MATLAB FUNDAMENTALS: CONTROL STRUCTURES – LOOPS HP 100 – MATLAB Wednesday, 10/1/2014

Quote and Video…  “Check Blackboard for the new homework. I know none of you will, but it makes me feel better.”  Sumona Mundal  Statistics  Video: 

Introduction to Loops  Loops: Simply a way for a program to execute certain values for a function  In other words, repeat a section of code  Can be a pattern or sporadic  Used when vectorization is not possible or impractical  We will learn for and while loops

Loops  MATLAB construct which allows multiple executions of the same block of code multiple times.  The while loop is repeated an indefinite number of times until a logical expression (like we learned a week ago) is satisfied  The for loop is repeated a predefined number of times  Visualize: While Loop For Loop Statement = true Repeat 3 times

For Loops – Basic Vocab  For construction:  Always contains for and end command  Code is between the for and end for variable = parameters Code line 1; Code line 2;...“Loop Index” end

Sporadic For Loops  For a preset vector of variables for x = [1,3,13,90] value = 2*x^2; disp(value); end  Output:

Simple Vector Loops  When your variables are in a pattern: factorial = 1; for ii = 1:5 factorial = factorial * ii; fprintf(‘%5.0f \n’, factorial); end  Output:

But……..  What if you want to store some values in an array to access later in the program after the loop is complete?

Loops – Defining an Array  You must index your array for ii = 1:5 a(ii) = 4*ii; disp(a); end  a is redefined each time (“iteration”) through the loop  Output  When your loop is done you have:  a = [ ]

Nested logicals in For Loops  You can have logical statements in loops count = 0; for ii = [ ]; grade = ii./60.*100; if grade >= 90 count = count + 1; end disp([‘In this group, ’ num2str(count) ‘ students got an A on the first physics exam!’]);  Output In this group, 2 students got an A on the first physics exam

Incremental For Loops  You can have loops in increments for a = start:incr:end Expression 1 Expression 2... Expression n end  Just be careful with your vectors. MATLAB will automatically fill empty slots in your vector with zeros

For Loops with Arrays  You can execute arrays in loops MatLab uses each column for each iteration format bank for ii = [1 2 3; 4 5 6; 7 8 9]; transpose = ii’; disp(transpose); end  Output

Nesting Loops  You can put one loop inside of another!  Make sure you use different loop indexes  ExampleMeaning: disp('# *# =#'); for ii = 1:3 1 * 1 = 1 for jj = 1:3 1 * 2 = 2 product = ii * jj; 1 * 3 = 3 disp([num2str([ii jj product])]); 2 * 1 = 2 end 2 * 2 = 4 end 2 * 3 = 6 Ends the inner most loop3 * 1 = 1 Ends the outer loop3 * 2 = 6 3 * 3 = 9  Remember: The end’s will close the innermost loop first and work out Any “ break ” or “ continue ” will also refer to the innermost loop which contains it – we’ll discuss this more later.

While Loops - Basic  While construction:  Always contains while and end command  Code is between the while and end while logical_expression Code line 1; Code line 2;... end  Logical Examples (Two weeks ago): x >= 5, x 5, x==1 || x==10

While Loops  Concept similar to for loops m = 0; while (m < 5) m = m + 1; disp(m); end  Output

Indexed Arrays  Like for loops, can store value as an indexed array m = 0; while(m < 4) m = m + 1; array(m) = m^2; end disp(array);  Output Useful when you don’t know how many iterations (“cycles”) you will need to complete

The Physics Example – Again!  While Construct: scores = [ ];student = 0;count = 0; while count < length(scores); count = count + 1; grade = scores(count) / 60 * 100; if grade >= 90 student = student + 1; end fprintf(‘In this group, %1.0f students got an A on the first physics exam \n’, student);  Output In this group, 2 students got an A on the first physics exam.

Break and Continue  Break: Can be used to stop a loop % This program accepts 10 positive input values n = 0; while n < 10; n = n + 1; a = input(‘Enter a positive number ’); if a < 0 disp(‘Has to be positive. You broke it! ’); break end disp(n) end  Output: Program displays all positive numbers until a negative number was entered.

Another Example  Guess Joe’s favorite number in 5 guesses guess = 0; while guess < 5; guess = guess + 1; fprintf(‘\nThis is guess number %1.0f \n’,guess); a = input(‘Make a guess! ’); if a == 8 disp(‘You guessed it! ’); break else disp(‘Nope. Try again.’); continue end

Good Programming Practice:  Indent the bodies of loops Not necessary but you should always do it. To make your code pretty, highlight all code, right click, smart indent  Never modify your loop index within the loop. You will produce errors that you will never find!  Preallocate all arrays before using them in a loop. Your code will run much faster! Do this: square = zeros(1,100); for ii = 1:100 square(ii) = ii^2; end Rather than this: for ii = 1:100 square(ii) = ii^2; end

The for Loop - Vecotorization (cont.)  Good Programming Practice:  Vectorize your code when you can. To perform the same calculations a vectorized code can perform 15 times faster than code using loops. Ex: Using a Loop Vecotrized for ii = 1:100 ii = 1:100; square(ii) = ii^2; square = ii.^2 square_root(ii) = ii^(1/2); square_root(ii)= ii.^(1/2); cube_root(ii) = ii^(1/3); cube_root(ii) = ii.^(1/3) end

Questions and Homework: Questions?  Homework: 8.9, 8.23, 8.24, 8.25 (with plot) Due next Wednesday at 5 (like normal). Get started early!