Data Structures in Python

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Data Structures in Python

• By Christopher Todd

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Lists in Python

• A list is a group of comma-separated values
  between square brackets.
• The values can be of multiple types such as
  strings, ints, or other lists.
• Lists start at the indices 0
• Defining a list
• A hello, 5, test, 8
• B 0, 1, test2, A

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Methods of a List

• len()
• Gives the size of the list
• append()
• Adds an item to the end of the list
• extend(L)
• Extend the list by appending all the items
• remove(x)
• Remove the first item from the list whose value
  is x. It is an error if there is no such item.
• pop(i)
• Remove the item at the given position in the list

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Methods of a List

• index(x)
• Return the index of the first item whose value is
  equal to x
• count(x)
• Return the number of times x is in the list
• sort()
• Sort the items of the list
• reverse()
• Reverse the items of the list

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Lists as Stacks

• List methods make it easy to use lists as stacks
• The Stacks are formed with the last element being
  added is the first element retrieved
• (last-in, first-out)
• Use append() to add to the top of the stack
• Use pop() to retrieve from the top of the stack

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Lists as Queues

• Unlike sets, lists are not as efficient for
  forming Queues
• Appending and popping from the front of the list
  is slower than doing the same from the back
• The Queues are formed with the first element
  being added is the first element retrieved
• (first-in, first-out)
• Use collections.deque to implement a queue
• This allows for faster access to both sides of
  the list

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Tools when using Lists

• filter(function, sequence)
• Returns a sequence consisting of items from the
  sequence that returns a true value for
  function(item)
• Outputs either a string, tuple, or list
• If sequence is a string or tuple, it returns data
  of the same type
• If sequence is anything else it returns a list
• map(function, sequence)
• Calls function(item) for each of the sequences
  items
• Returns a list of return values from the
  function(item) call
• reduce(function, sequence)
• Returns a single value constructed by calling the
  first two items of a sequence, then the result
  and the next item until the sequence is spent

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List Comprehensions

• List comprehensions provide a concise way to
  create lists without resorting to the use of the
  aforementioned methods.
• Each list comprehension consists of an expression
  followed by a for clause, then 0 or more for or
  if clauses.
• The result will be a list evaluated from the
  expression of the for and if clauses.
• If the resulting list is to be converted into a
  Tuple, it must be parenthesized.
• List comprehensions are much more flexible that
  map() and can be applied to complex expressions
  and nested functions

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Tools when using Lists

• filter(function, sequence)
• Returns a sequence consisting of items from the
  sequence that returns a true value for
  function(item)
• Outputs either a string, tuple, or list
• If sequence is a string or tuple, it returns data
  of the same type
• If sequence is anything else it returns a list
• map(function, sequence)
• Calls function(item) for each of the sequences
  items
• Returns a list of return values from the
  function(item) call
• reduce(function, sequence)
• Returns a single value constructed by calling the
  first two items of a sequence, then the result
  and the next item until the sequence is spent

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The del Statement

• The del statement is a way to remove an item from
  a list given its index, instead of its value
• The del statement can be used to remove slices
  from a list or clear an entire list
• del can also be used to delete variables
• del a
• If you reference the variable a after this, the
  program returns an error
• Unlike pop(), del does not return a value

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Tuples

• Tuples ordered list separated by commas
• Usually used in databases and formatting
• Some information about Tuples
• output Tuples are always enclosed in parentheses
• Tuples are immutable
• Defining Tuples
• Empty Tuples
• Empty ()
• Tuples with one value
• One hello,

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Sets

• Sets Unordered list that contain varying
  elements
• Usually used in membership testing and
  eliminating duplicate information
• Some information about sets
• Cannot contain duplicate information
• Set objects support mathematical operation

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Dictionaries

• Dictionaries Unordered associative arrays
• Dictionaries indexed by keys which can be any
  immutable type such as int, string, or tuple
• You cannot use lists as keys
• Dictionarys keys must be unique
• Dictionaries are unordered sets with a keyvalue
  relationship

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Dictionaries Cont.

• Defining Dictionaries
• Use curly brackets
• Separate keys and values by
• A) dictest first 1st, second 2nd
• B) dict(first1st, second2nd)
• A and B create the same Dictionary
• key()
• Returns all of the keys in the Dictionary

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Looping Techniques

• iteritems()
• Used for printing the keys and values of a
  dictionary while looping
• enumerate()
• Used for printing the index and value of a list
  while looping
• zip()
• To loop over two or more lists at the same time
• reverse()
• To loop over a list in reverse
• sorted()
• To loop over a list in sorted order

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Conditions

• The conditions used in while and if statements
  can contain any operators, not just comparisons.
• Comparison operators have a lower priority than
  mathematical operators
• Comparisons can be chained
• a lt b lt c
• Checks to see if a is less than b, than if b is
  less than c.

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Conditions Cont.

• In not in check to see if values exist in a
  list or sequence
• Is is not check to see if two values are equal
• AND, OR, and NOT can all be used to combine
  multiple comparisons
• Parenthesis can be used for grouping
• The operators are tested left to right