Files and File Management Scripts Logical Operations Conditional Statements

Transcription

1 Files and File Management Scripts Logical Operations Conditional Statements

2 Files and File Management Matlab provides a group of commands to manage user files pwd: Print working directory displays the full path of the present working directory. cd path: Change to directory (folder) given by path, which can be either a relative or absolute path. dir : Display the names of the directories (folders) and files in the present working directory. what: Display the names of the M-files and MAT-files in the current directory. delete file: Delete file from current directory type file: Display contents of file (text file only, such as an M-file).

3 Saving and Restoring Matlab Information It is good engineering practice to keep records of calculations. These records can be used for several purposes, including: To revise the calculations at a later time. To prepare a report on the project. Diary Command The diary commands allows you to record all of the input and displayed output from a Matlab interactive workspace session. The commands include: diary file: Saves all text from the Matlab session, except for the prompts (>>), as text in file, written to the present working directory. If file is not specified, the information is written to the file named diary. diary off: Suspends diary operation. diary on: Turns diary operation back on. diary: Toggles diary state

4 Saving and Restoring Matlab Information Example: >> diary roots >> a=1; >> b=5; >> c=6; >> x = -b/(2*a); >> y = sqrt(b^2-4*a*c)/(2*a); >> s1 = x+y s1 = -2 >> s2 = x-y s2 = -3 The file roots is written in your current working directory. It can be displayed by Matlab command type roots.

5 Storing and Loading Workspace Values save : Stores workspace values (variable names, sizes, and values), in the binary file matlab.mat in the present working directory save data :Stores all workspace values in the file data.mat save data_1 x y :Stores only the variables x and y in the file data_1.mat load data_1 :Loads the values of the workspace values previously stored in the file data_1.mat

6 Script M-Files Group of Matlab commands placed in a text file with a text editor. Matlab can open and execute the commands exactly as if they were entered at the Matlab prompt. The term script indicates that Matlab reads from the script found in the file. Also called M-files, as the filenames must end with the extension.m, e.g. example1.m.

7 Script M-Files Script M-Files

8 Script M-Files Example : Create the file named qroots.m in your present working directory using a text editor: %qroots:quadratic root finding script format compact; a=1; b=5; c=6; x = -b/(2*a); y=sqrt(b^2-4*a*c)/(2*a); s1 = x+y s2 = x-y To execute the script M-file, simply type the name of the script file qroots at the Matlab prompt: >> qroots a = 1 b = 5 c = 43 6 s1 = -2 s2 = -3

9 Effective Use of Script Files 1. The name must begin with a letter and may include digits and the underscore character. 2. Do not give a script file the same name as a variable it computes 3. Do not give a script file the same name as a Matlab command or function. To check existence of command, type exist( rqroot ). This command returns one of the following values: 0 if rqroot does not exist 1 if rqroot is a variable in the workspace 2 if rqroot is an M-file or a file of unknown type in the Matlab search path.

10 Effective Use of Script Files 4. All variables created by a script file are defined as variables in the workspace. After script execution, you can type who or whos to display information about the names, data types and sizes of these variables. 5. You can use the type command to display an M-file without opening it with a text editor. For example, to view the file rqroot.m, the command is type rqroot.

11 Matlab Search Path, Path Management Matlab search path: Ordered list of directories that Matlab searches to find script and function M-files stored on disk. Commands to manage this search path: matlabpath: Display search path. addpath dir: Add directory dir to beginning of matlabpath. If you create a directory to store your script and function M-files, you will want to add this directory to the search path. rmpath dir: Remove directory dir from the matlabpath.

12 Part II Different types of Variables Logical operators, Conditional Statements

13 Different types of Variables Numerical Variables Integer. positive whole numbers {1, 2, 3,... } negative whole numbers {-1, -2, -3,... } zero {0} Real. real number. Matrix index (ex: B(2,1) ) Counters All calculus results Complex. a + b i a and b are real numbers i is an imaginary number. ( i 2 = -1 ) Matlab Notation N= a+bi or N=a+bj Complex calculus Geometry Vector calculus Examples: A= 5+10i; B= j;

14 Different types of Variables Character/string Variables Character/string. Strings of alphanumeric elements Matlab Notation A= string Strings and characters follow the same rules as other matrices, with each character counting for one element. All labels and titles. Filenames Examples: name= James ; Date= October 7th ; Example: >>myname= James ; >>whos myname Name Size Bytes Class myname 1x5 10 char array

