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Which mark belongs to which student?You must remember positionsData can easily get mismatched
You don’t say: “Give me contact at position 5”You say: “Give me Rahul’s number”
Keys are uniqueValues can repeatOrder is not important (for beginners)
Dictionary does not guessKey must exist
Word countCharacter countVotesLogsAnalytics
Check if key existsIf yes → incrementIf no → initialize
Forgetting to check key existenceConfusing keys and valuesUsing mutable types as keysExpecting orderOverusing lists where dict is better
Use if key in dictStart frequency count from 1Use .items() to loop key + valueFor max: compare valuesUse meaningful key namesThink in mapping, not position
Store meaningful data using keysLook up data safelyCount frequenciesProcess real-world structured dataChoose dictionary over list correctly
Why do keys need to be unique?What happens if key does not exist?When should you prefer dictionary over list?Why is frequency counting so common?
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Day 5 — Dictionaries - Hashmap
🟢 Day 5 — Dictionaries (Key–Value Thinking)
1️⃣ Why Lists Are Not Enough
So far, we used lists:
names = ["Amit", "Rahul", "Neha"]
marks = [85, 90, 78]
Problem ❌
💡 Lists work well when order matters, but fail when meaning matters.
2️⃣ Real-World Analogy
Think of a phone contacts app:
That is key → value.
🧠 Dictionary = lookup by meaning, not position
3️⃣ What Is a Dictionary?
A dictionary stores data as:
key → value
Example:
marks = {
"Amit": 85,
"Rahul": 90,
"Neha": 78
}
4️⃣ Accessing Values
print(marks["Amit"])
🧠 You don’t “search”. You directly look up using key.
5️⃣ What If Key Does Not Exist? (Very Important)
print(marks["Suresh"])
❌ Program crashes.
Why?
Safer Way
if "Suresh" in marks:
print(marks["Suresh"])
else:
print("Student not found")
🧠 Always check before accessing.
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.
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.
8️⃣ Frequency Counting Pattern (EXTREMELY IMPORTANT)
This pattern appears everywhere:
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
🧠 This is one of the most important thinking patterns in programming.
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
⚠️ Common Beginner Mistakes
🧪 Practice Questions (Think Before Coding)
Q1
Create a dictionary to store student → marks and print all students with their marks.
Q2
Given a dictionary of items and prices, calculate total cost.
Q3
Count frequency of numbers in a list.
Example:
[1, 2, 1, 3, 2, 1]
→ {1: 3, 2: 2, 3: 1}
Q4
Count frequency of words in a sentence.
Example:
"the cat and the dog and the cat"
Q5
Find the student with highest marks.
Q6
Merge two dictionaries.
If key exists in both, add values.
Q7 (Thinking Question)
Why is dictionary better than list for storing marks?
Answer in words.
🔑 HINTS (Read Together, Only If Stuck)
✅ What You Should Be Able to Do Now
After Day 5, you should:
🧠 Reflection Questions (Answer in Words)
⭐ Golden Rule from Day 5
If data has meaning, it deserves a dictionary.
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