Python

Basic Object Types

• Python has four: Numbers, Strings, Lists, and Dictionaries

Numbers in Python

• Integers do not have a fractional part
• Example integers include 1, 2, 3, -7, and 5381
• Floating-point numbers ('floats') have fractional parts
• Example floats include 3.14159, -20.7, and 1.00
• Normal mathematical operations can be performed on numbers

Mathematical Operations

• Addition: 123 + 222 = 345
• Subtraction: 65 - 17 = 48
• Multiplication: 1.5 * 4 = 6.0
• Exponentiation: 2 ** 3 = 8

Strings in Python

• String objects contain textual information
• Example: S = 'Spam!'
• Strings are sequences of one-character strings
• The length of a string can be found using the function len()
• For example, len(S) would return 5 for the string 'Spam!'

String Indexing

• Left-to-right indexing starts at 0
• For example, if S = 'Spam!', then S[0] is 'S', S[1] is 'p', and S[2] is 'a'
• Python allows backward indexing
• For example, S[-1] is '!', S[-2] is 'm', and S[-3] is 'a'
• Slicing extracts a larger section
• For instance, S[0:5] gives 'Spam!' (up to, but not including index 5) and S[1:3] gives 'pa' (index 1 & 2)
• Excluding the first or last character: S[:] is 'Spam!' (the whole thing), S[:-1] is 'Spam' (everything but the last character), and S[1:] is 'pam!' (everything but the first character)

Basic String Operators

• String concatenation: 'abc' + 'def' results in 'abcdef'
• String repetition: S * 8 results in 'Spam!Spam!Spam!Spam!Spam!Spam!Spam!Spam!' if S = 'Spam!'
• Polymorphism is when the meaning of an operator depends on the objects being operated on

String Methods

• Strings have type-specific methods
• Methods are called by typing object.method(argument)
• The find method looks for a substring and returns its location
• For example, S.find('am') returns 2
• The find method returns -1 if the substring doesn't exist
• For example, S.find('zp') returns -1
• The replace method replaces a specified substring
• For example, S.replace('am', 'XYZ') results in 'SpXYZ!'
• The split method splits a string into a list of substrings using a specified delimiter
• Example: line = 'aaa,bbb,ccc,ddd' turns to line.split(',') equaling ['aaa', 'bbb', 'ccc', 'ddd']
• Python documentation has a list of string methods
• S.upper() converts a string to uppercase
• For example, S.upper() results in 'SPAM!'
• S.lower() converts a string to lowercase
• For example, S.lower() results in 'spam!'

Special String Characters

• The newline character is '\n'
• The tab character is '\t'

Lists

• Lists are positionally ordered collections of objects
• Objects in lists may be of different types
• An example of a list is L = [123, 'spam', 1.23]

Lists in Python

• Sequence operations also apply to lists
• len(L) returns the number of objects in a list
• L[0] returns the first element
• List slicing is possible with L[:-1]
• List concatenation: L + [4, 5, 6]
• List repetition: L * 2

List Methods

• Lists are mutable, and can grow and shrink
• L.append('NI!') adds 'NI!' to the end of the list
• L.pop(2) removes the element at index 2 and returns it
• List contents can be changed
• List contents can be sorted using L.sort()
• List contents can be reversed using L.reverse()

Dictionaries in Python

• Dictionaries are also known as mappings
• Dictionaries map keys to associated objects
• They are useful for databases
• Dictionaries are represented as "key:value" pairs

Mapping Operations

• Dictionaries are represented as "key:value" pairs
• Values are accessed using their key as an index
• Create new entries incrementally
• Dictionaries can be used to update quantitative information
• x += 1 is short for x = x + 1

The if/else Statement

• Numbers: if x == 3: print(’three’)
• Strings: if x == ’ja’: print(’yes!’) else: print(’no!’)

The for Loop

• The for loop provides a way to step through all items in a sequence and apply an operation on each item
• Items in a dictionary can be sorted for iteration
• Example of looping through numbers: for x in range(1, 11): print(x) # Numbers 1 to 10 will be printed
• Strings can be looped though for x in ’spam’: print(x.upper())

Debugging

• A programming error is called a 'bug'
• Detecting and eradicating bugs is important
• Two challenges in debugging are: detecting what’s wrong and how to fix it
• Read the error message
• Insert print statements to track progression and check values
• Always run intermediate versions of your code during software development
• Errors are often single-character typos, so be precise
• Familiarize yourself with online resources and tutorials
• Try one thing at a time
• Always try; don't be shy
• Debugging is a puzzle, and is part of computational work.
• Be patient and keep trying