C++ Program to Count Frequency Using Map with Explanation
C++
Medium
STL
33 views
1 min read
188 words
This problem helps you practice core C++ fundamentals in a practical way. It builds intuition around element, count, frequency. Let’s break it down step by step so you can implement it confidently.
Problem Statement
Count how many times each element appears.
Real Life: Vote counting, inventory management.
Input Format
An integer n (number of elements)
Then n integers.
Output Format
Each unique element with its count.
Constraints
• n ≥ 1
• Elements can repeat
• Order does not matter
Concept Explanation
We need to count how many times each value appears.
This is useful in real life for vote counting or inventory management.
Step-by-Step Logic
1.Take input size n.
2.Read n elements one by one.
3.Use a map to store:
• key → element
• value → count
4.For each element:
• If it already exists in map, increase count.
• If not, insert it with count 1.
5.Traverse the map.
6.Print each element with its frequency.
Code Solution
This explanation is written for learning purposes and to help beginners understand the concept clearly.
void stl_q9_frequency_map() {
vector<int> numbers = {1, 2, 2, 3, 1, 4, 2, 3, 1};
map<int, int> frequency;
// Count frequencies
for(int num : numbers) {
frequency[num]++;
}
// Print frequencies
cout << "Element frequencies:" << endl;
for(auto pair : frequency) {
cout << pair.first << " appears " << pair.second << " times" << endl;
}
}
Output Example
Output:
1 -> 2
2 -> 3
3 -> 1
4 -> 1
Common Mistakes
- Misreading input/output format.
- Not handling constraints and edge cases.
- Off-by-one errors in loops.
- Forgetting to reset variables between test cases (if any).
Solution Guide
Problem
Count how many times each element appears.
Real Life: Vote counting, inventory management.
Input / Output
Input
An integer n (number of elements)
Then n integers.
Output
Each unique element with its count.
Constraints
• n ≥ 1
• Elements can repeat
• Order does not matter
Examples
Output:
1 -> 2
2 -> 3
3 -> 1
4 -> 1
Explanation
Concept Explanation
We need to count how many times each value appears.
This is useful in real life for vote counting or inventory management.
Step-by-Step Explanation
1.Take input size n.
2.Read n elements one by one.
3.Use a map to store:
• key → element
• value → count
4.For each element:
• If it already exists in map, increase count.
• If not, insert it with count 1.
5.Traverse the map.
6.Print each element with its frequency.
Details
Common Mistakes
- Misreading input/output format.
- Not handling constraints and edge cases.
- Off-by-one errors in loops.
- Forgetting to reset variables between test cases (if any).
Official Solution
void stl_q9_frequency_map() {
vector<int> numbers = {1, 2, 2, 3, 1, 4, 2, 3, 1};
map<int, int> frequency;
// Count frequencies
for(int num : numbers) {
frequency[num]++;
}
// Print frequencies
cout << "Element frequencies:" << endl;
for(auto pair : frequency) {
cout << pair.first << " appears " << pair.second << " times" << endl;
}
}
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