This document discusses lists and dictionaries in Python. It provides examples of creating lists containing other lists, accessing values within nested lists using indexes, and printing the values using for loops. It also demonstrates initializing an empty dictionary, adding key-value pairs, updating dictionaries by adding new pairs or another dictionary, and deleting items from a dictionary. The key advantages of dictionaries over lists are that dictionaries have faster lookups and do not allow duplicate keys.

1. Computer Science 151
An introduction to the art of computing
Dictionary Objects
Rudy Martinez

2. CS151 Spring 2019
Homework 2 part A
!!!Due by midnight!!!!
1. Create a file (homework2.py). ← This is what you submit on learn.
2. Your program should open the data file above.
3. Read the file into a list.
4. When you see a missing data point in either TMax or TMin you should delete the row.This row [1994-04-04,70, ] is missing TMin so
remove it from the list or do not add it to the list.
5. You should print a count of the rows (minus the removed rows with missing data).
6. Write a comma separated value (CSV) file to disk (homework2.csv)

3. CS151 Spring 2019
Lists
● Lists
sea_creatures = ['shark', 'cuttlefish', 'squid', 'mantis shrimp', 'anemone']
Index 0 1 2 3 4
List item 'shark' 'cuttlefish' 'squid' ‘mantis shrimp' 'anemone'

4. CS151 Spring 2019
Lists containing Lists
sea_creatures = [[‘shark’,’Predator’],[‘cuttlefish’,’Predator’],[ ‘squid’,’Predator’],[ ‘mantis shrimp’,’Prey’]]
List Index List Value
0 [‘shark’,’Predator’]
1 [‘cuttlefish’,’Predator’]
2 [ ‘squid’,’Predator’]
3 [ ‘mantis shrimp’,’Prey’] index 0 1
Value ‘mantis shrimp’ ’Prey’
index 0 1
Value ‘shark’ ’Predator’
index 0 1
Value ‘cuttlefish’ ’Predator’
index 0 1
Value ‘squid’ ’Predator’

5. CS151 Spring 2019
List of lists
● Addressing
print(sea_creatures[0][0])
print(sea_creatures[0][1])
Output:
shark
predator
● The first index is for the first list, the second index is for the embedded list
Name of list [index i][index j]
Sea_creatures [0] [0]

6. CS151 Spring 2019
Printing all values in a list of lists
Manually indexing
print('sea_creatures[0][0]:', sea_creatures[0][0])
print('sea_creatures[0][1]:', sea_creatures[0][1])
print('sea_creatures[1][0]:', sea_creatures[1][0])
print('sea_creatures[1][1]:', sea_creatures[1][1])
print('sea_creatures[2][0]:', sea_creatures[2][0])
print('sea_creatures[2][1]:', sea_creatures[2][1])
print('sea_creatures[3][0]:', sea_creatures[3][0])
print('sea_creatures[3][1]:', sea_creatures[3][1])
Output:
sea_creatures[0][0]: shark
sea_creatures[0][1]: Predator
sea_creatures[1][0]: cuttlefish
sea_creatures[1][1]: Predator
sea_creatures[2][0]: squid
sea_creatures[2][1]: Predator
sea_creatures[3][0]: mantis shrimp
sea_creatures[3][1]: Prey

7. CS151 Spring 2019
Printing all values in a list of lists
Print using for loops:
for i in range(0, len(sea_creatures),1):
print('sea_creatures[i] = ', sea_creatures[i])
for j in range(0, len(sea_creatures[i]),1):
print('t sublist = ', sea_creatures[i][j])
Output:
sea_creatures[i] = ['shark', 'Predator']
sublist = shark
sublist = Predator
sea_creatures[i] = ['cuttlefish', 'Predator']
sublist = cuttlefish
sublist = Predator
sea_creatures[i] = ['squid', 'Predator']
sublist = squid
sublist = Predator
sea_creatures[i] = ['mantis shrimp', 'Prey']
sublist = mantis shrimp
sublist = Prey
Ind
ex i
Ind
ex j
list[i] list[i][j]
0 [Shark , predator]

8. CS151 Spring 2019
Printing all values in a list of lists
Print using for loops:
for i in range(0, len(sea_creatures),1):
print('sea_creatures[i] = ', sea_creatures[i])
for j in range(0, len(sea_creatures[i]),1):
print('t sublist = ', sea_creatures[i][j])
Output:
sea_creatures[i] = ['shark', 'Predator']
sublist = shark
sublist = Predator
sea_creatures[i] = ['cuttlefish', 'Predator']
sublist = cuttlefish
sublist = Predator
sea_creatures[i] = ['squid', 'Predator']
sublist = squid
sublist = Predator
sea_creatures[i] = ['mantis shrimp', 'Prey']
sublist = mantis shrimp
sublist = Prey
Ind
ex i
Ind
ex j
list[i] list[i][j]
0 [Shark , predator]
0 0 [Shark , predator] shark

