Announcements

1. Announcements • Project 4: First Team Project • Homework: Pick a team or let me know you need a team • Teams of 3 • If you want a challenge: • Write a function that will break the Caesar cipher • User input • Frequency Analysis

2. Project 3: TODO 2 • Before starting make sure you understand how to “traverse” a matrix (list of lists) in various ways • B = [[1, 0, 0], [0.5, 3, 4], [-1, -3, 6]] • for x in range(3): • for y in range(3): • print (B[x][y])

3. CQ: Are these programs equivalent? • B = [[1, 0, 0], [0.5, 3, 4], [-1, -3, 6]] • for x in range(3): • for y in range(3): • print (B[x][y]) • B = [[1, 0, 0], [0.5, 3, 4], [-1, -3, 6]] • for y in range(3): • for x in range(3): • print (B[x][y]) A: yes B: no

4. What does this imply? • The order in which we write our loops affects the order in which we print the elements • What else affects the order in which we print the elements? • The values assigned to x and y! • What if we count down instead of up? • Instead of 0,1,2 for our indexes we us 2, 1, 0 • What if we count down only for one of the variables?

5. Input/Output • Thus far we have only been able to get input from the user and produce output to the screen • Limits the scope of our programs • What if we wanted to encrypt a book? • What if we wanted to encrypt text we already have? Do we have to type it into our program? • Our output was limited by what we could display to the screen • After our program completed the output was gone!

6. Files: Multi-line Strings • A file is a sequence of data that is stored in secondary memory (disk drive). • Files can contain any data type, but the easiest to work with are text. • A file usually contains more than one line of text. • Python uses the standard newline character (\n) to mark line breaks.

7. Multi-Line Strings • HelloWorldGoodbye 32 • When stored in a file:Hello\nWorld\n\nGoodbye 32\n

8. Multi-Line Strings • This is exactly the same thing as embedding \n in print statements. • Remember, these special characters only affect things when printed. They don’t do anything during evaluation.

9. File Processing • The process of opening a file involves associating a file on disk with an object in memory. • We can manipulate the file by manipulating this object. • Read from the file • Write to the file

10. File Processing • When done with the file, it needs to be closed. Closing the file causes any outstanding operations and other bookkeeping for the file to be completed. • In some cases, not properly closing a file could result in data loss.

11. File Processing • Reading a file into a word processor • File opened • Contents read into RAM • File closed • Changes to the file are made to the copy stored in memory, not on the disk.

12. File Processing • Saving a word processing file • The original file on the disk is reopened in a mode that will allow writing (this actually erases the old contents) • File writing operations copy the version of the document in memory to the disk • The file is closed

13. File Processing • Working with text files in Python • Associate a disk file with a file object using the open function<filevar> = open(<name>, <mode>) • Name is a string with the actual file name on the disk. The mode is either ‘r’ or ‘w’ depending on whether we are reading or writing the file. • There are also other modes • Infile = open("numbers.dat", "r")

14. File Methods • <file>.read() – returns the entire remaining contents of the file as a single (possibly large, multi-line) string • <file>.readline() – returns the next line of the file. This is all text up to and including the next newline character • <file>.readlines() – returns a list of the remaining lines in the file. Each list item is a single line including the newline characters.

15. File Processing # Prints a file to the screen. def main(): fname = input("Enter filename: ") infile = open(fname,'r') data = infile.read() print(data) main() • First, prompt the user for a file name • Open the file for reading • The file is read as one string and stored in the variable data

16. File Processing • readline can be used to read the next line from a file, including the trailing newline character • infile = open(someFile, "r")for i in range(5): line = infile.readline() print (line[:-1]) • This reads the first 5 lines of a file • Slicing is used to strip out the newline characters at the ends of the lines

17. File Processing • Another way to loop through the contents of a file is to read it in with readlines and then loop through the resulting list. • infile = open(someFile, "r")for line in infile.readlines(): # Line processing hereinfile.close()

18. File Processing • Python treats the file itself as a sequence of lines! • infile= open(someFile, "r")for line in infile: # process the line hereinfile.close()

19. File Processing • Opening a file for writing prepares the file to receive data • If you open an existing file for writing, you wipe out the file’s contents. If the named file does not exist, a new one is created. • Outfile = open("mydata.out", "w") • print(<expressions>, file=Outfile)

20. Example Program: Batch Usernames • Batch mode processing is where program input and output are done through files (the program is not designed to be interactive) • Let’s create usernames for a computer system where the first and last names come from an input file.

21. AnotherString Method • One of these methods is split. This will split a string into substrings based on spaces. >>> "Hello string methods!".split() ['Hello', 'string', 'methods!']

