1

GEEN 1300Introduction to Engineering Computing

Class meeting #13Wednesday, October 7th

Excel & Visual Basic for Applications (VBA)

Program flowo sequence

1Homework #6, posted, due next Wednesday

o selectiono repetition

Program Flow

3 fundamental structures

sequence

selection

repetitionSequence

execute one statement at a time, in sequential order

This is the encompassingstructure of all programs:

Begin

2

End

2

Selection

program branching based on decisions

Program does one thingth d di

logicalcondition

TF

do this,if TRUE

do this,if FALSE

or another dependingwhether a logical conditionevaluates to TRUE or FALSE

3

There a several specific structures that providefor selection.

Repetition

provides for looping or iteration

there must be something inthe loop that affects the decision;

donel i

TF

p ;otherwise you’ll never get out!

4

looping?

There are several specific types ofrepetition structures

3

Selection Structures in VBA

One-way If

logical TF

If <condition> Then<do this> statements

End If

General form:“If-Then”

condition

do this,if TRUE

otherwise,just go on

d

Example:

If Xval > 10 ThenYval = Xval – 10Xval = 1

End If

5

One-line, one-statement form:

If n = 2 Then MsgBox “Yo, bubba”

Note: no End If req’d

Indentation shownis not req’d, but isgood style. Do it.

One-way If

“If-Then”

Example: Create a function that squaresa numerical quantity and carries the signof that quantity to its square.

f = x2

x < 0?

TF

f = - f

6

4

Two-way If

“If-Then-Else”

logical TF

General form:

If <condition> Then<statements>

Else< t t t >condition

do this,if TRUE

else do this,if FALSE

<statements>End If

Example:

If Switch ThenM B “Li ht i ON”

7

MsgBox “Light is ON”Else

MsgBox “Light is OFF”End If

“Switch” would be a Boolean (logical)variable here, previously set to TRUE or FALSE

This is the mostcommon Ifstructure used

Two-way If

“If-Then-Else”

Example: a 2-piece engineering formula

16

R 2000Rf R0.079

R 2000

1 4

R 2000R

R <2000 ?

TF

Fluid = 16/R0.079R1/4

Fluid =

8

5

Multi-alternative If“If-Then-ElseIf ”

condition1

TF

code

If <condition 1> Thencode block 1

ElseIf <condition 2> Thencode block 2

El If < diti > Th condition2

TF

conditionn

TF

codeblock

1codeblock

2

ElseIf <condition n> Thencode block n

Elseelse code block

End If

9

else do this ifall conditions

fail

codeblock

n

Notice: subsequent if tests come off theFALSE branch of the previous tests

Multi-alternative If Example

Develop a VBA function to compute temperature (C) fromvoltage (mV) for a thermocouple (Type J, iron-constantan).

2 3 4

5 6 7

T 19.528 V 1.2286 V 1.0752 V 0.59087 V

0 17257 V 0 028132 V 0 0023963 V

5 6 7

5 8

0.17257 V 0.028132 V 0.0023963 V

8.3823 10 V 8.095 V 0.000 mV

A

2 3 4 4

6 5 8 6 10 7

T 19.784 V 0.20012 V 0.010370 V 2.5497 10 V

3.5852 10 V 5.3443 10 V 5.0999 10 V

0.000 V 42.919 mV

B

10

Regressed polynomials from NIST

2 3 4

6 5

T 3113.6 300.54 V 9.9477 V 0.17028 V 0.0014303 V

4.7389 10 V 42.919 V 69.553 mV

C

6

mV <-8.095 ?

TF

mV <=0 ?

TF "mV out ofrange low"

Multi-alternative If Example

0 ?

mV <=42.919 ?

TF useformula

Auseformula

B

g

mV <=69.553 ?

TF

useformula

C"mV out of

hi h"

11

Crange high"

Note: the Else part is optional anddoesn’t need to be there if there isno Else consequence

Multi-alternative If Example

12

7

Multi-alternative If Example

13

Select-Case Select Case <test expression>Case <list 1>

code block 1Case <list 2>

code block 2

whichcase

?

Case 1 Case 2 Case 3 Case else

Case Elseelse code block

End Select

14[often used for “menu”

alternatives]

Case 1 Case 2 Case 3 Case else

Case Elseis optional

8

Example of Select-Case

Moves to a new linewithin a text string

15

Example of Select-Case

16

9

Repetition Structures in VBA

general loop: Do . . . Loop

count-controlled loop: For . . . Next

General Do Loop

Dopre-test code block

If <condition> Then Exit Dopost-test code block

Loop

General Do . . . Loop“mid-test loop”

17

Loopdone

looping?

TF

Dox = Sqr(x)

If x < 1.1 Then Exit Dox = x^1.5

Loop

Example:

Example of Mid-test Loop -- Input Validation

inputvalid

?

TF

get input

correctionmessage

18

message

10

Special Cases of the General Do . . . Loop

Do – While “pre-test loop” Do While <condition>loop code

Loop

conditionTF

loop code

Do While AOKAOK = FALSE

Loop

Example:

19

p

There is no “pre-test” code,and the loop is continued onTRUE / exited on FALSE

Do – Until “post-test loop”

Doloop code

loop code

donelooping

TF

pLoop Until <condition>

Doi = i + 1

Loop Until i > ilim

Example:

20

looping?

There is no “post-test” code.

11

Count-controlled Iteration

For . . . Next

index = startY

For index = start To limit Step incrementloop code

Next index

Example:

if Step incrementis left out, a stepof 1 is default

index = start

index = index+ increment

indexpasseslimit ?

loop code

N

sum = 0For i = 1 to n

sum = sum + x(i)Next i

Here, x is an arraywith n elements and

21

Used frequently with arrays

with n elements andx(i) refers to the ith

element of the array

We will see more of this structure in coming classes

Count-controlled Iteration

For . . . Next

Example

n

i 1

n n 1i

2

Validate

22

12

By using combinations of the sequence, selection andrepetition structures, we can construct algorithms toaccomplish a wide variety of computations requiredin engineering and science.

Where to from here?

implementing numerical methods like bisectionand linear interpolation automatically with VBA

working with array data types

23

array-based numerical methods

user interfaces: input boxes, message boxesand user forms