Selection Statements

Transcription

1 Selection Statements 1

2 Introduction Matlab has two basic selection statements: the if statement and the switch statement. The if statement has optional else and elseif. The relational/comparison and logical operators are used to form conditions to be evaluated in selection and repetition statements. The conditions are logically, either true or false. Types of Operators Arithmetic Comparison/Relational Logical Control Structures Sequential Selection Repetition 2

3 Relational/Logical Expressions Operator Meaning < less than > greater than <= less than or equals >= greater than or equals == equality ~= not equal to Relational operators in MATLAB Logical operator Description && And for scalars Or for scalars ~ Not Logical operators in MATLAB 3

4 The Conditions - introduction The conditions can use both relational operators and logical operators, that are logically either true or false. Example: (10>(20/4+2)) (8/4<2*0) Ans: 1 (true) Description: logical operators Condition 1 Condition 2 (10>(20/4+2)) (8/4<2*0) relational operators What is true and what is false in Matlab? o 1 is used to represent true and 0 is used to represent false o Special cases: Empty string Array with false values [1 0 1] Blank matrix [] 4

5 The Conditions - introduction o What about NaN (Not a Number)? NaN represents Not true and not false If (NaN) what to do??? NaN s cannot be converted to logical option: delete or change NaN to mean, zero etc Array a contains NaN that cannot be converted to logicals Replace the NaNs with zeros The function xor, yields true if exactly one (but not both) of two conditions is true. 5

6 The Conditions the order of precedence The rules for the numerical operators, in the order of precedence is shown in Table 2. Table 2: Operator Precedence Rules 7

7 Examples of the Conditions Matlab Statement Return Value 1 <= 2 1 (true) 1 >= 2 0 (false) a = 4; b = 3; ( a + b ) < 2 * a ('a' < 's') ('a' > 's')&&(4 < 5) 1 (true) 1 (true) 0 (false) (5<4) (3~=4) 1 (true) xor(3<5,'a'>'c') xor(3<5, 'a <'c') 1 (true) 0 (false) ~((5<4) (3~=4)) 0 (false) (10>30/4) (2+8<2*3) 1 (true) 8

8 The If Statement 9

9 The If Statement The If statement allows a program to decide on the course of action based on whether a certain condition is true or false. Three If statement types are: If Statement If-Else Statement If-ElseIf-Else Statement 10

10 The If Statement 1. If statement, the general form is; if condition action1 action2 end Condition? True Steps to process if Condition is True False Example: % IfExample1.m a = input('please enter the value of a: '); if a < 0 a=abs(a); end fprintf('the sqrt of %.1f is %.1f\n',a, sqrt(a)) Ans: 11

11 The If-Else Statement 2. If-Else statement, the general form is; if condition action1 action2 else action3 action4 end Actions executed if condition is True Actions executed if condition is False Condition? True Steps to process if Condition is True False Steps to process if Condition is False 12

12 The If-Else Statement - example CheckCalRadius.m two actions: printing error message or calculate the area using radius Ans: 13

13 The If-Else Statement - example Example: Flow Chart: Start x P Input a, x, L and P RA a RB Ra= P(L-a)/L Rb=P-Rb Bending Moment Diagram X<=a? True False Mx = RA * x Mx = RA*x - P*(x-a) Mx=Ra(x) Mx=Ra(x)-P(x-a) Print Mx End 14

14 The If-Else Statement - example SimBeamPt.m Ans: 15

15 3. If-ElseIf-Else The If-ElseIf-Else Statement The if-elseif-else statement allow for more than two possible alternatives. The general form is if condition1 action1(s) elseif condition2 action2(s) elseif condition3 else end action3(s) action4(s) Only executed if condition1 is False and condition2 is True Note: Else is optional No condition after Else 16

16 The If-ElseIf-Else Statement Condition 1 False Condition 2 False Condition 3 True True True Steps to process if condition 1 is True Steps to process if condition 2 is True Steps to process if condition 3 is True False Steps to process if all above Conditions are False The conditions may be unrelated 17

17 CaseCPA.m The If-ElseIf-Else Statement - example What is the value of message for each of the following case? 1. CPA=3.7;GPA= 3.2; Sem=7; Asal='Fantasy Island'; Ans: Conditions are unrelated 2. CPA 3.5, GPA 3.7 Sem 5, Kapas 3. CPA 3.3, GPA 3.8 Sem 7,Pangkor 4. CPA 3.6, GPA 3.2 Sem 7, Tioman 5. CPA 3.2, GPA 3.9 Sem 5, Tioman 18

