Hossain Shahriar

Announcement and reminder! Tentative date for final exam need to be fixed! Topics to be covered in this lecture(s) List File Regular expression

List A flexible data structure that can store one or more items L = [1, 2, 3] Some useful methods of list list.append(x): Add an item to the end of the list list.insert(i, x): Insert an item at a given position list.remove(x): Remove the first item whose value is x. It is an error if there is no such item (use with exception)

List (cont.) list.pop([i]): Remove the item at the given position in the list, and return it. If no index is specified, a.pop() removes and returns the last item in the list. list.index(x): Return the index in the list of the first item whose value is x. list.count(x): Return the number of times x appears in the list. list.sort(): Sort the items of the list, in place. list.reverse(): Reverse the elements of a list.

List -operators (cont.) Slice operator allows to choose a subset from a list L = [1, 2, 3, 4, 5, 6] Slice [ :3] = [4, 5, 6] #select elements from 0 to (3-1) Slice [3 :3] = [ ] #select elements from 3 to (3-1) Slice [2 : ] = [3, 4, 5, 6] #select elements from 2 to (6-1) Concatenating two lists (+) L1= [1, 2, 3], L2= [4, 5, 6] L = L1+ L2 = [1, 2, 3, 4, 5, 6] Multiplication (*) L1= [1, 2, 3] L2 = L1*2 = [1, 2, 3, 1, 2, 3]

List -operators (cont.) del operator deletes an element L = [1, 3, 5, 7] del L[1] = [1, 5, 7] in tests whether an element is in a list or not Returns true or false L = [1, 3, 5, 7] 3 in L = true 3 not in L = false # not operator

File Stores data permanently How to read a file? Open a file Read data by each line Close the file Do these steps in try-exception blocks def fileread(name): try: f = open(name, "r") lines = f.readlines() f.close() for line in lines: print line except IOError: raise ValueError("Filename does not exist, or cannot be read.") fileread("C:/Users/shahriar/Desktop/CISC101/test.txt")

File How to write a file? Open a file Write data Close the file Do the steps in try-exception blocks def filewrite(name): try: f = open(name, “w") # this will erase any previous content f.write(‘hello world') f.close() except IOError: raise ValueError("Filen cannot be open for writing") fileread("C:/Users/shahriar/Desktop/CISC101/test.txt")

File More one opening mode f = open(name, “w") 'r‘: file will be opened for read only 'w‘: file will be opened for writing only and it will erase all the contents if a file already exists 'a‘: fill will be opened for appending; any data written to the file is automatically added to the end

Regular expression Very useful for string data processing Tokenization First, we import regular expression library re We need to know how to write regular expression Match any sequence of alpha numeric characters– ‘\w+’ str = “I know that you do not know” Mylist = re.findall (‘\w+’, str) ['I', 'know', 'that', 'you', 'do', 'not', 'know'] We have just performed string tokenization!! Each element of the list is a token that we obtained from str We could have obtained Mylist using str.split() too!

Regular expression \d – identifies all numbers in a string and return in a list How can we recognize numbers in a string? -- \d+ numlist = re.findall ('\d+', "i am 19 years old ") print numlist >>> [19] Take home assignment Can you find two more examples of meta characters and show examples how they provide outputs?

Regular expression \d: Matches any decimal digit; [0-9]. \D: Matches any non-digit character; [^0-9]. \s: Matches any whitespace character; [ \t\n\r\f\v]. \S: Matches any non-whitespace character; [^ \t\n\r\f\v]. \w: Matches any alphanumeric character; [a-zA-Z0-9_]. \W: Matches any non-alphanumeric character; [^a-zA-Z0- 9_]. + : One or more character *: zero or more character

Regular expression match(): Determine if the RE matches at the beginning of the string. search(): Scan through a string, looking for any location where this RE matches. findall(): Find all substrings where the RE matches, and returns them as a list.