1.4. Strings#

A string in programming is a sequence of characters and is used to handle text data. A string in Python is declared using single or double quotation marks, so 'hello world' is the same as "hello world".

Declaring a string variable is simply done by assigning a variable equal to a string.

variable = "string"

For example, enter the following into the console.

In [30]: string = "hello world"

To output a string we can use the print() command (printing is explained in more detail here).

print(variable)

Lets print our string variable, enter the following into the console

In [31]: print(string)
Out[31]: hello world

To define a multiline string we can use triple single or double quotation marks. For example, enter the following into the console[1] (you will need to press the enter key after the word velocity to continue onto the next line).

In [31]: multiline_string = """What is the air-speed velocity 
    ...: of an unladen swallow?"""

In [32]: print(multiline_string)
What is the air-speed velocity 
of an unladen swallow?

1.4.1. Modifying strings#

Python has the following built-in functions that can be used to modify a string.

Table 1.2 String modification functions#

Function

Description

string.upper()

Converts the characters of a string to uppercase

string.lower()

Converts the characters of a string to lowercase

string.strip()

Remove spaces before and after the characters in a string

string.replace(<old string>, <replacement string>)

Replaces a string with another string

To demonstrate these enter the following code into the console.

In [33]: string = "   Hello World   "

In [33]: print(string.upper())
   HELLO WORLD   

In [32]: print(string.lower())
   hello world   

In [33]: print(string.strip())
Hello World

In [34]: print(string.replace("l", "x"))
   Hexxo Worxd

1.4.2. Concatenating strings#

To concatenate (merge) two or more strings we use the + operator.

merged_string = string1 + string2

To demonstrate this enter the following into the console.

In [35]: string1 = "hello"

In [36]: string2 = "world"

In [37]: merged_string = string1 + " " + string2

In [38]: print(merged_string)
hello world

Note that we needed to include a space " " when concatenating the two words, if we didn’t do this the concatenated string would be helloworld.

1.4.3. Indexing characters in a string#

The characters in a string can be indexed using the character position starting at 0 for the first characeter.

string[ index ]

To demonstrate this enter the following into the console.

In [39]: string = "What have the Romans ever done for us?"

In [40]: print(string[0])
W

In [41]: print(string[10])
t

Here we have printed the 1st and 11th character in string.

To index a range of characters in a string we use a colon to separate the first and last characters in the range.

string[ first_character_index : last_character_index + 1 ]

To demonstrate this enter the following into the console.

In [42]: print(string[14:25])
Romans ever

Here we have printed the string which consists of the 15th to the 25th character in string.

Note

A Python string is actually an array of characters so we can use array slicing commands which are covered later to index strings.

1.4.4. Length of a string#

The length of a string is the number of characters in the string and can be determined using the len() function.

len(string)

To demonstrate this enter the following into the console.

In [43]: print(len(string))
38

So our string is 38 characters long.

1.4.5. Exercise#

Exercise 1.3

Define two string variables for the following:

  • String 1: “Your mother was a hamster”

  • String 2: “and your father smelt of elderberries!”

Use your strings to answer the following:

  1. Print string 1 using all lowercase characters

  2. Print string 2 using all uppercase characters

  3. Print string 2 with the word “elderberries” replaced with “roses”

  4. Create another string variable by concatenating string 1 and string 2 and print it

  5. Print the length of your concatenated string

  6. Print the last 30 characters of the concatenated string