1 5.3 sub procedures, part ii passing by value passing by reference sub procedures that return a...

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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.