Beginning Programming in Python

Fall 2019

Agenda

  • A couple of simple list algorithms:
    • Linear search
    • Binary search
    • A very brief introduction to runtime analysis and Big-O Notation
  • Dictionaries
  • Set

Linear search

  • Linear search or sequential search is a search method for finding an element within a list
  • It sequentially checks each element of the list until a match is found or the whole list has been searched

Linear search example

Binary search

  • In binary search, for a sorted list, we start from the middle of the list and then check if the middle word is less than, equal to or greater than the search query string.

  • Based on the result we can generally halve the size of the search, and so recursively solve the problem.

binary search example

Linear vs binary search

dictionaries

dictionary

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 Advanced issues found
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  • Dictionary is an unordered collection of key-value pairs
    • Each value can be accessed using its key
    • Key can be any immutable object
eng2sp = {}
eng2sp["one"] = "uno"
eng2sp["two"] = "dos"

sp2eng = {"uno":"one", "dos":"two"}

practice with dictionary

5 minutes break!

why use dictionaries?

  • Convenient
  • Fast
    • The cost of looking up a key in the dictionary is O(1) comparing to searching a list in O(N)
    • Removal and update operations are also O(1)

word counting

sets

set

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 Advanced issues found
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  • Set is a collection of unique objects
  • Objects in set are not ordered 
x = set(["Internet", "Radio", "TV"]) 
print(x)
# Prints {'Internet', 'TV', 'Radio'} 

y = {"Internet","Radio","Internet"} 
print(y)
# Prints {'Internet', 'Radio'}

set example

Lecture 12 challenge

Questions?