15 Different types of Variables Logical Variables Logical Variables or Boolean. Logical expression with 2 states: which means: 0 or 1 false or true Condition statements Decision making Example: >>A=true A = 1 >> whos A Name Size Bytes Class A 1x1 1 logical array Example: >>B=false B = 0 >> whos B Name Size Bytes Class B 1x1 1 logical array

16 Relational Operators Decision making uses comparison of logical variables Comparison is done by creating logical expressions Format of SIMPLE Logical Expressions: ****** expression1 relational-operator expression2****** relationaloperator Comparison == Is equal to > Is greater than < Is smaller than >= Is greater or equal to <= Is smaller or equal to ~= Is not equal to Example: >> A=1; B=2; >> A==B ans = 0 >> A>B ans = 0 >> A<B ans = 1 >> A>=B ans = 0 >> A<=B ans = 1 >> A~=B ans = 1

17 For example, suppose that x = [6,3,9] and y = [14,2,9]. The following MATLAB session shows some examples. >>z = (x < y) z = >>z = (x ~= y) z = >>z = (x > 8) z = 0 0 1

18 4-21 The relational operators can be used for array addressing. For example, with x = [6,3,9] and y = [14,2,9], typing z = x(x<y) finds all the elements in x that are less than the corresponding elements in y. The result is z = 6.

19 4-22 The arithmetic operators +, -, *, /, and \ have precedence over the relational operators. Thus the statement z = 5 > is equivalent to z = 5 >(2+7) and returns the result z = 0. We can use parentheses to change the order of precedence; for example, z = (5 > 2) + 7 evaluates to z = 8.

20 Accessing Arrays Using Logical Arrays When a logical array is used to address another array, it extracts from that array the elements in the locations where the logical array has 1s. So typing A(B), where B is a logical array of the same size as A, returns the values of A at the indices where B is

21 Logical Operators Format of COMPOUND Logical Expressions: (exp1 relational-op exp2) Logical operator (exp3 relational-op exp4) Logical operator & operation and or xor or (exclusive) ~ not A B C= A B ~(A B) Truth Table A B C= A&B ~(A&B) 1 A B C= xor(a,b) ~xor(a,b)

22 Logical Variables Examples: >> A=1; B=2; >> (A==B) & (A>B) ans = 0 >> (A<B) & (A==B) ans = 0 >> (A==B) (A>B) ans = 0 >> (A<B) (A==B) ans = 1 >> xor( (A==B), (A<B) ) ans = 1 >> xor( (A~=B), (A<B) ) ans = 0 >> ~(A<B) ans = 0 >> ~(A>B) ans = 1 >> (A>0) & (B>A) ans = 1 >> (A>0) & (B>A)&(B<0) ans = 0

23 Order of precedence for operator types. Precedence Operator type First Parentheses; evaluated starting with the innermost pair. Second Arithmetic operators and logical NOT (~); evaluated from left to right. Third Fourth Fifth Relational operators; evaluated from left to right. Logical AND. Logical OR. 4-29

24 Logical functions: Logical function all(x) all(a) any(x) any(a) finite(a) 4-30 Definition Returns a scalar, which is 1 if all the elements in the vector x are nonzero and 0 otherwise. Returns a row vector having the same number of columns as the matrix A and containing ones and zeros, depending on whether or not the corresponding column of A has all nonzero elements. Returns a scalar, which is 1 if any of the elements in the vector x is nonzero and 0 otherwise. Returns a row vector having the same number of columns as A and containing ones and zeros, depending on whether or not the corresponding column of the matrix A contains any nonzero elements. Returns an array of the same dimension as A with ones where the elements of A are finite and zeros elsewhere.

25 Logical function ischar(a) isempty(a) isinf(a) isnan(a) Definition Returns a 1 if A is a character array and 0 otherwise. Returns a 1 if A is an empty matrix and 0 otherwise. Returns an array of the same dimension as A, with ones where A has inf and zeros elsewhere. Returns an array of the same dimension as A with ones where A has NaN and zeros elsewhere. ( NaN stands for not a number, which means an undefined result.) 4-31

26 isnumeric(a) Returns a 1 if A is a numeric array and 0 otherwise. isreal(a) logical(a) xor(a,b) Returns a 1 if A has no elements with imaginary parts and 0 otherwise. Converts the elements of the array A into logical values. Returns an array the same dimension as A and B; the new array has ones where either A or B is nonzero, but not both, and zeros where A and B are either both nonzero or both zero. 4-32

27 The find Function find(a) [u,v,w] = find(a) 4-33 Computes an array containing the indices of the nonzero elements of the array A. Computes the arrays u and v containing the row and column indices of the nonzero elements of the array A and computes the array w containing the values of the nonzero elements. The array w may be omitted.

