Lecture 3 MATLAB programming (1) Dr.Qi Ying

Transcription

1 Lecture 3 MATLAB programming (1) Dr.Qi Ying

2 Objectives Data types Logical operators/functions Branching Debugging of a program

3 Data types in MATLAB Basic: Numeric (integer, floating-point, complex) Logical: True, false Characters and string Advanced (will be discussed later): Structure Function handles Look-up table User-defined data types

4 Logical Data Type Assign a logical data to a variable: a1 = true; (equivalent to: a1 = 1) b2 = false; (equivalent to: b2 = 0) c3 = 5 > 4; % relational operators return logical data

5 Relational operators Operators with two operands (typically numerical or string) that yield a logical result A1 OP A2 List of OPs == equal to ~= not equal to > greater than < less than >= greater than or equal to <= less than or equal to

6 Relational operators Relational operators on arrays the result of the operation is an array of logical values If a1 and a2 are both arrays, they have to be of the same size It is also valid if one of the operands is a scalar Relational operators on strings The strings have to be of the same length Relational operators are evaluated after all arithmetic operators have been evaluated < is equivalent to (7+3)<(2+11) Be careful of round off error when comparing two numerical values e.g. sin(pi) = e-16 (which is not zero!).

7 Logic operators Operators with one or two logical operands that yield a logical results & Logical AND && Logical AND with shortcut evaluation Logical inclusive OR Logical inclusive OR with shortcut evaluation xor Logical exclusive OR ~ Logical NOT

8 Short-circuit evaluation Example: evaluate whether a/b is greater than 10 x = a/b>10.0 Better code to avoid potential dividing-by-zero error: x = (b ~= 0) && (a/b>10.0) a/b>10.0 will not even be tested when b=0, and the entire expression returns 0 Example 2: please compare: [1 0 1] [0 0 1] [1 0 1] [0 0 1]

9 Hierarchy of operation 1. Parentheses 2. All arithmetic operators are evaluated first 3. All relational operators are evaluated, from left to the right 4. All ~ operators are evaluated 5. All & and && operators are evaluated from left to right 6. All, and xor operators are evaluated from left to right

10 Logical functions isnan(a) isnumeric(a) isinf(a) true if a is NaN (not a number) true if a is a numerical variable true if a is Inf

11 Flow control of a program One of the most important concepts of programming is the ability to control the flow of a program. The mechanisms that allow us to control the flow of execution are called control structures. Sequence Branching Iteration (looping)

12 Branches Statements that permit selective execution of specific parts of code. The if construct The switch construct The try/catch construct

13 The If construct if control_expr_1 statement 1 statement 2 elseif control_expr_2 statement 1 statement 2 else statement 1 statement 2 end Example: Solution of a quadratic equation: 2 ax + bx c + = 0 2 b b 4ac 2 x = when b 4ac 0 (two real roots) 2a b x = 2a 2 when b 4ac = 0 (two identical roots) r=b^2 4*a*c; if r>0 disp( two distinct real roots ); elseif r==0 disp( two identical real roots); else disp( two complex roots ); end

14 A Simple MATLAB program

15 The If construct Nested if construct Nesting If x>y If y<100 X=y; Else X=2*y; end Else X=0; end

16 The switch construct switch (switch_expr) case case_expr_1, statement 1 statement 2 case case_expr_2, statement 1 statement 2 otherwise, statement 1 statement 2 end Example: switch (value) case {1,3,5,7,9}, disp ( The value is odd. ) case {2,4,6,8,10}, disp ( The value is even. ) otherwise, disp( The value is out of range. ) end

17 Switch Example: [daynum, daystring] = weekday(date, 'long', 'en_us'); switch daystring case 'Monday' disp('start of the work week') case 'Tuesday' disp('day 2') case 'Wednesday' disp('day 3') case 'Thursday' disp('day 4') case 'Friday' disp('last day of the work week') otherwise disp('weekend!') end

18 The try/catch construct try statement 1 statement 2 catch statement 1 statement 2 end Example: a=[ ]; try index = input( Enter subscript of element to display: ); disp( [ 'a(' int2str(index) ')=' num2str(a(index)) ] ); catch % if we get here, an error occurred disp( [ Illegal subscript: int2str(index) ] ); end

19 Debugging a program It is equally important to debug as to code. It is very important for new learners to trace through the entire code for variable changes and logic flow to make sure they behave as expected. Use the debug tool in the editor window. Set break point at where you would the program to pause to check intermediate results of variables Execute the program step by step. Variables and their current values are available at the workspace.

20 Program documentation % Wixon Valley City Boundary Project % Lab #2 % file name Wang_Code.m % % Purpose: % This program reads in the locations of the boundary corners of the Waxon % Valley City, Texas, converts them to state plane coordinates, and then % checks whether random points are inside or outside of an area of interest % within the city boundary. % Record of revisions: % Date Programmer Description of Changes % ========== ============ ======================== % 2/3/2010 B. Wang Original Code % % Define variables: % points --- coordinates of boundary corners % WV-Boundary --- spherical coordinates of the boundary corners % p2 --- duplicate of the boundary corners with the first % and last being the identical to make convenient % drawing c closed shape % i --- index variable % n --- intermediate variable % latp --- latitude coordinates of points % longp --- longitude coordinates of points