bruce mayer, pe licensed electrical & mechanical engineer bmayer@chabotcollege

[email protected] • ENGR-25_Programming-3.ppt1

Bruce Mayer, PE Engineering/Math/Physics 25: Computational Methods

Bruce Mayer, PELicensed Electrical & Mechanical Engineer

[email protected]

Engr/Math/Physics 25

Chp4 MATLAB

Programming-3



Learning Goals Write MATLAB Programs That can

MAKE “Logical” Decisions that Affect Program Output

Write Programs that Employ LOOPing Processes• For → No. Loops know a priori• while → Loop Terminates based on

Logic Criteria



if Decision Statement

The if Statement Basic Form

if logical expression

statements

end Every if statement must

have an accompanying end statement. • The end statement marks

the terminous of the statements that are to be executed if the logical expression is true.

Start

if LogicalExpression

Statements

end

Statements

True

False



else Decision Statement

else Statement Basic Form

if logical expression

Statement Group-1

else

Statement Group-2

end

Start


Statements-1

end

Statements

True

False

Statements-2



Logical Expression on Arrays

When the test, if logical expression, is performed, where the logical expression may be an ARRAY, the test returns a value of true only if ALL the elements of the logical expression evaluated as true• “Logic Gate” equivalent statement (“and”)

– ALL High → Output High– ELSE Output Low



Logical Array Example

For example, if we fail to recognize how the test works, the following statements may not perform as expected

x = [4,-9,25];if x < 0

disp(’Some elements of x are negative.’)else

y = sqrt(x)end

Because the test if x < 0 is false, when this program is run it gives the resulty = 2.0000 0 + 3.0000i 5.0000



Logical Array Example cont

Now Fine-Tune to Test for Positive xx = [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 if x < 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



Combine Decisions & Logicals

The following statementsif logical expression 1 if logical expression 2

statementsend

end

Can be written more Concisely with

if logical expression 1 & logical expression 2statements

end



Example: Decisions & Logicals

The Coder = 7; s = -11;% Double ifif r < 13 if s > -17 t = log(abs(r)+abs(s)) endend% if-n-&if (r < 13)&(s > -17) q = log(abs(r)+abs(s))end

The OutPut t = 2.8904q = 2.8904



The elseif Statement The elseif Statement Basic

Formif logical expression 1 statement group 1elseif logical expression 2 statement group 2else statement group 3end

• The else and elseif statements may be omitted if not required. However, if both are used, the else statement must come AFTER the elseif statement to since the else yields the DEFAULT result



elseif FlowChartStart


Statements-1

end

Statements

elseif Logical Expr

True

else

Statements-2

True

Statements-3

else



elseif Example

Given• y = log(x) for

x > 10 • y = sqrt(x)

for 0 <= x <= 10,

• y = exp(x) - 1 for x < 0

The Following finds y for given user input for x

x = input('enter scalar: x = ')if x > 10; z = log(x);elseif x >= 0 z = sqrt(x);else z = exp(x) - 1;end% output to usery = z

The InPut & OutPut x = -3.7000y = -0.9753



Nested if FlowChartStart

x < 13?

y = sinh(x)

Yes

y, z out

y > 43?

y = 7.3xz = -19x + 29

y 17?

z = √y z = y1.89

z = 0

No

No No

Yes Yes

Nesting if statements is a alternative to the elseif form



Strings

A STRING is a variable that contains Characters. • 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



Strings cont

The following string, digits, is not the same as the variable number created by typing digits = 987.>> digits = '987'digits =987

>> p = log(digits)??? Function 'log' is not defined for values of class 'char'.

No Indent for String



Strings & the input Statement

The prompt program on the next slide uses the 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 string variable x.• The function returns an empty matrix if you

press the Enter key without typing anything.

x = input(’prompt’, ’string’)



Strings and Decisions

The following prompt program is a script file that allows the user to answer Yes by typing either Y or y or by pressing the Enter key. • 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 = ’Y’else

response = ’N’end



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 “Spring Constant” in N/m

– x the Compression Distance in m

dxdxkxkW

dxxkW

:2

:

21

1



Problem 4-20 cont

Find or Createa. A Function to find x given W, k1, k2, and db. A Plot of x vs. W for 0 W 3000 N given

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

Solve Force-Balance Eqns for x dx

kk

dkWx

dxkWx

:2

2

:

21

2

1



The Prob 4-20 Plots

Before Brk-Pt After Brk-Pt

111

1

kdW

dxm

2122 2

1

kkdW

dxm



All Done for Today

A NiceSpring

Platform



Bruce Mayer, PELicensed Electrical & Mechanical Engineer

[email protected]

Engr/Math/Physics 25

Appendix 6972 23 xxxxf



Prob 4-20: spring_plat.m



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



.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 SECTIONWmax = 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 = ');



.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 <= Wmax % All 3 springs engaged W=[0, Wcrit, Wmax]; % 3element row vector % make corresponding 3-pt output vector x = [spring_plat(W(1),k1,k2,d),spring_plat(W(2),k1,k2,d),... spring_plat(W(3),k1,k2,d)]; plot(W,1000*x,'-s'), xlabel('Wt (N)'), ylabel('Compression (mm)'),... title('Above-Critical Load') % Plot 2pts for one Spring Linear Rangeelse W = [0,Wmax] x = [spring_plat(W(1),k1,k2,d), spring_plat(W(2),k1,k2,d)]; plot(W,1000*x,'-s'), xlabel('Wt (N)'), ylabel('Compression (mm)'),... title('Below-Critical Load')end

bruce mayer, pe licensed electrical & mechanical engineer bmayer@chabotcollege

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