What are the risks of declaring a variable in the middle of the code?-CodePudding

I usually see in almost all of VBA codes all variables are declared after e.g. Sub/Function name line

I know and I used variable declaration in the middle of some of my codes (Not inside a loop) and saw no problems.

I usually avoided that because I see most of VBA example codes have them declared right after the first line. I just want to know what are the risks from an expert/experienced VB programmer point of view.

CodePudding user response：

There are no risks of declaring it in the middle.

The effect of declaring a variable in the middle is that it can only be used after that point and not before (which is scope).
The lifetime of the variable is different: the variable is created (allocated and initialized to its respective flavour of zero) when you enter the procedure, but you may not actually use it until you reach its scope (the point in the procedure where it's declared).

Declaring inside or outside a loop does not make a difference in VB6/A as they do not have block scope, unlike VB.NET.

So there is no performance difference between the two approaches (because all variables are created when you enter the procedure), but there is a difference in usage (you may not use a created variable before its declaration line). If you think that distinction is helpful in making sure you are not using a variable wrongly, declare your variables only where needed. Otherwise you are free to pick any of the two approaches, just apply it consistently (it's probably not a good idea to declare most of the variables in the beginning and then some in the middle).

CodePudding user response：

Declare your variables, when you actually need them. When you have all declarations lumped at the top of the procedure, refactoring becomes much harder. And when you want to double check your declaration as you read your code (or, perhaps, someone else), searching it at the top may be again quite inconvenient, unless you procedure is short.

CodePudding user response：

Declaring your variables up front, at the top of your sub/function makes it easy for other (and for you if you come by the code after, say a month) to read and understand what your code needs to calculate, and what placeholders/variables are required for the code to function.

You can of course declare variables anywhere (as long as you remember not to use the variable unless you have actually declared it first). That can work, and it has no effect whatsoever on the performance of your code. It just isn't good practice to declare some variables at the top then do some work, then declare another set of variables mid-code.

Sub CalculateAge()
    Dim BirthYear As Integer
    Dim CurrentYear As Integer

    'Code to fetch current year 
    'Code to get BirthYear from user/or document
    'Code to report result
    
End Sub

Compare that with the following:

Sub CalculateAge2()
    Dim BirthYear As Integer
    'Code to ask the user or fetch the birth year from the document

    Dim CurrentYear As Integer
    'Code to populate currentYear 

    'Code to do the calculation and report result
End Sub

In the first example, there is a clear separation from variables and logic. In the second, everything is mixed.

The first example is a lot easier to read and understand, especially if you use a good naming convention.

If you look at how classes are written, defined, you will see properties usually are first declared, then methods/logic below. This is the common practice used to write code.

PS: In other languages, you can declare and assign variables in the same line. in C# or VB.Net you could say something like:

int Age = CurrentYear - BirthYear;

But that's not possible in VBA. You need a separate line for the decalration and another for the assignment. Since your variables are split, you might as well move the declaration to somewhere common.