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GEEN 1300Introduction to Engineering Computing

Class meeting #15Wednesday, Oct 14th

Excel & Visual Basic for Applications (VBA)

accessing a spreadsheet range directly from VBA

1Homework #7, posted, due next Wednesday

cell ranges as arguments to functions

creating array variables in VBA

Array Data Structures

Example: a set of pH measurements on treated wastewater

6.2 7.1 7.2 7.6 7.1 7.5 7.0 8.57.0 6.5 7.0 6.8 7.7 7.8 8.1

These measurements can be stored on the spreadsheetThese measurements can be stored on the spreadsheetand they can be stored in an array variable in VBA.

In either case, it is convenient to refer to the set ofmeasurements by a single identifier and then refer toindividual measurements using a subscript.

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pH4 the 4th item in the pH array

This type of referencing is handled differently in Excel andVBA and cross-referencing from Excel to VBA is distinctalso.

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Accessing a spreadsheet range directly from VBA

Note: It is not necessary toote t s ot ecessa y toreference a particularworksheet since a rangename is global throughoutthe workbook.

The "ActiveCell" is at the topof the "Selection" and its value

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can be referenced in VBA by

Note: "Value" is the defaultproperty and can be left out

Accessing a spreadsheet range directly from VBA

What then is the result of

What if we want the value inthe cell below the ActiveCell?

What if we want toh th A ti C ll

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No. of rows "away"from the current row No. of columns

"away" from thecurrent column

change the ActiveCellto the cell below thecurrent ActiveCell?

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Accessing a spreadsheet range directly from VBA

Moving the ActiveCell to the end of a range of filled cells(record a macro to see how this is done)

Selecting from the ActiveCell to the endf f fill d ll

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of a range of filled cells

(again, figure this out by recording a macro)

Accessing a spreadsheet range directly from VBA

Selecting an entire range of cells (Ctrl - * in Excel)

Determining the size (no.of rows and columns) of aselected range of cells

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This is important in writing VBA code for an arbitraryselected range of cells.

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Cell ranges as arguments to functions

It is common to want to write a function that makescalculations for one or more arguments that are rangesof cells.

General pattern:General pattern:

Invoke function on spreadsheet with pH argument:Trace execution of function with

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breakpoint -- check value of nRows:

nRows = 15

Cell ranges as arguments to functionsExample: Find the second largest item in a

range of cellsNote: We should take advantage of Excel's (not VBA's)

Max function.

find the maximumfind the maximumvalue in DataRange

to start, set the second maximumvalue = the first value in DataRange

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use nested For-Next loops to go through all theelements of DataRange -- take no action whenthe MaxVal is found -- check all other values againstthe current second maximum, and, if a new secondmaximum is found, update the stored value

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Cell ranges as arguments to functions

Simple example: Find the second largest item in arange of cells

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Creating array variables in VBA

allocates space for the array in VBA,variant type

assigns the value of 3 to an element of X

Track what’s going on using the Locals windowTrack what s going on using the Locals window

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Notice that elements 0 to 10 (11 total) have beenallocated to X

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Creating array variables in VBA

So, when you Dim X(10), the arrayorigin is 0

Shows that element 1

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Expand X in the Locals Windowby clicking on the + next to it

Shows that element 1of X has the value 3

Creating array variables in VBAUse the declaration Option Base 1

to force the array origin to be 1

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Now, only 10 elements, indexed 1 to 10have been allocated to X

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Creating array variables in VBA

13Now, origin of X is 1

Creating array variables in VBA

Two-dimensional array(rows and columns, like a matrix, correspondsto a rectangular range of cells on the spreadsheet)

14Shows 1 to 4, 1 to 4

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Creating array variables in VBATwo-dimensional array

By expanding X, then row branch 2,note value of 3 assigned to elementin row 2, column 3

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Creating array variables in VBA

Creating arrays with variable dimensions(when you don’t know an integer value or values

to put into the Dim statement for the array)

Start by “Dim”-ing array X as emptySta t by g a ay as e pty

Get value of allocation for X fromsomewhere, here the user

Use the value stored in n to“ReDim” the X array

You can “ReDim” an array many times during

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You can “ReDim” an array many times duringa VBA program, thus creating a dynamic allocationof space for the array.

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Cell ranges as arguments to functions

Example application: compute the median absolute deviation(MAD)

The MAD of a set of data is found in the following way:

1) find the Median of the data set (sort the data and pickthe one in the middle -- for even-numbered set,average the two in the middle)

2) calculate the absolute values of the differences betweenthe data and the Median value of the data

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3) find the Median of that set of absolute-value differences

Note: Excel (not VBA) has a function that computes theMedian, so we will want to take advantage of that.

Cell ranges as arguments to functionsExample application: compute the median absolute deviation

(MAD)

declare array forabsolute deviations

begin

RC = 1RC >R

Flowchart

AbsDev

get nRows & nColsfor DataRange

allocate space forAbsDev(nRows,

nCols)

nRows?

RC = RC + 1

CC = 1CC >nCols?

CC = CC + 1

use Excel’s Median

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use Excel’s Medianfunction to get

DataMed

AbsDev(RC,CC) =|DataRange(RC,CC)-

DataMed|

function to getMAD as median of

AbsDev array

end

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Cell ranges as arguments to functionsExample application: compute the median absolute deviation

(MAD)necessary so arraysubscript origin is1,1

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Referencing array elements on the spreadsheet

array of cells named “pH”

Use of INDEX function toselect one element ofpH array with row and

l i di

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column indices

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Referencing array elements on the spreadsheet from VBAborrowing the Index function from Excel

Transferring the array from the spreadsheet to a VBAarray variable

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When assigningan object, mustuse the Setcommand

Building an array in VBA from user input

This is called a “sentinel” value

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The keyword “Preserve” extends the size of the arrayfor each data value entered, without wiping out theprevious values entered

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Once you are able to program VBA to accessranges of cells and treat them as arrays, orto create arrays within VBA, the possibilitiesfor programming applications are virtuallyendless.endless.

These include

all kinds of user-defined functions thatarise from needs, and where Excel hasno built-in function

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vector-matrix math calculations

VBA manipulations of tables of data that cansave hours of spreadsheet operations