28 Logical Operators and the find Function Consider the session >>x = [5, -3, 0, 0, 8];y = [2, 4, 0, 5, 7]; >>z = find(x&y) z = Note that the find function returns the indices, and not the values. 4-34

29 Note that the find function returns the indices, and not the values. In the following session, note the difference between the result obtained by y(x&y) and the result obtained by find(x&y) in the previous slide. >>x = [5, -3, 0, 0, 8];y = [2, 4, 0, 5, 7]; >>values = y(x&y) values = >>how_many = length(values) how_many =

30 Structured Programming Sequential Programming Structured Programming Initialization Initialization Input Input Initialization Calculation Calculation Results Decision making? Calculation 1 Calculation 2 Condition statements Result 1 Result 2

31 Structured Programming Format of if statement: if Logical Expression Statements end if True Statement False Format of if else statement: if Logical Expression Statement 1 else Statement 2 end if True Statement1 False Statement2

32 Structured Programming Example: iftest1.m % Program to test the if statement #1 Initialization Input X If X>=0 Calculate True X False Display Result End of script X=input( Enter value for x: ); if X>=0 Y=sqrt(X); fprintf( The squareroot of %3.2f is %4.3f,X,Y) end >>iftest1 Enter value for x: 9 The squareroot of 9.00 is >>iftest1 Enter value for x: -2 >>

33 Structured Programming Example: iftest2.m % Program to test the if statement #2 X=input( Enter value for x: ); Initialization Input X If X>=0 False if X>=0 Y=sqrt(X); fprintf( The squareroot of %3.2f is %3.4f,X,Y) else disp( x is negative: there is no real result ) end Calculate True X Display Result End of script Display NO Result >>iftest2 Enter value for x: 3 The squareroot of 9.00 is >>iftest2 Enter value for x: -2 x is negative: there is no real result >>

34 Structured Programming Format of if elseif else statement: if Logical Expression Statements 1 elseif Logical Expression Statements 2 else Statements 3 end if True Statement1 False elseif True Statement2 False Statement3

35 Structured Programming Example: iftest3.m % Program to test the if statement #3 X=input( Enter value for x: ); Initialization if X>0 disp( x is positive ); elseif X<0 Input X disp( x is negative ); False else If X>0 disp( x equal 0 ); end True Display Result >0 End of script If X<0 True False Display Result <0 Display Result = 0 >>iftest3 Enter value for x: 3 x is positive >>iftest3 Enter value for x: -2 x is negative

36 Structured Programming nesting Problem: Pick a random number N (-2<N<2) Calculate B= log( N) If N positive calculate: A=log(N) if A positive calculate: B=sqrt(A) % nested if statements example N= rand(1)*4-2; if N>=0 A=log(N); if A>0 if A>0 B=sqrt(A); B=sqrt(A); end end end Use indentation (Tab key)

37 Structured Programming The SWITCH structure switch variable case test1 Statement 1 case test2 Statement 2.. otherwise Statement n end Switch Statement1 Statement3 Statement n Statement2 Statement4

38 The SWITCH structure % program to test switch A=input('Your choice [1,2 3]? '); switch A case 1 disp('choice 1') case 2 disp('choice 2') case 3 disp('choice 3') otherwise disp('wrong choice') end >> Testswitch Your choice [1,2 3]? 1 Choice 1 >> Testswitch Your choice [1,2 3]? 2 Choice 2 >> Testswitch Your choice [1,2 3]? 3 Choice 3 >> Testswitch Your choice [1,2 3]? 7 Wrong choice

39 Example1 Write a script example.m to find roots of a second order equation ax 2 +bx+c=0. When the script is executed it will ask the user enter the coefficients a,b,c calculate discriminant calculate the roots and display the case according to sign of discriminant.

40 Example2 Write a script that allows a user to enter a string containing a day of a week ( Sunday, Monday etc) uses a switch construct to convert the day to its corresponding number, where Monday is the first day of the week. Print out the resulting day number. Also be sure to handle the case of an illegal day name.