CPSC120B
Fundamentals of Computer Science

Genetic Algorithms

• Evolved Virtual Creatures
• MarI/O

Reading Questions

Quiz

String Operators

• We’ve already seen strings, they are text inside of quotes

  text = "this is a string"

• Now we are learning how to write programs that use and manipulate strings
• One way is with string operators
• Numerical operators are +, -, *, /, etc.
• Comparison operators are >, <, ==, etc.
• Logical operators are and, or, not
• String operators are + and *

• The + operator doesn’t add, it joins two strings together into a new string, this is called concatenation

  print("hello" + "everybody")

• Note, this is different than using a comma in a print statement in two ways
• First, it doesn’t add the space automatically

• Second, it can be used outside of a print statement

  greeting = "hello"
  audience = "everybody"
  greeting = greeting + " " + audience
  print(greeting)

• Also note, operands must both be stings

• Can also compare whether two strings are equal

  name = input("enter name: ")
  if name == "nobody":
      print("hello nobody")

Index Operator

• Each character in a string has a unique location
• The location, or index, of a character is the characters distance from the first character
• For example, in the string “hello”, h has the index 0, e has the index 1, etc.

• The index operator can be used to copy a character out of a string

  text = "hello"
  first_character = text[0]
  print(first_character)

• Negative indices can be used to access characters from the end

  text = "hello"
  last_character = text[-1]
  print(last_character)

• Indices that are too large or too small will produce an error

  text = "hello"
  nonexistent_character = text[5] # Error
  print(nonexistent_character)

String Traversal

• Use a loop and the index operator to do something for every character in a sting

  text = "hello"
  for i in range(len(text)):
      print(text[i])

• Can even have accumulators with strings

  text = "hello"
  new_text = ""
  for i in range(len(text) - 1):
      new_text = text[i] + "-"
  print(new_text)

• Note that the initial value of the accumulator variable is a string with no characters, the empty string

Activity