ME 142Engineering Computation I

Using Subroutines Effectively

Key Concepts

Functions vs. Subprograms

Using Subroutines Effectively

Declaring Variables

Functions vs. Subprograms

A Function is a self-contained program that receives input through arguments (or parameters), performs calculations, and returns a result

A Subprogram is similar to a function, except it does not return a result

Functions vs. Subprograms

Functions may be used in formulas or expressions

Excel provides hundreds of built-in functions Functions return a result but may not write to

other cells in the spreadsheet Subprograms may write directly to cells in

the spreadsheet

Functions vs. Subprograms

Functions are launched from a formula Subprograms may be launched from a button

or as any macro Subprograms offer the most flexibility and

power, including the use of custom dialog boxes

Using Subroutines Effectively

Longer programs are commonly split up into smaller programs, sometimes referred to as subroutines

Typical scenario: Main program passes information to a

subroutine via argument list The subroutine performs calculations and returns

results to main program

Sub MathOp()'Demonstrates using subroutines'Key Variables:' Oper - math operation (* - + / ^)

‘Get input from spreadsheet A = Cells(1, 2) B = Cells(2, 2) Oper = Cells(2, 1)

'Call subroutine to perform calculation Call Calculate(A, B, Oper, C)

'Output results to spreadsheet Cells(3, 2) = C

End Sub

Sub Calculate(x, y, Oper, z)'subroutine to perform math operationIf Oper = "*" Then z = x * yElseIf Oper = "-" Then z = x - yElseIf Oper = "+" Then z = x + yElseIf Oper = "/" Then z = x / yElseIf Oper = "^" Then z = x ^ yElse MsgBox "Invalid Operation" z = "Error"End If End Sub

Declaring Variables

VBA supports a variety of Data Types, the most common being: Integer Single precision real Double precision real String Variant

Declaring Variables

If you don’t declare the data type for a variable, VBA uses the default data type of variant

Data stored as variant changes types depending on what you do with it

With variant data types, VBA automatically handles conversion

Trade-off is you sacrifice speed and memory

Why Declare Variables

Declaring variables makes your program run faster and use memory more efficiently

You may “force” yourself to declare all variables by including “Option Explicit” as the first statement of your VBA module Option Explicit

When this statement is present, you won’t be able to run your code if it contains any undeclared variables

Variable Declaration Examples

Dim x As Integer Dim y As Single Dim z As Double Dim q As String Dim a As Single, b As Single, c As Single

Life of Variables

Variables may declared at 3 levels Procedure Module All Modules

For our purposes, Procedure variables will be sufficient

Procedure Variables

Most efficient because VBA frees up the memory they use when the procedure ends

To declare, simply use the Dim statement within the procedure Dim x as Single

Static Variables

Static variables retain their value when the procedure ends Static B as Integer

Could be used to keep a running total or track the number of times a program is called

Variables are purged/reset when The workbook is closed or reopened The reset toolbar button is clicked When “end” statement/error message is executed