I210 review

Fall 2011, IUB

Python is• High-level programming

– High-level versus machine language• Interpreted Language

– Interpreted versus compiled

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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)

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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)4

Mathematical Operators

Indexing & Slicing Sequences

inventory = ["sword", "armor", "shield", "healing potion"]

inventory[0] ?inventory[0][1]?inventory[0:2]?

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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 ()

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

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Lists versus Tuplesinvent_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 immutableword = "sword”

word[0] = "S" ERRORword = "Sword" is OK!!

Using Dictionaries

• Dictionary: A mutable collection of key-value pairs

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"]

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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

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Branching Structures

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Branches based on a condition/conditions

A block of code

Using Indentation to Create BlocksCorrect:if password == "secret":

print "Access Granted"else:

print "Access Denied”

Incorrect:if password == "secret":

print "Access Granted” else:

print "Access Denied"

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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

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The while Loopwhile 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

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The continue & break Statementswhile True:

count += 1# end loop if count is greater than 10if count > 10:

break# skip 5if count == 5:

continueprint count

• continue jumps to top of loop to check condition

• break causes a loop to end immediately

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Using for Loops

• for loop – Like while 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

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Counting Forward, By Fives, and Backwards

# counting forwardfor i in range(10):

print i,

# counting by fivesfor i in range(0, 50, 5):

print i,

# counting backwardsfor i in range(10, 0, -1):

print i,

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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 functionsdef my_func():print "this function does nothing"my_func()

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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 filetext_file = open("read_it.txt", "r”)line1 = text_file.readline()text_file.close()

• Loop through a filetext_file = open("read_it.txt", "r")for line in text_file: print line

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

Handling Exceptionstry: 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

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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

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