9. CS151 Spring 2019
Printing all values in a list of lists
Print using for loops:
for i in range(0, len(sea_creatures),1):
print('sea_creatures[i] = ', sea_creatures[i])
for j in range(0, len(sea_creatures[i]),1):
print('t sublist = ', sea_creatures[i][j])
Output:
sea_creatures[i] = ['shark', 'Predator']
sublist = shark
sublist = Predator
sea_creatures[i] = ['cuttlefish', 'Predator']
sublist = cuttlefish
sublist = Predator
sea_creatures[i] = ['squid', 'Predator']
sublist = squid
sublist = Predator
sea_creatures[i] = ['mantis shrimp', 'Prey']
sublist = mantis shrimp
sublist = Prey
Ind
ex i
Ind
ex j
list[i] list[i][j]
0 [Shark , predator]
0 0 [Shark , predator] shark
0 1 [Shark , predator] predator

10. CS151 Spring 2019
Printing all values in a list of lists
Print using for loops:
for i in range(0, len(sea_creatures),1):
print('sea_creatures[i] = ', sea_creatures[i])
for j in range(0, len(sea_creatures[i]),1):
print('t sublist = ', sea_creatures[i][j])
Output:
sea_creatures[i] = ['shark', 'Predator']
sublist = shark
sublist = Predator
sea_creatures[i] = ['cuttlefish', 'Predator']
sublist = cuttlefish
sublist = Predator
sea_creatures[i] = ['squid', 'Predator']
sublist = squid
sublist = Predator
sea_creatures[i] = ['mantis shrimp', 'Prey']
sublist = mantis shrimp
sublist = Prey
Inde
x i
Inde
x j
list[i] list[i][j]
1 [cuttlefish , predator]

11. CS151 Spring 2019
Printing all values in a list of lists
Print using for loops:
for i in range(0, len(sea_creatures),1):
print('sea_creatures[i] = ', sea_creatures[i])
for j in range(0, len(sea_creatures[i]),1):
print('t sublist = ', sea_creatures[i][j])
Output:
sea_creatures[i] = ['shark', 'Predator']
sublist = shark
sublist = Predator
sea_creatures[i] = ['cuttlefish', 'Predator']
sublist = cuttlefish
sublist = Predator
sea_creatures[i] = ['squid', 'Predator']
sublist = squid
sublist = Predator
sea_creatures[i] = ['mantis shrimp', 'Prey']
sublist = mantis shrimp
sublist = Prey
Index
i
Index
j
list[i] list[i][j]
1 [cuttlefish , predator]
1 0 [cuttlefish , predator] cuttlefish

12. CS151 Spring 2019
Printing all values in a list of lists
Print using for loops:
for i in range(0, len(sea_creatures),1):
print('sea_creatures[i] = ', sea_creatures[i])
for j in range(0, len(sea_creatures[i]),1):
print('t sublist = ', sea_creatures[i][j])
Output:
sea_creatures[i] = ['shark', 'Predator']
sublist = shark
sublist = Predator
sea_creatures[i] = ['cuttlefish', 'Predator']
sublist = cuttlefish
sublist = Predator
sea_creatures[i] = ['squid', 'Predator']
sublist = squid
sublist = Predator
sea_creatures[i] = ['mantis shrimp', 'Prey']
sublist = mantis shrimp
sublist = Prey
Index
i
Index
j
list[i] list[i][j]
1 [cuttlefish , predator]
1 0 [cuttlefish , predator] cuttlefish
1 1 [cuttlefish , predator] predator

13. CS151 Spring 2019
Dictionary
In python a dictionary is a key:value pair where a value is associated with a key!
Syntax:
#myDictionary = {key:value}
myDictionary = {} # Initialize an empty dictionary to a variable
myDictionary = {0: ‘Monday’} # key = 0, value = Monday
myDictionary = {1: ‘Tuesday’} # key = 1, value = Tuesday
myDictionary = {2: ‘Wednesday’} # key = 2, value = Wednesday
myDictionary = {3: ‘Thursday’} # key = 3, value = Thursday
myDictionary = {4: ‘Friday’} # key = 4, value = Friday
myDictionary = {5: ‘Saturday’} # key = 5, value = Saturday
myDictionary = {6: ‘Sunday’} # key = 6, value = Sunday
Key 0 1 2 3 4 5 6
Value Monday Tuesday Wednesday Thursday Friday Saturday Sunday

14. CS151 Spring 2019
Adding to a dictionary programmatically
Call update and pass a key value pair.
myDictionary.update({1:'Tuesday'})
Call update and pass it another dictionary!
# Create a second Dictionary
myDictionary2 = {3:'Wednesday'}
# Call update on it passing it a second dictionary
myDictionary.update(myDictionary2)

15. CS151 Spring 2019
Deleting items from a dictionary
Call Delete and pass a key
del myDictionary[0] # Notice square brackets like list
Call pop and pass a key
myDictionary.pop(0) # Function call so parens here plus key
Let’s clear the entire Dictionary
myDictionary.clear() # Removes all key:value pairs

16. CS151 Spring 2019
Why use a Dictionary over a List?
List Dictionary
Keeps order of items Not guaranteed ordering
Can have duplicates No Duplicates
Slower lookups O(1) O(1) lookups

17. CS151 Spring 2019
Let’s code
Open spyder3