Dictionaries & Sets

# Creating a dictionary
student = {
    "name"   : "Fatima Malik",
    "age"    : 20,
    "gpa"    : 3.92,
    "active" : True
}

# Accessing values by key
print(student["name"])    # Fatima Malik
print(student["gpa"])     # 3.92

# .get() — safe access (no KeyError)
print(student.get("age"))         # 20
print(student.get("email"))       # None (key missing)
print(student.get("email", "N/A")) # N/A (default provided)

# Add or update a key
student["email"] = "fatima@bwb.com"  # add new
student["age"]   = 21                # update existing

# Delete a key
del student["active"]
removed = student.pop("gpa")       # remove + return
print(f"Removed GPA: {removed}")

# Check if key exists
print("name" in student)    # True
print("phone" in student)   # False

print(len(student))          # number of key-value pairs

scores = {"Ali": 88, "Sara": 95, "Zara": 72}

# Loop over keys
for name in scores:
    print(name, end=" ")   # Ali Sara Zara

# Loop over values
for score in scores.values():
    print(score, end=" ")  # 88 95 72

# Loop over key-value pairs — most common
for name, score in scores.items():
    grade = "A" if score >= 80 else "B"
    print(f"  {name:8} {score}  {grade}")

# Merge two dicts
extra  = {"Omar": 80, "Sara": 98}   # Sara's score updated
scores.update(extra)
print(scores)
# {'Ali': 88, 'Sara': 98, 'Zara': 72, 'Omar': 80}

# Dict comprehension
doubled = {k: v * 2 for k, v in scores.items()}
passing = {k: v for k, v in scores.items() if v >= 80}
print(passing)   # {'Ali': 88, 'Sara': 98, 'Omar': 80}

# ── Nested dict — user record ─────────────────
user = {
    "id"      : 1001,
    "name"    : "Ali Hassan",
    "address" : {
        "city"   : "Lahore",
        "country": "Pakistan"
    },
    "courses" : ["Python", "Web Dev"]
}
print(user["address"]["city"])   # Lahore
print(user["courses"][0])         # Python

# ── Word frequency counter ────────────────────
text  = "apple banana apple mango banana apple"
freq  = {}
for word in text.split():
    freq[word] = freq.get(word, 0) + 1
print(freq)   # {'apple': 3, 'banana': 2, 'mango': 1}

# ── Grouping items ────────────────────────────
students = [
    {"name": "Ali",  "grade": "A"},
    {"name": "Sara", "grade": "B"},
    {"name": "Zara", "grade": "A"},
]
by_grade = {}
for s in students:
    g = s["grade"]
    by_grade.setdefault(g, []).append(s["name"])
print(by_grade)   # {'A': ['Ali', 'Zara'], 'B': ['Sara']}

# Creating sets — duplicates auto-removed
colours = {"red", "blue", "green", "red"}
print(colours)    # {'blue', 'green', 'red'} — order not guaranteed

empty_set = set()   # NOT {} (that's an empty dict!)

# Set from a list — fast deduplication
nums   = [1, 2, 2, 3, 3, 3, 4]
unique = set(nums)
print(unique)      # {1, 2, 3, 4}

# Set operations
python_students = {"Ali", "Sara", "Zara", "Omar"}
webdev_students = {"Sara", "Bilal", "Zara"}

print(python_students | webdev_students)   # Union — all students
print(python_students & webdev_students)   # {'Sara', 'Zara'} — in both
print(python_students - webdev_students)   # {'Ali', 'Omar'} — Python only
print(python_students ^ webdev_students)   # {'Ali', 'Omar', 'Bilal'} — not shared

# Membership test — O(1) speed
print("Sara" in python_students)    # True — instant lookup

# Add and remove
python_students.add("Nadia")
python_students.discard("Omar")   # safe — no error if missing

# Subset / superset checks
print({"Sara", "Zara"}.issubset(python_students))    # True

# list — ordered, can change, can repeat
cart = ["apple", "milk", "apple"]   # duplicates fine

# tuple — fixed record, hashable (can be dict key)
coord = (33.6844, 73.0479)   # Islamabad lat/lon
locations = {coord: "Islamabad"}  # tuple as dict key

# dict — label your data
config = {"debug": False, "port": 8000, "host": "0.0.0.0"}

# set — fast membership, remove duplicates
visited_pages = {"/home", "/about", "/home"}  # only 2 stored
if "/contact" not in visited_pages:
    print("First visit to /contact")

# Practical: find students enrolled in ALL three courses
python = {"Ali", "Sara", "Zara", "Omar"}
webdev = {"Sara", "Zara", "Bilal"}
data   = {"Zara", "Sara", "Nadia"}
all_three = python & webdev & data
print(f"Enrolled in all 3: {all_three}")   # {'Sara', 'Zara'}