18 p The If-ElseIf-Else Statement - example Ans: PverA.m a Equation: 0<a<10 p = a 10 a<25 p = a<35 p = *(a-25) a 35 p = *(a-35) 19

19 Collection of IF Statements Flow Charts Condition? True Steps to process if Condition is True False Condition 1 False Condition 2 True True Steps to process if condition 1 is True Steps to process if condition 2 is True False Condition 3 True Steps to process if condition 3 is True False True Condition? False Steps to process if all above Conditions are False Steps to process if Condition is True Steps to process if Condition is False 20

20 The If-ElseIf-Else Statement - example Example: Write a program that will calculate the support reactions and bending moment at any point on a simply supported beam partially loaded with a uniformly distributed load. Algorithm: 1. Inputs: a, b, x, L and W 2. Calculate Rb=Wb(a+b/2)/L and Ra=Wb-Rb; 3. if 0 x < a then Mx=Ra*x elseif a x (a+b) then Mx=Ra*x-W*(x-a)^2/2 else Mx=Rb*(L-x) 4. Print Ra, Rb and Mx 21

21 The If-ElseIf-Else Statement - example SimpBeamPartLoad.m 22

22 The If-ElseIf-Else Statement - example Ans: 23

23 The If-ElseIf-Else Statement - example Example: Consider implementing the following continuous mathematical function y=f(x). The value of y is based on the value of x, which could be in one of three possible ranges. Ans: CalcY.m 24

24 Nested Decisions A branch of one decision structure includes another complete decision structure inside it. The inside most decision must always end before the outer decision ends. True Steps A Sports? False Steps P True Bowling Indoors? False Football True Free Tickets Opera? False Shopping vouchers Steps D Steps T 25

25 Nested If - example StdDev ConChrStr.m 8 4 A B 20 ChrStr Determine the value of StdDev: a) Curve a ChrStr 15 StdDev 6 You can use logical operators to modify the code without having to nest the If statements. b) B 15? Ans: c) B 20? d) A 30? 26

26 Tips Indentation improves the readability. Make If block easy to understand. The following If statements do the same thing. if cond1 if Cond2 Action end end if Cond1 && Cond2 Action end If programs call for selecting among many possibilities or conditions, the switch statement is often a better alternative. The switch statement: Tests each condition separately Conditions must be mutually exclusive 27

27 The Switch Statement 28

28 The Switch Statement The general form of Switch statement is; switch switch_expression case caseexp1 action1; case caseexp2:caseexp3 action2; case {caseexp4,caseexp5} action3; otherwise actionn; end single value range of values list of values The evaluated switch_expression can be numbers, strings or objects that support the function. The switch block tests each case until one of the cases is true. If the value of the switch_expression does not match any of the case expressions (e.g, caseexp1, caseexp2 etc), the action after the word otherwise is executed. 29

29 Switch Flow Chart The Switch Statement Evaluate switch Value set 1? False Value set 2 False Value set 3 False Steps to process if all above conditions False True True True Steps to process if Expression =Value set 1 Is True Steps to process if Expression =Value set 2 is True Steps to process if Expression =Value set is True Can avoid complex IF statements by using Switch statement Use Switch structure when the decision variable has more than two or three possible values Switch is an efficient decisionmaking structure that simplifies choosing from several actions 30

30 The Switch Statement - example The switch statement is easier when a lot of choices are possible. SwitchMatClass.m tukar % This program allows student to find % the day(s) for Matlab lecture and tutorial day = input('please enter the day: '); switch day; end case 'Mon'; disp('matlab class'); case {'Tue','Wed','Thu'} disp('matlab tutorial'); otherwise disp('free'); Ans; 31

31 The Switch Statement - example SwitchDay.m Ans; 32

32 The Switch Statement - Menu Function The general form of Switch statement for menu function is; Switch_expression = menu( instructions, option1, option2, option3 ) switch switch_expression case caseexp1 action1; case caseexp2:caseexp3 action2; case {caseexp4,caseexp5} action3; otherwise action4; end action5; 33

33 Example Menu Function There is three buttons to choose from the menu function and how the otherwise get selected. firstmenu.m 34

34 Example Menu Function Ans; 35

35 The END