EGR 106 – Week 2 – Arrays & Scripts Brief review of last week Arrays: – Concept – Construction – Addressing Scripts and the editor Audio arrays Textbook , chapter

Review of Last Week Variables: placeholders for numerical data – equal sign is an assignment operator – naming restrictions (not pi, etc. ) – can be complex valued ( x = i ) Basic math on numbers and variables: + – * / ^

Array Concept The fundamental data unit in Matlab – Rectangular collection of data – All variables are considered to be arrays Data values organized into rows and columns

Size or dimension of an array: – number of rows and columns – written as R by C or R x C where R = number of rows C = number of columns e.g. yield is 3 by 4 test is 1 by 5

– Array size (or value if a scalar or small) is shown in the workspace window

Names for special sizes – scalar: 1 x 1 array 4 or [4] – row vector: 1 x C array [ ] is a 1 x 5 row vector – column vector: R x 1 array is a 3 x 1 column vector

– matrix: R x C array with R > 1, C > 1 If R = C  square matrix – each row must have the same number of entries If R = C = 0  null matrix [ ] (a pair of empty brackets)

Array Construction Direct specification: – Name followed by an equal sign ( = ), just like variables – List values within a pair of brackets ( [ ] ) – Enter data one row at a time left to right, top to bottom order space or comma between the values rows separated by semicolons or the enter key

– For example, to get type b = [ 4,5,3,9; 10,4,66,20; 18,-3,2,0 ] or b = [ 4, 5, 3, 9 10, 4, 66, 20 18, -3, 2, 0 ] enter

– Can use simple math operations as well as numerics as the entries: – Note the common format of all entries in the response (exp(1) = e = , log 10 (100) = 2, = ) – M ATLAB scales the exponent to the largest entry !!

– This scaling is sometimes deceptive: Not really zero Really zero

Concatenation – gluing arrays together if a = [ ] b = [ ] – Attaching left to right – use a comma [ a, b ] – Attaching top to bottom – use a semicolon [ a; b ] semicolon comma

– Note that sizes must match for this to work: if a = [ ] then [ a, b ] = ?? [ a; b ] = ?? – Size needs for concatenation: # of rows the same for side by side (comma) # of columns the same for top to bottom (semicolon)

Evenly spaced vectors – the colon operator first : increment : maximum yields a row vector of equally spaced values – examples: 0 : 2 : 10 [ ] 1 : 5 [ ] 7 : -2 : -3 [ ] 1 : 2 : 8 [ ] – default for increment is 1 Note – does not hit 8!! Recall Assignment 1, #10 xv = –3:0.1:3;

Addressing We indicate a particular element within an array by it’s row/column position: – use parentheses after the array name – e.g. yield(2,4)

Used to read a value from an array (right hand side of = )

How about more than one entry? Can specify a rectangular sub-array – again, use parenthesis after the array name – list desired rows, comma, desired columns as separate vectors, typically in brackets – e.g. yield([1 2],[3 4])

Examples sample(3,2) sample (1,2:4) sample (3,[4,3,2,1]) sample ([1,3],[2,4])

Used to read a sub-array ( rhs of =) Note – scalar row choice does not need brackets!

Avoid some common addressing errors:

Rules of the road for arrays: – Symbols to use: brackets to glue elements together to make an array (left to right or top to bottom) comma (or space) and semicolon (or enter) for separating column/row elements parentheses after the array name for addressing – Be careful to match array sizes – Remember – rows first, then columns in addressing

Scripts – Simple Programs So far, commands have been typed in the command window: – Executed by pressing “enter” – Edited using the arrow keys or the history window

Script (m-file) Concept A file containing Matlab commands – Can be re-executed – Is easily changed/modified or ed to someone Commands are executed one by one, sequentially – File is executed by typing its name (without.m) – Results appear in the command window (or use ; ) Can be created using any text editor –.m extension – Listed in Current Directory window

Matlab’s Built-in, Color Editor: – Can create a new file or open an existing M file (icons or click on file name) – Color used to aid in file creation (command types, typos, etc.)

– typical Windows menu – line numbers – “run” button or F5 – debug capability – comment lines – note use of semicolons – note use of colors

How scripts can get data: – From arrays in the current workspace – From arrays defined in the script – Using the “input” command: Numeric: x = input(' how many? ') String: x = input(' name? ', 's')

How scripts can show data: – Command of the array name – Using the display command: Existing array (a single array only – if necessary, use [ ] !!) disp(x) or disp([x y]) Text disp(' The task is done ')

Example : Note that disp shortens the resulting output by dropping the array name and removing blank lines

Other useful script commands: – clc – clears the command window – pause – stops operation and waits for a key press – pause(n) – stops operation and waits for n seconds

Sample Scripts

Audio Arrays (not in the book!) MatLab can interface to microphones and speakers through the computer’s sound card (sampled and digitized) ….e.g. “maple”

– Matlab represents sounds using arrays (actually, as column vectors) – Equipment needed: Microphone Speakers and software: 2 Matlab scripts: init_sound.m sound_in.m Headsets available in ECC Files available on the egr106 website; save them in your “work” directory

– How to use them: Connect headset (is it muted?) Type init_sound at the command prompt (only needed once per session) Type sound_in at the command prompt to record 1 second of sound (waits for your input) generates array named “data” Type sound(data,10000) at the prompt to play array “data” – Can plot “data”, manipulate “data” before replaying, etc.

init_sound – sets up M ATLAB to sound card interface sound_in – prompts you to speak and records one second of input sound(.,.) – a regular MatLab function What are these tools?

Time Sampling – Speech

Whistling a Scale

Normal versus too loud an input