Python Concepts for Data Science: Introduction to Strings

Words play a vital role in how we communicate, so it is in the right sense that we would want our computers to be able to work with words and sentences as well.

In Python, something like a word or sentence is known as a string. A string is a sequence of characters contained within a pair of 'single quotes' or "double quotes". A string can contain any letters, numbers, symbols and spaces.

In this post, our objective is to learn more about strings and how they are treated in Python. We will learn how to slice strings, select specific characters from strings, iterate through strings and use strings in conditional statements. Now let's zoom in.

A string can be thought of as a list of characters. Like any other list, each character in a string has an index. Consider the string:

name = 'Kwame'

Specific letters from this string can be selected using the index. Let’s say we decide to select the first letter of the string.

print(name[0])

This would output:

K

What about the third letter?

  print(name[2])
>>a

Not only can a single character be selected from a string, but entire chunks of characters can also be selected. This can be done with the following syntax:

string[first_index:last_index]

This process is known as slicing. When a string is sliced, a new string that starts at the first_index and ends at (but excludes) the last_index is created. An example would be:

print(name[1:4])

This would result in:

wam

There can also have open-ended selections. If the first index isn't included, the slice starts at the beginning of the string and if we remove the second index the slice continues to the end of the string. Here are some examples:

print(name[:3])
print(name[2:])

These lines result in:

Kwa
ame

We can also concatenate, or combine, two existing strings together into a new string. Consider the following two strings:

firstname = 'Nana'
lastname = 'Yaw'

A new string is formed by concatenating the above strings as follows:

fullname = firstname + lastname
print(fullname)

The resulting concatenation will be:

NanaYaw

Notice that there are no spaces added here. We have to manually add in the spaces when concatenating strings if we want to include them.

fullname = firstname + ' ' + lastname
print(fullname)

And the result is:

Nana Yaw

Python comes with some built-in functions for working with strings. One of the most commonly used of these functions is len() and this returns the number of characters in a string:

  print(len(name))
>>5

If you are taking the length of a string with spaces, the spaces are counted as well.

  print(len(fullname))
>>8  

len() comes in handy when we are trying to select characters from the end of a string which has an index of len(string_name) - 1. The following line of code will return the last character in the string.

  print(fullname[len(fullname)-1])
>>w

Because strings are lists, that means we can iterate through them using 'for' or 'while' loops. This opens up a whole range of possibilities of ways that can be manipulated to analyze strings. Let's take a look

for letter in fullname:
    print(letter)

This will iterate through each letter in fullname word and will print it as follows:

N
a
n
a

Y
a
w

When we iterate through a string we perform an action with each character. By including conditional statements inside of these iterations, we can manipulate the characters extra. Let's analyze what the following code does:

counter = 0
for letter in fullname:
    if letter == 'a':
        counter += 1
print(counter)

This code will count the number of 'a; in fullname. First, a counter is initialized to zero. The 'for' loop iterates through each character in fullname and compares it to the letter 'a'. Each time a character equals 'a' the code will increase the variable counter by one. Once all characters have been checked, the code will print the counter, telling us how many 'a' are in fullname.

Another method to determine if a character is in a string is using the keyword 'in'. 'in' checks if one string is part of another string. Here is how the syntax looks like:

substring in string

This is a boolean expression that is True if the substring is in the string.

Here is the link to the repo on GitHub