Looping through the 3 Main Data Structures in Python

Looping through the 3 Main Data Structures in Python

Situation: An introduction to loops and data structures.

In Python, three main ones are lists, sets and dictionaries. Here are some vignettes to show how they can be looped.

Three ways to loop

# set
data = {'Caleb', 'Jimmy', 'Sam', 'Monica', 'Caleb'}

# list
data = ['a', 'b', 'c', 'd', 'e']

# dictionary
data = {
    'Caleb': 5,
    'Jimmy': 10,
    'Sam': 12,
    'Monica': 20,
    'Caleb': 4
}


# direct
for i in data:
   print(i)
   
# range & len
for i in range(len(data)):
   print(i)


# enumerate
# grab index and key, not value
# remove duplicate keys
for i, letter in enumerate(data):
   print(i, letter)
   

Iterating through Dictionaries

data = {
    'Caleb': 5,
    'Jimmy': 10,
    'Sam': 12,
    'Monica': 20,
    'Caleb': 4
}

# .items() method returns key & values
for key, val in data.items():
    print(key, val)
    
# enumerate gets index & key (no values)
for index, value in enumerate(data):
    print(index, value)

Updating Dictionaries

You have a dictionary with key, value pairs, you want to update values of existing keys and find new keys and determine value.

drinks_in_storage = {
    "grape": 5, 
    "black cherry": 3, 
    "cola": 4
}

# update with List of Lists
drink_purchased = [['grape', 3], ['black cherry', 1], ['cola', 5], ['orange', 12]]

# can also update with List of Sets
drink_purchased = [('grape', 3), ('black cherry', 1), ('cola', 5), ('orange', 12)]

# checking if a key is present before updating value
for drink in drink_purchased:
    if drinks_in_storage.get(drink[0]):
        drinks_in_storage[drink[0]] += drink[1]
    else:
        drinks_in_storage[drink[0]] = drink[1]

# see that drinks_in_storage added 'orange':12 to the dictionary
print(drinks_in_storage) 

Using defaultdict

# -------------------------- without defaultdict
drink_purchased = [('grape', 3), ('black cherry', 1), ('cola', 5), ('orange', 12)]

# checking if a key is present before updating value
for drink in drink_purchased:
    if drinks_in_storage.get(drink[0]):
        drinks_in_storage[drink[0]] += drink[1]
    else:
        drinks_in_storage[drink[0]] = drink[1]
        
        
# -------------------------- with defaultdict
drinks_in_storage = defaultdict(int, drinks_in_storage)

drink_purchased = [['grape', 3], ['black cherry', 1], ['cola', 5], ['orange', 12]]

# cut out the 'else'
for drink in drink_purchased:
        drinks_in_storage[drink[0]] += drink[1]