Programming in MATLAB

Transcription

1 2. Scripts, Input/Output and if Faculty of mathematics, physics and informatics Comenius University in Bratislava October 7th, 2015

2 Scripts

3 Scripts script is basically just a sequence of commands the same kind we enter into the command window scripts are stored as a plain text files with an.m extension so called m-files can also contain functions more on that later name of the script can be used as a command only works in the current directory, to change: GUI method: a drop-down edit box, top left console method: cd [path] to change cd to display S = cd to store

4 Script example let s have a script file called randmul.m: % Test matrix multipication. % generate random inputs A = rand(5); x = rand(1,5); % test res = x * A we can run it by typing it s name: >> randmul res =

5 Comments the lines starting with % are comments they are ignored (not executed) they serve as documentation the first block of uninterrupted comments is special: >> help randmul Test matrix multiplication. help command is text-only (and quick) use doc for complete HTML documentation can sometimes crash or freeze save often maybe prefer searching the web

6 Input

7 input() function asking for user input: >> a = input('enter a number: ') Enter a number: 4.7 a = the 4.7 was entered by keyboard the rest is MATLAB output

8 input() function the input is actually an expression: >> b = input('enter an expression: ') Enter an expression: 2*sin(pi/3) b = even variables can be used: >> c = input('enter an expression: ') Enter an expression: 2 * b c =

9 input() function so we can input vectors and matrices: >> A = input('enter a matrix: ') Enter a matrix: [ 1 2 3; 4 5 6; ] A =

10 input() function and strings: >> s = input('your name: ') Your name: 'Thomas' s = Thomas but more conveniently: >> t = input('favorite color: ', 's') Favorite color: blue t = blue

11 Output

12 Output mentioning a variable prints it: >> e = exp(1); >> e e = this is equivalent to display(e) alternatives: disp fprintf

13 disp() function display the value of an expression/variable: >> disp(e) >> disp('text') text >> disp([1 2 3; 4 5 6]) basically just omits the name of the variable / ans =

14 fprintf() function file print formatted function from the C s standard library formatted output optionally to a file (more on that later) precise control of formatting not-so-easy to use

15 fprintf() function syntax: fprintf(format string, arguments) example: >> fprintf('e is %f\n', e) e is the %f is a placeholder there should be one for each argument the \n is an escape sequence this particular one outputs a newline

16 Format strings (most) placeholders: %o integers (octal) %d integers (decimal) %x, %X integers (hexadecimal) %c character %s string %f real number (fixed-point) %e, %E real number (scientific) %g, %G real number (compact)

17 Format strings padding output to n digits with spaces or zeros: >> fprintf('[%d]\n', 4); [4] >> fprintf('[%3d]\n', 4); [ 4] >> fprintf('[%03d]\n', 4); [004] forcing the printing of a sign (minus is always printed) >> fprintf('[%+03d]\n', 4); [+04]

18 Format strings controlling the precision: >> disp(e) >> fprintf('%f\n', e) >> fprintf('%.2f\n', e) 2.72 >> fprintf('%+07.2f\n', e)

19 Escape sequences some characters have special meaning to print them, we must use an escape sequence: \n newline \t tabulator \\ backslash '' single quote %% percent sign

20 Printing vectors and matrices printing a vector: >> fprintf('%d\n', [1, 2, 3]) the print command executes for each element in matrices, this means in column-order it s easier to just use disp()

21 Logic

22 The logical type there are only two logical values: false, 0 true, 1 we can convert any number to a logical value: >> logical([ pi Inf ]) ans = zero is false and everything else is true

23 Relational operators we can also get logical values from comparisons: == equality ~= inequality <, > strict comparison <=, >= weak comparison for example: >> 1 == 2 ans = 0 >> 3 >= 2 ans = 1

24 Logic operators we can manipulate them using logic operators: ~, not negation, or disjunction, alternative &&, and conjuction xor exclusive or for example: >> [ ~0 not(1) ] ans = 1 0 >> or(1, 0) ans = 1 >> 1&&0 ans = 0 >> xor(1,0) ans = 1

25 Conditionals

26 if statement if syntax: if condition command command... command end example: >> temp = 120; >> if temp>100; disp('boiling!'); end >> if temp<0; disp('freezing!'); end Boiling!

27 Example: sgn script sgn.m x = input('enter a number: '); if x > 0 disp('positive') end if x == 0 disp('zero') end if x < 0 disp('negative') end

28 if-else statement if-else syntax: if condition command(s) else command(s) end example: >> temp = 83; >> if temp>100; disp('boiling!');... else disp('not boiling.'); end Not boiling.

29 Example: sgn script #2 nested ifs: sgn.m x = input('enter a number: '); if x > 0 disp('positive') else if x == 0 disp('zero') else disp('negative') end end

30 if-elseif statement if-elseif syntax: if condition command(s) elseif condition commmand(s) elseif condition command(s)... else command(s) end any number of elseif clauses the else clause is optional

31 Example: sgn script #3 using elseif: sgn.m x = input('enter a number: '); if x > 0 disp('positive') elseif x == 0 disp('zero') else disp('negative') end