Python — lists, loops and dictionaries

invoice_amounts = [1200.5, 1500.0, 980.8, 1720.2]

total = 0
for amount in invoice_amounts:
    total += amount
print(total / len(invoice_amounts))     # average

large = [a for a in invoice_amounts if a > 1200]   # list comprehension

len() gives the count, invoice_amounts[0] the first item, invoice_amounts[-1] the last.

A dictionary maps key → value. Perfect for counting:

products = ["coffee", "tea", "coffee", "juice", "coffee"]
counts = {}
for p in products:
    counts[p] = counts.get(p, 0) + 1
# {'coffee': 3, 'tea': 1, 'juice': 1}

Almost all data you encounter in AI consists of lists of numbers (vectors), lists of lists (matrices), or dictionaries (records with named fields). Getting comfortable here makes the rest much easier.