Week 7 - Programming I Relational Operators A > B Logical Operators A | B For Loops for n = 1:10 –commands end

Week 7 - Programming I Introduce programming: –“scripts” and “functions” are programs of sequentially evaluated commands Today, extend features to: additional operators branches to control operations loops to repeat operations Textbook chapter 7, (sections 7.1, 7.2.1, 7.2.2, 7.4.1, 7.6)

Relational Operators Used to compare A and B: A op B A and B can be: –Variables or constants or expressions to compute –Scalars or arrays (watch the sizes on arrays!) –Numeric or string Result is true (1) or false (0) – perhaps an array Operators: >> == = << =~ =

Examples: expressionresult 5 < 71 [ ] > = [ ]1 1 0 max( 1:6 ) < = 7 1 [3 pi -12 ] > 'Tom' = = 'Bob'0 1 0 'Tom' = = 'm'0 0 1 Note – arrays and strings need to be the same size

Notes: Can compute using the result: e.g. –“how many of a particular letter in a state name?

Don’t confuse = = and = Round off errors can impact ~ = sin(0) = = 01 sin(pi) = = 00 instead, test size abs(sin(pi)) < = eps 1

Logical Operators Typically, used to combine A and B: A op B A and B can be: –Variables or constants or expressions to compute –Scalars or arrays, numeric or string A and B are interpreted as binary: –Numeric 0 is interpreted as false –All else is interpreted as true (equal to 1) Result is true (1) or false (0) – perhaps an array

Basic operators: and & or | xor not ~ ABA&BA|BA|Bxor(A,B)~A~A “truth table”“unary” operator

Examples: expressionresult (5 9)1 'Tom'= ='m' | 'Tom'= ='o' Also consider: name = input('enter name','s'); name = = 'Tom' | name = = 'Bob' or roll = sum(ceil(6*rand(1,2))); roll = = 7 | roll = = 11

Operator Precedence (left to right) 1.Parentheses ( ) 2.Transpose(') and power(.^) 3.Negation (-) and logical negation (~) 4.Multiplication (.*) and division (./), 5.Addition (+) and subtraction (-) 6.Colon operator (:) 7.Relational operators (, >=, = =, ~=) 8.Logical AND (&) 9.Logical OR (|)

Branches Conditional Statements Commands to select and execute certain blocks of code, skipping other blocks. Three types in Matlab: –if/else –switch –try/catch this week

“If/Else” Use relational and logical operators to determine what commands to execute: if expression {commands if true } else {commands if false } end evaluate this use of blue in editor; also, auto indentation

Example 1 – output whether a variable x is positive or not: x = … { computed somehow }; if x > 0 disp('the value is positive') else disp('the value is negative or zero') end

Example 2 – output a warning if the variable x is negative (note that there is no “else” portion in this test): x = … { computed somehow }; if x < 0 disp(‘Warning: negative value’) end no else component

Example 3 – ask if a plot should be drawn: x = input(‘Plot now? ‘,’s’); if x = = ‘yes’ | x = = ‘YES’ plot( ….. ) end more complicated expression

Loops Commands to repeatedly execute certain blocks of code Two types in Matlab: –for –while this week

The “for” Loop Used for a specific number of repetitions of a group of commands: for index = array { commands to be repeated go here } end Rules: One repetition per column of array index takes on the corresponding column’s values

Homework revisited – collect 7 digits of a telephone number and store in an array: for digit = 1:7 number(digit) = input('enter value '); end 7 repetitions since the array is [ ] digit cycles through the 7 values to create the 1 by 7 array “number”

Example 1 – calculating interest for 10 years: value = 1000; for year = 1:10 value = value * 1.08; fprintf('$ %6.2f after %2d years\n', value,year) end no need for a counter variable! year takes on the values 1 through 10

Example 2 – implement a count down timer (in seconds): time = input(‘how long? ‘); for count = time:-1:1 pause(1) disp([ num2str(count),’ seconds left’]) end disp(‘done’)

Example 3 – a general vector for array: for x = [ ] disp([ ' dial ', num2str(x) ]) pause(1) end

Example 4 – a matrix for array: board =(2*(rand(3,3)>.2)–1).*(rand(3,3)>.5) for x = board x end

Example 5 – even a string array: for x = 'EGR106' disp(x) end

Early Termination of Loops Use the break command: for variable = {array of length n} ….. break end Terminates the loop

Example – Hi-Lo: a guessing game with feedback select hidden number input guess correct? yes no provide hi/lo feedback 5 tries? yes no win lose

% HI-LO numb = round ( 100 * rand (1) ); for count = 1:5 guess = input('guess my number '); if guess = = numb disp( 'You got it !!!' ) break end if guess > numb disp('you are too high') else disp('you are too low') end if count = = 5 disp('Sorry, you lose! ') end

Example – calculating interest until the amount doubles using a for loop: value = 1000; for year = 1:1000 value = value * 1.08; fprintf('$ %6.2f after %2d years\n', value,year) if value >= 2000 break end will calculate up to 1000 years, if necessary if condition decides when to terminate loop