Python Lists Explained — Creation, Indexing, Methods and Common Mistakes

# --- Creating different kinds of lists ---

# A list of grocery items (strings)
grocery_list = ["milk", "eggs", "bread", "butter"]

# A list of daily temperatures in Celsius (floats)
weekly_temps = [18.5, 21.0, 19.3, 22.7, 20.1, 17.8, 23.4]

# A list of student ages (integers)
student_ages = [14, 15, 15, 16, 14, 17]

# A mixed list — Python allows different types in one list
user_profile = ["Alice", 30, True, 4.9]

# An empty list — created first, filled later
pending_tasks = []

# Print each list to see exactly what Python stores
print("Grocery list:", grocery_list)       # shows all 4 items in order
print("Weekly temps:", weekly_temps)       # shows all 7 temperatures
print("Student ages:", student_ages)       # shows 6 ages
print("User profile:", user_profile)       # shows mixed types
print("Pending tasks:", pending_tasks)     # shows empty brackets

# Check how many items are in a list using len()
print("Number of groceries:", len(grocery_list))   # len() counts the items

# Our list of planet names in order from the sun
planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
#  indexes:      0         1        2       3        4          5         6         7
# neg indexes:  -8        -7       -6      -5       -4         -3        -2        -1

# --- Positive indexing ---
print(planets[0])    # First planet  → Mercury
print(planets[2])    # Third planet  → Earth
print(planets[7])    # Last planet   → Neptune

# --- Negative indexing (count from the end) ---
print(planets[-1])   # Last planet         → Neptune
print(planets[-2])   # Second to last      → Uranus
print(planets[-8])   # Same as index 0     → Mercury

# --- Slicing: list[start:stop] — stop is EXCLUDED ---
inner_planets = planets[0:4]        # indexes 0, 1, 2, 3 (not 4)
print("Inner planets:", inner_planets)

outer_planets = planets[4:]         # from index 4 to the end
print("Outer planets:", outer_planets)

first_three = planets[:3]           # from the start up to (not including) index 3
print("First three:", first_three)

# --- Slicing with a step ---
every_other = planets[::2]          # start to end, jumping 2 at a time
print("Every other planet:", every_other)

reversed_planets = planets[::-1]    # step of -1 reverses the whole list
print("Reversed:", reversed_planets)

# --- Slicing creates a NEW list — original is unchanged ---
print("Original still intact:", planets)

# Start with a list of tasks in a to-do app
tasks = ["buy groceries", "call dentist", "write report", "call dentist"]

# --- append() — add one item to the end ---
tasks.append("pay electricity bill")   # adds to the tail of the list
print("After append:", tasks)

# --- insert() — add at a specific position ---
tasks.insert(1, "reply to emails")     # inserts at index 1, shifts others right
print("After insert at index 1:", tasks)

# --- extend() — merge another list onto the end ---
extra_tasks = ["book flight", "renew passport"]
tasks.extend(extra_tasks)              # each item of extra_tasks is added individually
print("After extend:", tasks)

# --- remove() — delete the first matching value ---
tasks.remove("call dentist")           # removes only the FIRST occurrence
print("After remove 'call dentist':", tasks)

# --- pop() — remove and RETURN the item at an index ---
finished_task = tasks.pop(0)           # removes the item at index 0 and gives it back
print("Popped task:", finished_task)
print("List after pop:", tasks)

# --- count() — how many times does a value appear? ---
duplicate_count = tasks.count("call dentist")   # still one 'call dentist' left
print("'call dentist' appears:", duplicate_count, "time(s)")

# --- index() — find the position of a value ---
position = tasks.index("write report")
print("'write report' is at index:", position)

# --- sort() — sorts the list IN PLACE (modifies original) ---
fruit_prices = [3.50, 1.20, 4.75, 2.00, 0.99]
fruit_prices.sort()                    # ascending order by default
print("Sorted prices:", fruit_prices)

# --- sorted() — returns a NEW sorted list, original untouched ---
unsorted_names = ["Zara", "Alice", "Mike", "Beth"]
alpha_names = sorted(unsorted_names)   # original list not changed
print("Sorted names (new list):", alpha_names)
print("Original names unchanged:", unsorted_names)

# --- reverse() — flip the list IN PLACE ---
fruit_prices.reverse()
print("Reversed prices:", fruit_prices)

# --- clear() — empty the entire list ---
temporary_cache = ["data1", "data2", "data3"]
temporary_cache.clear()
print("Cache after clear:", temporary_cache)

# A list of exam scores for a class
exam_scores = [72, 85, 91, 60, 78, 95, 55, 88, 74, 66]

# --- Basic for loop --- iterate over every score
print("All scores:")
for score in exam_scores:
    print(" ", score)   # each score printed on its own line

# --- enumerate() — gives you index AND value together ---
print("\nScores with student numbers:")
for student_number, score in enumerate(exam_scores, start=1):  # start=1 so counting begins at 1
    print(f"  Student {student_number}: {score}")

# --- List comprehension: transform every item ---
# Give every student a 5-point bonus
boosted_scores = [score + 5 for score in exam_scores]
print("\nBoosted scores:", boosted_scores)

# --- List comprehension: filter items with a condition ---
# Only keep scores that are a pass (70 or above)
passing_scores = [score for score in exam_scores if score >= 70]
print("Passing scores:", passing_scores)

# --- List comprehension: transform AND filter together ---
# Boost only failing scores (below 70) by 10 points
adjusted_scores = [score + 10 if score < 70 else score for score in exam_scores]
print("Adjusted scores:", adjusted_scores)

# --- Equivalent for loop (to show what comprehension does under the hood) ---
adjusted_scores_loop = []
for score in exam_scores:
    if score < 70:
        adjusted_scores_loop.append(score + 10)   # failing: add 10
    else:
        adjusted_scores_loop.append(score)         # passing: keep as-is

print("Same result via loop:", adjusted_scores_loop)
print("Both methods match:", adjusted_scores == adjusted_scores_loop)  # should be True