Strings in Python#

Goals of this lecture#

  • What is a string (str) type?

  • Why should we care about strings?

  • Working with str types in Python.

    • Assigning a string to a variable.

    • Indexing characters in a str object.

    • Looping through a str object.

  • Next time: more complex operations with strings.

What is a string?#

A string is a sequence of characters. It belongs to the str type in Python.

A string stores characters as text, and is created using either single ('') or double ("") quotes.

Note that although strings are often used to store words, this isn’t necessarily the case. A string could be:

"dog"

'dog'

"abcdef"

'abcdef'

"1 + 4"

'1 + 4'

With many more possibilities. Basically, any character that you wrap with quotes becomes part of a str in Python.

Multi-line strings#

Multi-line strings can be defined using """ """, as below.

long_str = """
This string spans multiple lines.
This is the second line.
This is the third line.
"""
print(long_str)

This string spans multiple lines.
This is the second line.
This is the third line.

Side note: a str is a kind of sequence#

A sequence is a collection of items (e.g., numbers, characters, etc.) with some determined order.

A list and str are both kinds of sequences.

We’ll discuss sequences more when we talk about lists, but there are a couple of important properties to remember:

  • Sequences have a particular order.

  • You can index into a sequence to obtain the item at a particular position.

Checking whether something is a str#

Recall that you can check the type of a variable using type.

type("This is a sentence.")

str

type("1 + 4")

str

type(1 + 4)

int

Check-in#

Which of the following variables would evaluate to a str?

x1 = 1.5
x2 = True
x3 = "2 * 100"

Why care about strings?#

Strings are incredibly useful and versatile, so it’s important to understand how they work and how to manipulate them.

Common uses of strings:

  • Pretty much all text data is stored as a str (e.g., a text corpus, a word, etc.).

  • Storing information that can’t be represented as int or bool, such as password.

  • Declaring features of an object in Python that can’t be represented as int or bool.

  • Representing a filename.

Strings are so useful that virtually all programming languages have something like a str type.

Working with strings: basic operations#

Today, we’re going to focus on a few basic operations we can use with strings. In a future lecture, we’ll talk about more complex operations.

The basic operations include:

  1. Getting the length (len) of a string.

  2. Indexing into a string (string_name[0]).

  3. Looping through a string (for ch in string_name...).

You’ll note that each of these operations can also be applied to a list type!

Calculating string length with len#

The len operator calculates the number of characters in a str (or list).

x1 = "CSS"
print(len(x1))

3

x2 = "class"
print(len(x2))

5

Check-in#

How many characters are in the string "2 + 2"?

Try answering before you try typing in the expression.

Spaces count as characters!#

An empty space (" ") counts as a character in Python.

Thus, the str "big dog" has one extra character than the str "bigdog".

len("big dog")

7

len("bigdog")

6

Check-in#

How many characters are in the str below?

str_test = "Computational Social Science is fun."

Putting quotes into a string#

Certain characters, like quotes, require an escape character if you want to put them into a string. Otherwise they’ll simply end the string.

quote_str = "Then he said, \"I love CSS!\""
print(quote_str)

Then he said, "I love CSS!"

Indexing into a str#

In programming, indexing into a sequence means retrieving the item at a particular position.

Because a str is a kind of sequence, we can retrieve the character at a particular position.

We can index into a str (or list) using the string_name[...] notation, where ... would be replaced with the index of the character we want to retrieve.

test_var = "computer"
test_var[0]

'c'

Note on indexing#

Python uses zero-indexing: the first element in a sequence is assigned the index 0, the second is assigned 1, and so on.

  • This can be hard to get used to at first!

  • But over time, it’ll start to seem more natural.

Check-in#

Which of the indexing operations below would return the letter "S"?

s = "CSS"
x1 = s[0]
x2 = s[1]
x3 = s[2]

Check-in#

Why does the code below return an error?

s = "CSS"
s[4]

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Input In [17], in <cell line: 2>()
      1 s = "CSS"
----> 2 s[4]

IndexError: string index out of range

Slicing into a str#

Slicing is like indexing, but allows you to return a subset within a sequence.

For example, rather than getting the n-th character of a str, you can return the characters between index 0 and index 2.

  • To slice, use the syntax [start_index:end_index].

  • start_index is the index of the first character you want to return.

  • end_index is the index of the final character you want to return, plus one.

    • Like range, the final index is not “inclusive”.

s = "programming"
s[0:4]

'prog'

Check-in#

How many characters would the following slice return? Which characters would they be?

s = "programming"
subset = s[5:7] ## how many characters is this?

Check-in#

Write a slice operation to return the str "humid" within the string "dehumidify".

original_str = "dehumidify"
### Your code here

Looping through strings#

Looping through a str means repeating some piece of code for each (or a subset) of the characters within a string.

We’ve already discussed loops in previous lectures, so this will be a brief review:

  • A for loop iterates through each item in a sequence (like a str), repeating some piece of code.

  • A while loop continues as long as some condition is met, and can also be used to iterate through a sequence.

Looping with a for loop#

seq = "CSS"
for i in seq:
    print(i)

C
S
S

Looping with a while loop#

i = 0
seq = "CSS"
while i < len(seq):
    print(seq[i])
    i += 1

C
S
S

Conclusion#

As you learn more about Python (or other programming languages), you’ll encounter strings more and more frequently. Next time, we’ll discuss more advanced operations we can use with strings, such as modifying the case of characters in a str, replacing certain characters with other characters, and so on.