Lists, Dictionaries, Iteration
An introduction to Data Abstraction using Python Lists [] and Python Dictionaries {}.
- Lists and Dictionaries
- List and Dictionary purpose
- Formatted output of List/Dictionary - for loop
- Alternate methods for iteration - while loop
- Calling a function repeatedly - recursion
- Hacks
Lists and Dictionaries
As a quick review we used variables in the introduction last week. Variables all have a type: Strings, Integer, Float, List and Dictionary are some key types. In Python, variables are given a type at assignment, Types are important to understand and will impact operations, as we saw when we were required to user str() function in concatenation.
- Developers often think of variables as primitives or collections. Look at this example and see if you can see hypothesize the difference between a primitive and a collection.
- Take a minute and see if you can reference other elements in the list or other keys in the dictionary. Show output.
# variable of type string
name = "Noor Grewal"
print("name", name, type(name))
# variable of type integer
age = 17
print("age", age, type(age))
# variable of type float
score = 90.0
print("score", score, type(score))
print()
# variable of type list (many values in one variable)
langs = ["Python", "JavaScript", "Java", "Bash"]
print("langs", langs, type(langs))
print("- langs[3]", langs[3], type(langs[3]))
print()
# variable of type dictionary (a group of keys and values)
person = {
"name": name,
"age": age,
"score": score,
"langs": langs
}
print("person", person, type(person))
print('- person["name"]', person["name"], type(person["name"]))
List and Dictionary purpose
Our society is being build on information. List and Dictionaries are used to collect information. Mostly, when information is collected it is formed into patterns. As that pattern is established you will collect many instances of that pattern.
- List is used to collect many
- Dictionary is used to define data patterns.
- Iteration is often used to process through lists.
To start exploring more deeply into List, Dictionary and Iteration we will explore constructing a List of people and cars.
- As we learned above, List is a data type: class 'list'
- A 'list' data type has the method '.append(expression)' that allows you to add to the list
- In the example below, the expression appended to the 'list' is the data type: class 'dict'
- At the end, you see a fairly complicated data structure. This is a list of dictionaries. The output looks similar to JSON and we will see this often, you will be required to understand this data structure and understand the parts. Easy peasy ;).
InfoDb = []
# Append to List a Dictionary of key/values related to a person and cars
InfoDb.append({
"FirstName": "John",
"LastName": "Mortensen",
"DOB": "October 21",
"Residence": "San Diego",
"Email": "jmortensen@powayusd.com",
"Owns_Cars": ["2015-Fusion", "2011-Ranger", "2003-Excursion", "1997-F350", "1969-Cadillac"]
})
# Append to List a 2nd Dictionary of key/values
InfoDb.append({
"FirstName": "Sunny",
"LastName": "Naidu",
"DOB": "August 2",
"Residence": "Temecula",
"Email": "snaidu@powayusd.com",
"Owns_Cars": ["4Runner"]
})
# Append to List a 3rd Dictionary of key/values
InfoDb.append({
"FirstName": "Noor",
"LastName": "Grewal",
"DOB": "October 27",
"Residence": "San Diego",
"Email": "noorg60381@stu.powayusd.com",
"Owns_Cars": ["Mazda cX9"]
})
# Print the data structure
print(InfoDb)
Formatted output of List/Dictionary - for loop
Managing data in Lists and Dictionaries is for the convenience of passing the data across the internet or preparing it to be stored into a database. Also, it is a great way to exchange data inside of our own programs.
Next, we will take the stored data and output it within our notebook. There are multiple steps to this process...
- Preparing a function to format the data, the print_data() function receives a parameter called "d_rec" short for dictionary record. It then references different keys within [] square brackets.
- Preparing a function to iterate through the list, the for_loop() function uses an enhanced for loop that pull record by record out of InfoDb until the list is empty. Each time through the loop it call print_data(record), which passes the dictionary record to that function.
- Finally, you need to activate your function with the call to the defined function for_loop(). Functions are defined, not activated until they are called. By placing for_loop() at the left margin the function is activated.
def print_data(d_rec):
print(d_rec["FirstName"], d_rec["LastName"]) # using comma puts space between values
print("\t", "Residence:", d_rec["Residence"]) # \t is a tab indent
print("\t", "Birth Day:", d_rec["DOB"])
print("\t", "Cars: ", end="") # end="" make sure no return occurs
print(", ".join(d_rec["Owns_Cars"])) # join allows printing a string list with separator
print()
# for loop iterates on length of InfoDb
def for_loop():
print("For loop output\n")
for record in InfoDb:
print_data(record)
for_loop()
Alternate methods for iteration - while loop
In coding, there are usually many ways to achieve the same result. Defined are functions illustrating using index to reference records in a list, these methods are called a "while" loop and "recursion".
- The while_loop() function contains a while loop, "while i < len(InfoDb):". This counts through the elements in the list start at zero, and passes the record to print_data()
def while_loop():
print("While loop output\n")
i = 0
while i < len(InfoDb):
record = InfoDb[i]
print_data(record)
i += 1
return
while_loop()
Calling a function repeatedly - recursion
This final technique achieves looping by calling itself repeatedly.
- recursive_loop(i) function is primed with the value 0 on its activation with "recursive_loop(0)"
- the last statement indented inside the if statement "recursive_loop(i + 1)" activates another call to the recursive_loop(i) function, each time i is increasing
- ultimately the "if i < len(InfoDb):" will evaluate to false and the program ends
def recursive_loop(i):
if i < len(InfoDb):
record = InfoDb[i]
print_data(record)
recursive_loop(i + 1)
return
print("Recursive loop output\n")
recursive_loop(0)
Hacks
- Add a couple of records to the InfoDb
- Try to do a for loop with an index
- Pair Share code somethings creative or unique, with loops and data. Hints...
- Would it be possible to output data in a reverse order?
- Are there other methods that can be performed on lists?
- Could you create new or add to dictionary data set? Could you do it with input?
- Make a quiz that stores in a List of Dictionaries.