Python Program for String Compression

String compression is a technique used to reduce the size of a string by replacing repeated characters with the character followed by its count.

It is a common problem that helps in understanding loops, grouping, and basic data compression concepts.

1. Understanding the Problem

Given a string, compress it by replacing consecutive repeated characters with the character and its count.

Input: aaabbc
Output: a3b2c1

2. Method 1: Using Loop

              
            
Python

          Basic loop method
        
string = input("Enter a string: ")
compressed = ""
count = 1

for i in range(len(string)):
    if i < len(string) - 1 and string[i] == string[i + 1]:
        count += 1
    else:
        compressed += string[i] + str(count)
        count = 1

print(compressed)

This method counts consecutive characters and appends them to the result.

3. Method 2: Using itertools.groupby

              
Python

          Using groupby
        
from itertools import groupby

string = input()
compressed = ""

for char, group in groupby(string):
    compressed += char + str(len(list(group)))

print(compressed)

groupby groups consecutive characters efficiently.

4. Method 3: Using Function

              
            
Python

          Reusable function
        
def compress_string(s):
    result = ""
    count = 1
    
    for i in range(len(s)):
        if i < len(s) - 1 and s[i] == s[i + 1]:
            count += 1
        else:
            result += s[i] + str(count)
            count = 1
    
    return result

print(compress_string("aaabbc"))

Encapsulating logic inside a function improves reusability.

5. Method 4: Using List and Join

              
            
Python

          Efficient string building
        
string = input()
result = []
count = 1

for i in range(len(string)):
    if i < len(string) - 1 and string[i] == string[i + 1]:
        count += 1
    else:
        result.append(string[i] + str(count))
        count = 1

print("".join(result))

Using a list improves performance compared to string concatenation.

6. Method 5: Return Original if Not Smaller

              
            
Python

          Optimized compression
        
def compress(s):
    result = ""
    count = 1
    
    for i in range(len(s)):
        if i < len(s) - 1 and s[i] == s[i + 1]:
            count += 1
        else:
            result += s[i] + str(count)
            count = 1
    
    return result if len(result) < len(s) else s

print(compress("abc"))

This method ensures compression only if it reduces string size.

7. Algorithm

1. Take input string.

2. Initialize count = 1.

3. Traverse string.

4. Count consecutive characters.

5. Append character and count.

6. Print compressed string.

8. Common Mistakes

1. Not resetting count properly.

2. Incorrect indexing.

3. Missing last character group.

4. Inefficient string concatenation.

9. Applications

1. Data compression basics.

2. Encoding techniques.

3. Reducing storage size.

4. Algorithm design practice.

Conclusion

String compression is a classic problem that introduces you to optimization and grouping techniques in Python.

Using itertools.groupby is the most elegant method, while loop-based solutions help build strong fundamentals.