The Python programming language, along with many other languages, have a special notation when accessing an individual item in a sequence called subscript notation. This notation gives you the ability to ask a sequence for its “item at position int index”. The int index begins counting from 0, such that the item at index 0 is the first item, the item at index 1 is the second item, and so on.
Subscript Notation
Subscript notation uses square brackets [ ] to surround an int index expression. This notation follows a str expression. For example, "abcd"[0] evaluates to the character 'a' and "abcd"[1] evaluates to the character 'b'. Typically, you will subscript off of a variable storing a str and not a string literal. As an example, try the following in a Python REPL:
>>> a_str = "abcd"
>>> print(a_str[0])
a
>>> print(a_str[3])
d
>>> print(a_str[4])
IndexError: string index out of rangeNotice in the example above, if you ask for an index beyond its maximum index it will lead to an IndexError.
Use len to find a str’s length
It’s helpful to know how many characters are in a str, to avoid writing code that leads to an IndexError. Python has a built-in len function that will tell you the “length” of any str.
>>> print(len("abcd"))
4
>>> message = "a b c"
>>> print(len(message))
5
>>> print(message[len(message) - 1])
'c'Notice in the second example of len above the length of str "a b c" is 5. Remember, our emphasis is on characters and spaces are characters.
Also notice, since sequences begin indexing at 0, the last character’s index is always one less than the length of the sequence.
As a bit of foreshadowing, both the subscript notation and len will be used to work with many other sequences and collections of data beyond just strings.
Using int Expressions in Subscript Notation
It was mentioned in passing above, but must be addressed with emphasis now, the index number is an int expression. Beyond simple int literal values, you can form any expression you’d like as long as it evaluates to an int. This is a SUPER POWER!
This means you can subscript with int variables, which is very common, but also int arithmetic and function calls to functions that return an int. Let’s focus on the former with an example:
>>> s = "asdf"
>>> i = 0
>>> print(s[i])
a
>>> i += 1
>>> print(s[i])
s
>>> i += 1
>>> print(s[i])
d
>>> i += 1
>>> print(s[i])
f
>>> i += 1
>>> print(s[i])
IndexError: string index out of rangeNotice, since i is an int variable we can access it from within the subscript notation of s[i]. Since we can reassign new values to i while the program is running, that means we can control which index the expression s[i] refers to at runtime.
Also notice the same two statements are repeated and the effect was to move index-by-index through the characters of the string. When you find yourself writing repeating statements it is a strong indicator you can use a loop to achieve your goal. As you know, though, loops have a test condition that ends the looping behavior. To loop over all items in a str, you can use its len as part of the loop condition.
>>> s = "hiya!"
>>> i = 0
>>> while i < len(s):
... print(s[i])
... i += 1
...
h
i
y
a
!With this capability, you can write algorithms that are guided by the individual characters it is made of. For example, when a website checks your password to be sure it meets some criteria such as a minimum length and contains certain special characters, numbers, and letters… you could now write that function!
These same concepts of working with items in a sequence using subscript notation are applicable far beyond just string data. In the near future we will learn about lists which give you the ability to form sequences of items of any type not just characters. The same syntax, concepts, and general patterns for thinking apply there, too.