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Day 5 — Dictionaries - Hashmap

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🟢 Day 5 — Dictionaries (Key–Value Thinking)

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1️⃣ Why Lists Are Not Enough

So far, we used lists:
names = ["Amit", "Rahul", "Neha"]
marks = [85, 90, 78]

Problem ❌

Which mark belongs to which student?
You must remember positions
Data can easily get mismatched
💡 Lists work well when order matters, but fail when meaning matters.
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2️⃣ Real-World Analogy

Think of a phone contacts app:
You don’t say: “Give me contact at position 5”
You say: “Give me Rahul’s number”
That is key → value.
🧠 Dictionary = lookup by meaning, not position
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3️⃣ What Is a Dictionary?

A dictionary stores data as:
key → value
Example:
marks = {
"Amit": 85,
"Rahul": 90,
"Neha": 78
}
Keys are unique
Values can repeat
Order is not important (for beginners)
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4️⃣ Accessing Values

print(marks["Amit"])
🧠 You don’t “search”. You directly look up using key.
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5️⃣ What If Key Does Not Exist? (Very Important)

print(marks["Suresh"])
❌ Program crashes.
Why?
Dictionary does not guess
Key must exist

Safer Way

if "Suresh" in marks:
print(marks["Suresh"])
else:
print("Student not found")
🧠 Always check before accessing.
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6️⃣ Adding & Updating Values

Add New Key

marks["Suresh"] = 88

Update Existing Key

marks["Amit"] = 90
Same syntax. Python decides whether it’s add or update.
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7️⃣ Looping Over Dictionary

Loop Over Keys

for name in marks:
print(name)

Loop Over Values

for score in marks.values():
print(score)

Loop Over Both

for name, score in marks.items():
print(name, score)
🧠 .items() is the most used pattern.
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8️⃣ Frequency Counting Pattern (EXTREMELY IMPORTANT)

This pattern appears everywhere:
Word count
Character count
Votes
Logs
Analytics

Example: Count Characters

text = "programming"
freq = {}

for ch in text:
if ch in freq:
freq[ch] += 1
else:
freq[ch] = 1

print(freq)

How to Think

Check if key exists
If yes → increment
If no → initialize
🧠 This is one of the most important thinking patterns in programming.
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9️⃣ Dictionary vs List (Mental Model)

List
Dictionary
Access by index
Access by key
Order-based
Meaning-based
Position matters
Name matters
Slower search
Fast lookup
There are no rows in this table
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⚠️ Common Beginner Mistakes

Forgetting to check key existence
Confusing keys and values
Using mutable types as keys
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