engr-25 lec-11 sp12 programming-3 decisions

7/30/2019 ENGR-25 Lec-11 Sp12 Programming-3 Decisions

1/26

[email protected] ENGR-25_Programming-3.ppt1Bruce Mayer, PEEngineering/Math/Physics 25: Computational Methods

Bruce Mayer, PELicensed Electrical & Mechanical Engineer

[email protected]

Engr/Math/Physics 25

Chp4 MATLAB

Programming-3


2/26


Learning Goals

Write MATLAB Programs That canMAKE Logical Decisions that AffectProgram Output

Write Programs that Employ LOOPing

Processes

For No. Loops know a priori

while Loop Terminates based onLogic Criteria


3/26


if Decision Statement

The if Statement Basic Formiflogical expression

statements

end Every if statement must have

an accompanying end

statement.

The end statement marks theterminous of the statements that

are to be executed ifthe logical

expression is true.

Start

if Logical

Expression

Statements

end

Statements

True

False


4/26


else Decision Statement

else StatementBasic Form

iflogical expression

Statement Group-1

else

Statement Group-2

end

Start

if Logical

Expression

Statements-1

end

Statements

True

False

Statements-2


5/26

[email protected] ENGR-25_Programming-3.ppt5

Bruce Mayer, PEEngineering/Math/Physics 25: Computational Methods

Logical Expression on Arrays

When the test, iflogical expression, isperformed, where the logical

expression may be an ARRAY, the test

returns a value oftrue only ifALL the

elements of the logical expression

evaluated as true

Logic Gate equivalent statement (and) ALL High Output High

ELSE Output Low


6/26



Logical Array Example

For example, if we fail to recognize how thetest works, the following statements may not

perform as expectedx = [4,-9,25];

if x < 0disp(Some elements of x are negative.)else

y = sqrt(x)end

Because the test ifx < 0 is false, when this

program is run it gives the resulty =

2.0000 0 + 3.0000i 5.0000


7/26



Logical Array Example cont

Now Fine-Tune to Test forPositivexx = [4,-9,25];if x >= 0

y = sqrt(x)else

disp('Some elements of x are negative.')end

This File Produces Only the Message

Some elements of x are negative.

The test ifx < 0 is false, and the test if

x >= 0 also returns a false value because

x >= 0 returns the vector[1,0,1].

ENTIRE Statement is FALSE

since ONE element is False


8/26



Combine Decisions & Logicals

The following statementsif logical expression 1if logical expression 2

statementsend

end

Can be written more Concisely with

if logical expression 1 & logical expression 2

statementsend


9/26



Example: Decisions & Logicals

The Coder = 7; s = -11;% Double ifif r < 13

if s > -17t = log(abs(r)+abs(s))

endend% if-n-&if (r < 13)&(s > -17)

q = log(abs(r)+abs(s))end

The OutPutt =

2.8904q =

2.8904


10/26



The elseif Statement

Theelseif

Statement Basic Formif logical expression 1

statement group 1

elseif logical expression 2

statement group 2

else

statement group 3

end

The else andelseif statements

may be omitted if not

required. However, if

both are used, theelse statement must

come AFTER theelseif statement to

since the else yieldsthe DEFAULT result


11/26



elseif FlowChartStart

if Logical

Expression

Statements-1

end

Statements

elseif

Logical Expr

True

else

Statements-2

True

Statements-3

else


12/26



elseif Example

Given y = log(x) for

x > 10

y = sqrt(x) for

0 = 0z = sqrt(x);

elsez = exp(x) - 1;

end% output to usery = z

The InPut & OutPutx =

-3.7000y =

-0.9753


13/26



Nested if FlowChartStart

x < 13?

y = sinh(x)

Yes

y, z out

y > 43?

y = 7.3x

z = -19x + 29

y 17?

z = y z = y1.89z = 0

No

No No

Yes Yes

Nesting if statements

is a alternative to theelseif form


14/26



Strings

A STRING is a variable that containsCharacters.

Strings are useful for creating input prompts &messages, and for storing & operating on data

such as names and addresses To create a string variable, enclose the

characters in single quotes.

For example, Create string variable NYgiant:

>> NYgiant = 'Mel Ott'NYgiant =Mel Ott


15/26



Strings cont

The following string, digits, is not thesame as the variable number created bytyping digits = 987.

>> digits = '987'digits =987

>> p = log(digits)

??? Function 'log' is not definedforvalues of class 'char'.

No Indent for String


16/26



Strings & the input Statement

The prompt program on the next slide usesthe isempty(x) function, which returns a 1

if the array x is empty and 0 otherwise

The Program also uses the input function,

with syntax

This function displays the string prompt on

the screen, waits for input from the keyboard,

and returns the entered value in the stringvariable x.

The function returns an empty matrix if youpress the Enterkey without typing anything.

x = input(prompt, string)


17/26



Strings and Decisions

The following prompt program is a script filethat allows the user to answerYes by typingeitherY ory or by pressing the Enterkey.

Any other response is treated as the answer No.

response = input('Want to continue?Y/N:','s')if (isempty(response))|(response==Y)|response==y)

response = Yelseresponse = N

end


18/26



Example: Prob 4-20

Given Spring-Loaded Platform

In This Case the

ENGR36 Force-

Balance Model

In ENGR36 we

Learn that the

Spring Force

xkFs Where

k the SpringConstant in N/m

x the CompressionDistance in m

dxdxkxkW

dxxkW

:2

:

21

1


19/26



Problem 4-20 cont

Find or Createa. A Function to find x given W, k1, k2, and d

b. A Plot of x vs. W for 0 W 3000 Ngiven

k1 = 10 kN/m = 10 N/mm k2 = 15 kN/m = 15 N/mm d = 100 mm

SolveForce-BalanceEqns for x dx

kk

dkWx

dxkWx

:2

2

:

21

2

1


20/26



The Prob 4-20 Plots

Before Brk-Pt

After Brk-Pt

11

1

1

kdW

dxm

212

22

1

kkdW

dx

m


21/26



All Done for Today

A NiceSpring

Platform


22/26



Bruce Mayer, PELicensed Electrical & Mechanical Engineer

[email protected]

Engr/Math/Physics 25

Appendix 6972 23 xxxxf


23/26



Prob 4-20: spring_plat.m


24/26



Prob-20 Test Session

Case-1 W = 500N>> spring_plat(500,10e3,15e3,0.1)The Compression distance in m, x =

0.0500

Case-2 W = 2 kN

>> spring_plat(2e3,10e3,15e3,0.1)The Compression distance in m, x =

0.1250


25/26



.m-file = Prob4_20_plot.m

Documentation & Inputs% Bruce Mayer, PE * 26Sep11% ENGR25 * Problem 4-20% file = Prob4_20_plot.m

% Plot Displacement vs. Wt for Spring Platform%% INPUT SECTION

Wmax = input('Max Weight in N = ');k1 = input('Main-Spring Const in N/m = ');k2 = input('Secondary-Spring Const in N/m = ');d = input('distance to 2nd Springs in m = ');


26/26



.m-file = Prob4_20_plot.m cont%CALCULATION SECTION

% calc Critical-Load for Curve Break-PointWcrit = k1*d% Plot 3 pts for PieceWise Linear rangeif Wcrit

engr-25 lec-11 sp12 programming-3 decisions

Documents