09 1. Strings and Text Files

1. Strings and Text Files

1.1 String Basics

• A string is a sequence of characters (immutable).
  
• Subscript operator [] is used to access characters.
  
  • Example: s[2] returns the third character.
    
• Slicing: s[start:end] returns a substring.
  
• in operator checks for substring presence.
  

1.2 String Methods

• Strings have methods like .upper(), .lower(), .find(), .split(), .replace().
  

Strings are immutable: operations return new strings.

s = “sample”

s.upper() # returns ‘SAMPLE’

print(s)          # still 'sample'

s = s.upper() # now s = ‘SAMPLE’

1.3 Text File Handling

Writing:

with open("filename.txt", 'w') as f:

f.write(“Line 1\nLine 2”)

• Always convert non-string data (e.g., integers) using str() before writing.
  

Reading:

with open("filename.txt", 'r') as f:

text = f.read() # or f.readline(), or use a loop

Reading Numbers:

with open("integers.txt", 'r') as f:
    for line in f:

sum += int(line.strip())

CSV Files:

import csv
with open("file.csv", 'w', newline='') as f:

writer = csv.writer(f)

writer.writerow([“Item”, “Brand”, 1000])