Lecture Notes: Python Data Type - Strings

Introduction to Python Strings

• Strings are a data type in Python.
• Characters enclosed in single or double quotes.
• Applications in various fields, e.g., biology for sequences.

Creating Strings

• Assign strings to variables using single or double quotes.
• Example:

  my_string = "bioinformatics"
  

Printing Strings

• Use print() function to display strings.
• Can use direct string or variable.

String Case Conversion

• Convert strings to lowercase or uppercase using:
  • string.lower() for lowercase.
  • string.upper() for uppercase.
• Example:

  my_string.lower()
  my_string.upper()
  

Capitalizing Strings

• Use string.capitalize() to capitalize the first letter.
• Example:

  my_string.capitalize()
  

Finding Length of Strings

• Use len() function to find the number of characters.
• Space is counted as a character.
• Example:

  len(my_string)
  

Checking Character Presence

• Check if a character or substring is present using in keyword.
• Use string.find() to find the index of a substring.

Counting Characters or Substrings

• Use string.count() to count occurrences.
• Example:

  my_string.count("o")
  

Replacing Characters or Substrings

• Use string.replace(old, new) to replace parts of a string.
• Example:

  my_string.replace("old", "new")
  

Combining Strings

• Concatenation using + operator or string formatting.
• Example:

  full_name = first_name + " " + last_name
  

String Formatting

• Use placeholders %s or str.format() for formatting.
• Example:

  "Hello, %s!" % name
  "Hello, {}!".format(name)
  

Indexing and Slicing

• Access parts of strings using indexes.
• Python indexing starts at 0.
• Use slicing [start:end:step].
• Negative indexing counts from the end.

Practical Applications in Bioinformatics

• DNA sequences as strings.
• Calculating GC content using string operations.
• Example for GC content:

  gc = dna.count('G') + dna.count('C')
  gc_percent = (gc / len(dna)) * 100
  

Exercises

• Example problems related to string manipulations.
• Finding nucleotide positions, counting occurrences, etc.