1 5.3 sub procedures, part ii passing by value passing by reference sub procedures that return a...
TRANSCRIPT
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5.3 Sub Procedures, Part II
• Passing by Value
• Passing by Reference
• Sub Procedures that Return a Single Value
• Debugging
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ByVal and ByRef
• Parameters in Sub procedure headers are proceeded by ByVal or ByRef
• ByVal stands for By Value
• ByRef stands for By Reference
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Passing by Value
• When a variable argument is passed to a ByVal parameter, a copy of the value of the argument is passed.
• After the Sub procedure terminates, the original value of the actual parameter that was passed is unchanged, even if the procedure changes the formal parameter value.
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Passing by Reference• When a variable argument is passed to a
ByRef parameter, the actual parameter (argument) and the formal parameter are one and the same…they are the same location in memory.
• After the Sub procedure terminates, the original value of the actual parameter that was passed will be changed when the formal parameter is changed.
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Example 5.3.1
Sub procedure has a ByVal parameter.
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About to call.
Note the value in variable amt, the actual parameter (argument).
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In the called Sub procedure.
Formal parameter num has a copy of actual parameter amt.
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Formal parameter num’s value has changed.
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But the value in amt remains the same. This is because amt and val are two different memory locations.
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Therefore, any change to a ByVal formal parameter has no effect on its associated actual parameter.
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Example 5.3.1 (modified)
Let’s try the Sub procedure with a ByRef parameter.
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About to call.
Note the value in variable amt, the actual parameter (argument).
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In the called Sub procedure.
Formal parameter num is identical to actual parameter amt.
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Formal parameter num’s value has changed.
Formal parameter num’s value has changed.
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Since num is a ByRef parameter, the value in amt changes also. This is because amt and val
are the same memory location.
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Therefore, any change to a ByRef formal parameter remains in the
associated actual parameter after the called procedure terminates.
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Example 5.3.2
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Example 5.3.2
Sub (or Function) procedures with ByRef parameters can be used to “return” values. If you want a procedure to “return” more than one value,
you can use ByRef parameters for this.
Example 5.3.3
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Example 5.3.3
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If you want to swap the values in two variables, you need to have a temporary variable.