 6, 7 or 8 | |
− | + | Case 6, 7, 8, > 15, < 0 | |
− | + | ' ... Var is 6, 7, 8, greater than 15, or less than 0 | |
+ | Case Else | ||
+ | ' ... all other instances | ||
+ | End Select | ||
+ | </syntaxhighlight> | ||
+ | |||
+ | Now consider a misleading (advanced) example, and a common error: | ||
+ | |||
+ | <syntaxhighlight lang="oobas"> | ||
+ | Select Case Var | ||
+ | Case Var = 8 | ||
+ | ' ... Var is 0 | ||
+ | Case Else | ||
+ | ' ... all other instances | ||
+ | End Select | ||
+ | </syntaxhighlight> | ||
+ | |||
+ | The statement (Var = 8) evaluates to TRUE if Var is 8, and FALSE otherwise. TRUE is -1 and FALSE is 0. The Select Case statement evaluates the expression, which is TRUE or FALSE, and then compares that value to Var. When Var is 0, there is a match. If you understand the last example, then you also know why this example does not do what it appears | ||
+ | |||
+ | <syntaxhighlight lang="oobas"> | ||
+ | Select Case Var | ||
+ | Case Var > 8 And Var < 11 | ||
+ | ' ... Var is 0 | ||
+ | Case Else | ||
+ | ' ... all other instances | ||
+ | End Select | ||
+ | </syntaxhighlight> | ||
+ | {{InterWiki Languages BasicGuide|articletitle=Documentation/BASIC Guide/Branching}} | ||
{{PDL1}} | {{PDL1}} |
Latest revision as of 11:11, 30 January 2021
Use branching statements to restrict the execution of a code block until a particular condition is satisfied.
If...Then...Else
The most common branching statement is the If statement as shown in the following example:
If A > 3 Then B = 2 End If
The B = 2 assignment only occurs when value of variable A is greater than three. A variation of the If statement is the If/Else clause:
If A > 3 Then B = 2 Else B = 0 End If
In this example, the variable B is assigned the value of 2 when A is greater than 3, otherwise B is assigned the value of 0.
For more complex statements, you can cascade the If statement, for example:
If A = 0 Then B = 0 ElseIf A < 3 Then B = 1 Else B = 2 End If
If the value of variable A equals zero, B is assigned the value 0. If A is less than 3 (but not equal to zero), then B is assigned the value 1. In all other instances (that is, if A is greater than or equal to 3), B is assigned the value 2.
A complete If statement may be written on a single line, with a simpler syntax. The first example of this page may be written as:
If A > 3 Then B = 2
The second example of this page may be written as:
If A > 3 Then B = 2 Else B = 0
Select...Case
The Select...Case instruction is an alternative to the cascaded If statement and is used when you need to check a value against various conditions:
Select Case DayOfWeek Case 1: NameOfWeekday = "Sunday" Case 2: NameOfWeekday = "Monday" Case 3: NameOfWeekday = "Tuesday" Case 4: NameOfWeekday = "Wednesday" Case 5: NameOfWeekday = "Thursday" Case 6: NameOfWeekday = "Friday" Case 7: NameOfWeekday = "Saturday" End Select
In this example, the name of a weekday corresponds to a number, so that the DayOfWeek variable is assigned the value of 1 for Sunday, 2 for Monday value, and so on.
The Select command is not restricted to simple 1:1 assignments — you can also specify comparison operators or lists of expressions in a Case branch. The following example lists the most important syntax variants:
Select Case Var Case 1 To 5 ' ... Var is between the numbers 1 and 5 (including the values 1 and 5). Case > 100 ' ... Var is greater than 100 Case 6, 7, 8 ' ... Var is 6, 7 or 8 Case 6, 7, 8, > 15, < 0 ' ... Var is 6, 7, 8, greater than 15, or less than 0 Case Else ' ... all other instances End Select
Now consider a misleading (advanced) example, and a common error:
Select Case Var Case Var = 8 ' ... Var is 0 Case Else ' ... all other instances End Select
The statement (Var = 8) evaluates to TRUE if Var is 8, and FALSE otherwise. TRUE is -1 and FALSE is 0. The Select Case statement evaluates the expression, which is TRUE or FALSE, and then compares that value to Var. When Var is 0, there is a match. If you understand the last example, then you also know why this example does not do what it appears
Select Case Var Case Var > 8 And Var < 11 ' ... Var is 0 Case Else ' ... all other instances End Select
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