I210 review Fall 2011, IUB

1. I210 reviewFall 2011, IUB

2. Python is • High-level programming • High-level versus machine language • Interpreted Language • Interpreted versus compiled

3. Variables • Variable: Represents a value; provides way to get at information in computer memory • Assignment statement: Assigns a value to a variable; creates variable if necessary • Rules for legal variable names • Can contain only numbers, letters, and underscores • Can’t start with a number • Can’t be a keyword (del, elif, else, except, for , from, global, if, import, in, is, not, or, print, return, try, while, with)

4. Data Types • Data type: sequences (containers) • Strings (a sequence of letters) • Tuples (a sequence of elements of any type; immutable) • Lists (a sequence of elements of any type; mutable) • Dictionary is NOT a sequence • How to access the elements in a sequence • Sequential access (using for and while loops) • Indexing & Slicing • Removing and adding elements • Important sequence operators and functions, len([1, 2, 3]), ”a" in "abc", [1, 2, 3].index(1)

5. Mathematical Operators

6. Indexing & Slicing Sequences inventory = ["sword", "armor", "shield", "healing potion"] inventory[0] ? inventory[0][1]? inventory[0:2]?

7. List Methods languages = ["Python", "C++", "Java", "HTML"] • languages.append("FORTRAN") • languages.remove("C++") • languages.sort() • languages.count"C++") • languages.index("Python") • languages.insert(0, "PHP") • languages.pop()

8. Using the Right Types • Python does not need to specify the type of a variable in advance (by contrast, C does) Python: cost = 10 C: int cost = 10; • Important to know which data types are available • Equally important to know how to work with them • If not, might end up with program that produces unintended results • Converting values: e.g., int(“3”) = 3

9. Lists versus Tuples invent_list = ["sword", "armor", "shield", "healing potion"] invent_tuple = ("sword", "armor", "shield", "healing potion") invent_list[0] = "money" is OK!! invent_tuple[0] = "money" ERROR • Lists are mutable(we can dynamically change the individual elements!); but tuples are immutable • Strings are immutable word = "sword” word[0] = "S" ERROR word = "Sword" is OK!!

10. Using Dictionaries • Dictionary: A mutable collection of key-valuepairs geek = {"404" : "clueless.", "Uninstalled" : "being fired."} • Unlike tuples and lists, dictionaries don’t organize data into sequences, but pairs • Look up a key to get a value geek["404"]

11. Branching Structures • Branches based on a condition/conditions • if • if-else • if-elif-else • Condition: Expression that is True or False • Often create conditions by comparing values • Treating values as conditions: Any empty (None) or zero value is False • Compound conditions (logical operators) • and, or, not

12. Branching Structures Branches based on a condition/conditions A block of code

13. Using Indentation to Create Blocks Correct: if password == "secret": print "Access Granted" else: print "Access Denied” Incorrect: if password == "secret": print "Access Granted” else: print "Access Denied"

14. Loop Structure • Need to know when and how to use for and while loops correctly • for loops: iterate over the elements in a sequence • while loops: repeat a block of code as long as a condition is • insertion_sort.py

15. The while Loop while condition: <block> while response != "Because.": response = raw_input("Why? ”) • Repetition based on a condition • Allows you to repeat section of code as long as some condition is True • Like if statement, in that it tests a condition and executes associated block if condition True • But, after block, repeats condition test; if condition still True, repeats block • Continues process until condition tests False

16. The continue & break Statements while True: count += 1 # end loop if count is greater than 10 if count > 10: break # skip 5 if count == 5: continue print count • continue jumps to top of loop to check condition • break causes a loop to end immediately

17. Using for Loops • for loop • Likewhile loop, repeats a loop body • Unlike while loop, doesn’t repeat based on condition • Repeats loop body for each element in a sequence • Ends when it reaches end of the sequence • e.g., go through sequence of game titles and print each

18. Counting Forward, By Fives, and Backwards # counting forward for i in range(10): print i, # counting by fives for i in range(0, 50, 5): print i, # counting backwards for i in range(10, 0, -1): print i,

19. Functions • How to write functions • How to receive and return values • Not all functions take arguments, and not all functions return values!! • Understand local and global variables • And don’t forget to call functions defmy_func(): print "this function does nothing" my_func()

20. Working with Files • Open a file ("r", "w") • Read/write • Read from text files; readline(), readlines() • Write to text files (permanent storage); write(), writelines() • Close the file text_file = open("read_it.txt", "r”) line1 = text_file.readline() text_file.close() • Loop through a file text_file = open("read_it.txt", "r") for line in text_file: print line

21. Types of Errors • Syntax errors • “Computer doesn’t understand what I wrote, because I made a typo” • Logical Errors • Program does not perform correctly • Debugging • Runtime Error • The program saves and begins to execute, then crashes, usually after receiving input • try-except statements

22. Handling Exceptions try: num = float(raw_input("\nEnter a number: ")) except(ValueError): print "That was not a number!" else: print "You entered the number", num • Can add single else clause after all except clauses • else block executes only if no exception is raised • num printed only if assignment statement in the try block raises no exception

23. A Sample Question • Write a function that accepts a filename and reads in the comma-separated numbers from the text file. The function saves the numbers in a list, calculates and displays the average of the numbers, and return the list of numbers. • Please work out the practice midterm