Simple Algorithms. cs1

Transcription

1 Simple Algorithms cs1

2 Admin project due Thursday night. Questions?

3 First lets talk about folders Folders/directories Mechanism for separating groups of files You all know this Each system might separate folders differently C:\windows /usr/bin old:mac:way Python runs on lots of systems

4 Why subfolders Eventually you will want subfolders for your programs 'assets' Any art or sounds etc that your program uses Everything in the same folder will be a mess

5 Building a cross platform path Can't just build for your favorite platform Might not work somewhere else that python works So build cross platform To find the current working folder/directory of your program: file special variable contains the full pathname of the python file you are running

6 Os independent directory names Us os module import os dir = os.path.dirname( file ) os.path.join(dir, 'subfolder name') Python's os module will fill in the correct separator on what ever system you are using. Can use os.path.join for arbitrary directory names os.path.join(dir, 'subfolder name', 'sub-subfolder name')

7 Getting a list of filenames Python can get you a list of file names from a directory that match a pattern. Lets say we want all the files in a subdirectory that are gif files how do we do that? Lets say the subdirectory is called images

8 Getting a list of filenames Python can get you a list of file names from a directory that match a pattern. Lets say we want all the files in a subdirectory that are gif files how do we do that? os.path.join(os.path.dirname( file ), 'images', '*.gif') Show what this will produce using pycharm.

9 Glob The glob module helps you go from a 'wildcard' path expression to a list of filenames from glob import glob wildcardexp=os.path.join( os.path.dirname( \ file ), 'images', '*.gif') MyFilenameList = glob(wildcardexp) Lets take a look at what this produces in pycharm Lets take it a step farther with a list of graphics image objects.

10 Useful Data Now we can get some useful data On the perils of using numbers and strings all the time expounding upon Now that we have a bunch of image file names let's load the images for the next series of slides.

11 Simple Algorithms Need to introduce several simple algorithms here in this course. So first Algorithm Step-by-step procedure/formula for accomplishing a task. Recipe analogy

12 Counting How do we quickly find out how many images we have?

13 Counting How do we quickly find out how many images we have? Ok using the len() function, but pretend we don't have that.

14 Counting How do we quickly find out how many images we have? Ok using the len() function, but pretend we don't have that. Loop through the list and count items using an accumulator variable Start the accumulator at 0 (identity for addition) count = 0 for name in myfilenamelist: count = count+1

15 Find the largest First a philosophical question what does it mean to be the largest?

16 Find the largest First a philosophical question what does it mean to be the largest? With numbers that is easy right?

17 Find the largest First a philosophical question what does it mean to be the largest? With numbers that is easy right? Number furthest in the positive direction on the mythical number line What about strings?

18 Find the largest First a philosophical question what does it mean to be the largest? With numbers that is easy right? Number furthest in the positive direction on the mythical number line What about strings? The one with the most characters What about Images?

19 The largest Image Possible measures Largest file size Largest area Largest height Largest width Largest (x,y) location Lets use the largest area

20 How do we find the largest image Task: find the largest Image in a list of images. Solution Create a variable to hold the largest seen so far Loop through the list Compare the current list item to the largest so far If the new one is larger, update the largest so far Lets do this in code

21 Find the smallest image What change would we have to make to find the smallest rather than largest image

22 Average size? How would we find the average image size? Lets do that in code

23 Median Size Suppose we want the median size Remember median from high school/middle school? So what do we need to do to find the median?

24 Median Size Suppose we want the median size Remember median from high school/middle school? So what do we need to do to find the median? We need the list sorted otherwise we can't find the middle

25 Sorting a list Many times you will have to sort a list Lots of algorithms only work when data is sorted In some CS courses we will do that ourselves Need to know how a car works before we can design one In most of the discipline we will use the built in sorting Warning to those of you in MIS

26 So lets sort Python has two mechanisms for producing sorted lists List class has a sort method alist = [1,4,2,3] alist.sort() # list is now [1,2,3,4] Also sorted function Returns a sorted copy of the list leaving the original list unchanged

27 Sorting something useful Sorting a list of integers is not interesting What if we want to sort that list of images? How will that work?

28 Sorting something useful Sorting a list of integers is not interesting What if we want to sort that list of images? How will that work? Again how will python know what to sort on? What does it mean for one image to be larger than another?

29 Sorting something useful Sorting a list of integers is not interesting What if we want to sort that list of images? How will that work? Again how will python know what to sort on? What does it mean for one image to be larger than another? In the end, we have to tell python how we want it sorted.

30 Defining a sort key If we can come up with a numeric sortable representation of each object python can order the objects Define get_key Function should take an object (an image for us) and return a number for object to be sorted on What should we use for our number for Image objects?

31 Defining a sort key If we can come up with a numeric sortable representation of each object python can order the objects Define get_key Function should take an object (an image for us) and return a number for object to be sorted on What should we use for our number for Image objects? If we continue as we have return area of image

32 Defining get_key How will we define get_key? Make sure param and return value is right Remember we will be using image area for ordering

33 Using get_key Now we need to sort on the new key sorted(alist, key=get_key) alist.sort(key=get_key)

34 Now we can find the median Now we can find the median size image If even number lets just use the first of the two middle numbers yes I just made the math faculty twitch in terror

35 Summing How would be get the sum of all the items in the list?

36 Summing How would be get the sum of all the items in the list? First create variable An accumulator Loop through the list Add the new value to the accumulator When finished the loop huzzah done

37 Supposed we want a different size We've been looking at image area Because that is what we have. Can get file size simply in python os.path.getsize(<path>) <path> is some string with the path to the file Returns the size of the file in bytes

38 Lets look at your new project