3 Workshop January 26, 2012
Summary of Workshop
Review of conditionals Ranges/lists For Loop Intro For Loop Practice Problems Conditionals + For Loops Pictures! simple picture problem
Notes for students
Piazza checkup We have established many resources (office hours, workshop, piazza) for help. Please take advantage of them. If one of these channels does not seem appropriate, you may email your TA. Mini Project 1 (MP1) was due January 25th (Wed) at 8pm Jan 27: First QUIZ tomorrow!! hw01 is released and due next Friday
Review: Conditionals
if Tells JES to check IF condition is true, then perform commands underneath it. JES will run through EVERY if statement elif ("else if...") otherwise, if the statement under elif is true, perform the commands underneath it. if the condition is met underneath the "elif", JES will be done (mutually exclusive) else: the "catchall" for JES. this is a statement that will perform the commands underneath it no matter what
Conditional practice
Function Name: dataType() Parameters: x a variable representing one of 4 data types. Return: a string representing the data type Description: Write a function that takes in a parameter that is 1 of the 4 data types, asks which data type it is, and returns it as a string. E.g. if you entered 3 as a parameter, "integer" should be returned. Hint: use the function type() to determine what the data type is. Test Cases:
>>>dataType('hi') >>>dataType([3,3,3]) >>> 'string' >>> 'list'
Lists
Remember our 4 data types? str, int, float, and list. Lis ts a re a d a ta typ e th a t h o ld m ultip le ite m s b e twe e n b ra c ke ts , no t p a re nth e s is . e .g . [0 , 'a ', 2 5 5 , 'g re a t']
P ic ture a p p lic a tio n: lis ts a re h o w we s to re a ll th e p ixe ls o f a p ic ture in a lis t T o g e t th e p ixe ls s to re d in a lis t us e g e tP ixe ls () S h o w e xa m p le o f a lis t th a t's c re a te d c a lling g e tP ixe ls o f a p ic ture
Lists continued
range() is another function that creates a list. It has 3 parameters (3rd is optional) range(start, stop1, step) Try these out in JES: range(5) range(3,10) You can also create a list simply using the [] notation. You can also use this notation to add items to the new list or already created list.
Intro to For Loops Iteration
-For loops are great for repetitious events (e.g. drawing a 20 sided polygon with turtle commands). The simple things you need to know to use for loops: -how to use ranges : range(start,end-1,steps) -being able to trace through for loops Here is an example: def forLoops(): people = ["Kelsey", "Yusuf", "Kristin"] for item in people: print item
Example Problem:
Program should print out numbers from 1 to 4 (inclusive) and then return "the end". def anotherLoopExample (): toProcess = range (1,5) #(1,2,3,4) for val in toProcess: print val # why can't I use return val? return"the end" # explain why is return okay here.
For Loop In Class Activity
def forLoop(): vals = [3, 4, 5] temp = 0 for cur in vals: temp = temp + cur print temp (a) After running this code what is the value of the variable temp? (b) How many times is temp printed? (c) How many elements are in the collection being looped over? (d) What variable is considered the loop variable? (e) True/False: It is not necessary to initialize the variable "temp". (f) If print temp were indented under the for loop, how many times would temp be printed?
Practice Problem
Function Name: doubleOrNothing Parameters: list a list of numbers Return Value: None Description: Write a function that doubles each item in the list and then prints the new value for the item. For example, if the input list was [1,2,3] then the function would print 2, 4, 6.
Combining for loops & conditionals
Function Name: countBs Parameters: list Return value: number of Bs in list Test Cases: >>> list = [ 65, 90, 85, 100, 87, 82, 82] >>> countBs(list) 4
lets you loop over a collection of items and check each of the items against a specific condition
Description: Write a function that counts the number of grades in a list that are greater than 80 and less than 90
Pictures
Digital pictures are composed of pixels (individual squares that light up with a particular color) The human eye sees the pixels as one continuous image, but digitization allows us to manipulate pictures w/code Change pictures by changing the pixels Can change color, brightness, saturation, etc (this is what Photoshop, GIMP, etc do behind the scenes) Effects like sepia tone, posterize (limited colors) possible in JES!
Things to remember with pictures
The pixels in a picture are not numbered 1 to the end. it is similar to range() in that it begins at 0 and ends at 1 minus the length of the picture. e.g. a 200x200 picture has pixels 0199.
Pictures Color
JES uses the RGB system It consists of 3 color channels: Red, Green, Blue Each channel can have a value 0255 (0off, 255brightest) Over 16.5 million possible colors! Basic colors (r,g,b): Black: (0,0,0) all color channels off White: (255,255,255) all color channels max Pure Red (255,0,0) only red channel maxed Pure Green (0,255,0) Pure Blue (0,0,255) Cyan(0,255,255) Yellow(255,255,0) Magenta(255,0,255) Gray colors (x,x,x) where x is the same value All other colors are combinations of numbers in channels
Pictures & for loops example
Trace through the following and draw a picture on paper. TA's will explain the new functions and variables as you go. def changeColor(): pic=makeEmptyPicture(2,2, black) for pix in getPixels(pic): print getColor(pix) setColor(pix,white) print get color(pix)
PICTURE WORKSHEET!!! Work in groups
Group Quiz!
Pair up with a partner or two and answer the following question: Function Name: decreaseRed Parameters: 1. pic a picture object to be edited Return Value: None. The picture will be edited directly Description: Write a function that takes in a picture and decreases the blue channel value for each pixel by half. For example, if a pixel ahs RGB values (204, 20, 206), after the function is run the new RGB values should be (102,20,206).