9.9. Non-mutating Methods on Strings

There are a wide variety of methods for string objects. Try the following program.

ss = "Hello, World" print(ss.upper()) tt = ss.lower() print(tt) print(ss)

In this example, upper is a method that can be invoked on any string object to create a new string in which all the characters are in uppercase. lower works in a similar fashion changing all characters in the string to lowercase. (The original string ss remains unchanged. A new string tt is created.)

You’ve already seen a few methods, such as count and index, that work with strings and are non-mutating. In addition to those and upper and lower, the following table provides a summary of some other useful string methods. There are a few activecode examples that follow so that you can try them out.

Method	Parameters	Description
upper	none	Returns a string in all uppercase
lower	none	Returns a string in all lowercase
count	item	Returns the number of occurrences of item
index	item	Returns the leftmost index where the substring item is found and causes a runtime error if item is not found
strip	none	Returns a string with the leading and trailing whitespace removed
replace	old, new	Replaces all occurrences of old substring with new
format	substitutions	Involved! See String Format Method, below

You should experiment with these methods so that you understand what they do. Note once again that the methods that return strings do not change the original. You can also consult the Python documentation for strings.

ss = " Hello, World " els = ss.count("l") print(els) print("***"+ss.strip()+"***") news = ss.replace("o", "***") print(news)

food = "banana bread" print(food.upper())

Check your understanding

What is printed by the following statements?

s = "python rocks"
print(s.count("o") + s.count("p"))

• 0
• There are definitely o and p characters.
• 2
• There are 2 o characters but what about p?
• 3
• Yes, add the number of o characters and the number of p characters.

What is printed by the following statements?

s = "python rocks"
print(s[1]*s.index("n"))

• yyyyy
• Yes, s[1] is y and the index of n is 5, so 5 y characters. It is important to realize that the index method has precedence over the repetition operator. Repetition is done last.
• 55555
• Close. 5 is not repeated, it is the number of times to repeat.
• n
• This expression uses the index of n
• Error, you cannot combine all those things together.
• This is fine, the repetition operator used the result of indexing and the index method.

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