22. Another String Method • Split can be used on characters other than space, by supplying the character as a parameter. >>> "32,24,25,57".split(",") ['32', '24', '25', '57'] >>>

23. Example Program: Batch Usernames # userfile.py # Program to create a file of usernames in batch mode. def main(): print ("This program creates a file of usernames from a") print ("file of names.") # get the file names infileName = input("What file are the names in? ") outfileName = input("What file should the usernames go in? ") # open the files infile = open(infileName, 'r') outfile = open(outfileName, 'w')

24. Example Program: Batch Usernames # process each line of the input file for line in infile: # get the first and last names from line first, last = line.split() # create a username uname = (first[0]+last[:7]).lower() # write it to the output file outfile.write(uname+”\n”) # close both files infile.close() outfile.close() print("Usernames have been written to", outfileName)

25. Example Program: Batch Usernames • Things to note: • It’s not unusual for programs to have multiple files open for reading and writing at the same time. • The lower method is used to convert the names into all lower case, in the event the names are mixed upper and lower case. • We manually added a newline “\n” after each name, this ensures each name is on a separate line • What happens if we do not do this? • When we split the string we were “parsing”

26. Homework • Work on project 3 • Pick your groups for project 4

27. Announcements • For additional information on lists consult the python wiki book: • http://en.wikibooks.org/wiki/Python_Programming/Lists

28. Methods on Files • object.method() syntax: this time files are our object • Example: file = open(“myfile”, “w”) • file.read() -- reads the file as one string • file.readlines() – reads the file as a list of strings • file.readline() – reads one line from the file • file.write() – allows you to write to a file • file.close() – closes a file

29. Example: Writing to a File defformLetter(gender ,lastName,city): file = open("formLetter.txt",”w") file.write("Dear ") ifgender =="F": file.write("Ms. "+lastName+":\n") ifgender =="M": file.write("Mr. "+lastName+":\n") file.write("I am writingtoremindyouoftheoffer ") file.write("thatwesenttoyou last week. Everyone in ") file.write(city+" knowswhat an exceptionalofferthisis!") file.write("Sincerely ,\n") file.write("I.M. Acrook, Attorneyat Law") file.close()

30. Example: result >>> formLetter("M","Guzdial","Decatur”) Dear Mr. Guzdial: I am writing to remind you of the offer that we sent to you last week. Everyone in Decatur knows what an exceptional offer this is! Sincerely, I.M. Acrook, Attorney at Law

31. File Output is Important • Allows your programs to produce results that are viewable by other programs • Allows you to retain computed results after a program terminates

32. Python’s Standard Library • Python has an extensive library of modules that come with it. • The Python standard library includes modules that allow us to access the Internet, deal with time, generate random numbers, and…access files in a directory.

33. Accessing pieces of a module • We access the additional capabilities of a module using dot notation, after we import the module. • How do you know what code is there? • Check the documentation. • Python comes with a Library Guide. • There are books like Python Standard Library that describe the modules and provide examples.

34. The OS Module • The OS module provides an interface to the underlying operating system • Allows your program to request resources/actions to be performed on your program’s behalf • os.chdir(path) – changes the current working directory • os.listdir(path) – returns a list of all entries in the directory specified by path • os.curdir – returns what directory the program is executing in (ie the current directory)

35. The OS Path submodule • Once you import os – import os – you can also leverage the path module • os.path.isfile(path) – returns true if the path specified is a file, returns false otherwise • Use this method to perform a check to see if the user provided input for a valid file

36. Example: error checking >>> import os >>> path = input("please type a valid file name or path name:") please type a valid file name or path name:untitled >>> if os.path.isfile(path): file = open(path) … else: … # report that the file was not valid

37. The Random Module • random.random() – returns a random floating point number • random.choice(list) – returns a randomly selected element from a list of elements

38. >>> import random >>> for i in range(1,10): print (random.random()) 0.8211369314193928 0.6354266779703246 0.9460060163520159 0.904615696559684 0.33500464463254187 0.08124982126940594 0.0711481376807015 0.7255217307346048 0.2920541211845866

39. Randomly choosing words from a list >>> for i in range(1,5): print (random.choice(["Here", "is", "a", "list", "of", "words", "in","random","order"])) list a Here list

40. Randomly generating language • Given a list of nouns,verbs that agree in tense and number,and object phrases that all match the verb, • We can randomly take one from each to make sentences.

41. Random sentence generator import random def sentence(): nouns = ["Mark", "Adam", "Angela", "Larry", "Jose", "Matt", "Jim"] verbs = ["runs", "skips", "sings", "leaps", "jumps", "climbs", "argues", "giggles"] phrases = ["in a tree", "over a log", "very loudly", "around the bush", "while reading the newspaper"] phrases = phrases + ["very badly", "while skipping","instead of grading", "while typing on the Internet."] print (random.choice(nouns), random.choice(verbs), random.choice(phrases))

42. Running the sentence generator >>> sentence() Jose leaps while reading the newspaper >>> sentence() Jim skips while typing on the Internet. >>> sentence() Matt sings very loudly >>> sentence() Adam sings in a tree >>> sentence() Adam sings around the bush >>> sentence() Angela runs while typing on the Internet.

43. Many other Python Standard Libraries • datetime and calendar know about dates. • zipfile knows how to make and read .zip files • email lets you (really!) build your own spam program, or filter spam, or build an email tool for yourself. • Full listing: http://docs.python.org/modindex.html

44. A New Module: urllib • For more information see Python Documentation or the Python wiki book • http://en.wikibooks.org/wiki/Python_Programming/Internet • The urllib module provides functionality for getting information from a URL into python (or save it as a file) • urllib.urlopen(url) : opens the provided url in much the same way open, opens a file

45. Opening a URL and reading it >>> import urllib >>> connection = urllib.urlopen("http://www.ajc.com/weather") >>> weather = connection.read() >>> connection.close()

46. Now that we have the webpage what do we do? • The webpage will be encoded (typically HTML) • This is ok, its just text • How do we process text? • String methods! • We will take a look at parsing next week