Lecture 16, Thu 02/29

More on Strings cont.

Recorded Lecture: 2_29_24

String methods cont.

• .find(x, start, end) - returns the index of the first occurrence of x in the string. The parameters start and end are optional, but can be used to restrict the index range for finding x. If x is not found, -1 is returned
• .rfind(x) - same as find, but returns the index value of the last occurrence in the string.
• Example:

s = "how much wood can a woodchuck chuck"

print(s.find("wood")) # 9
print(s.find("wood", 13)) # 20
print(s.rfind("wood")) # 20
print(s.find("WOOD")) # -1

• .count(x) - returns the number of times x appears in the string
• Example:

s = "how much wood can a woodchuck chuck"

print(s.count("wood"))
print(s.count("w"))
print(s.count("z"))

• strip() - returns a string with leading and trailing whitespace removed
• Example:

s1 = "it's a lovely day"
s2 = "\n\t   Hello!\n\n "
print(s2.strip()) # Hello!

String Comparison (Lexicographical Order)

• We can determine if two strings are less than, equal to, or greater than each other
  • It uses ASCII values for each character to make this determination
• Python looks at the first character of each string to make the determination if one string is less than or greater than the other using ASCII values
  • If the first characters are equal, it goes on to the second character, and so on
• If one string is shorter than another string (but the shorter string contains all the same characters), then the shorter string is considered less than the longer string
• If all characters are equal in both strings, then Python considers these strings as equivalent
• Note that upper and lower case characters have different ASCII values, so "a" != "A"
  • If we didn’t want to compare cases and just alphanumeric characters, we could convert both strings to all upper case (using .upper()) or all lower case (using .lower())
• Example:

s1 = "abcd" 
s2 = "aeiou"
print(s1 < s2) # True
print(s1 > s2) # False
print(s1 == s2) # False
s1 = "abcd" 
s2 = "abc"
print(s1 < s2) # False
print(s1 > s2) # True
print(s1 == s2) # False

s1 = "ABC"
s2 = "abc"
print(s1 < s2) # True
print(s1 > s2) # False
print(s1 == s2) # False

print(s2.upper()) # ABC
print(s1.lower()) #abc
print(s1.upper() < s2.upper()) # False
print(s1.upper() > s2.upper()) # False
print(s1.upper() == s2.upper()) # True

s3 = "ant"
s4 = "Apple"

print(s3 < s4) # True
print(s3 > s4) # False
print(s3 == s4) # False
print(s3.lower() < s4.lower()) # False
print(s3.upper() > s4.upper()) # True 
print(s3.upper() == s4.upper()) # False

Boolean String Methods

• isalnum() - returns True if all characters are alphanumeric, returns False otherwise
• isdigit() - returns True if all characters are digits [0-9], returns False otherwise
• islower() - returns True if all alpha characters are lower case, returns False otherwise
• isupper() - returns True if all alpha characters are upper case, returns False otherwise
• isspace() - returns True if all characters are whitespace (includes \t and \n), returns False otherwise
• startswith(x) - returns True if the string starts with x, returns False otherwise
• endswith(x) - returns True if the string ends with x, returns False otherwise
• Example:

alpha = "ABCD"
num = "1234"
mixed = "AB12 ?!"
whitespaces = "\n \t"

print(alpha.isalnum()) # True
print(mixed.isalnum()) # False
print(alpha.islower()) # False
print(mixed.isupper()) # True
print(whitespaces.isspace()) # True
print(mixed.isspace()) # False
print(alpha.startswith("AB")) # True
print(mixed.endswith("!")) # True

split and join methods

• We’ve seen the string’s split() method that separates characters by whitespaces into elements in a list
• We can add a separator character (or sequence of characters) to split the string into elements separated by the separator
• Example:

s = "This is a sentence"
print(s.split()) # ['This', 'is', 'a', 'sentence']
print(s) # This is a sentence

# csv (or comma separated values)
s = "First,Last,CS8,100,90"
print(s.split(",")) # ['First', 'Last', 'CS8', '100', '90']

s = "Mississippi"
print(s.split("is")) # ['M', 's', 'sippi']

• The join() method is the inverse of the split() method. It takes a list of items and returns a single string that combines each element with a string
• Example:

s = "Mississippi"
splitList = s.split("is")
print(splitList) # ['M', 's', 'sippi']
newString = "IS".join(splitList)
print(newString) # MISsISsippi