Back to: Python Programming
Like many other popular programming languages, strings in Python are arrays of bytes representing unicode characters.
However, Python does not have a character data type, a single character is simply a string with a length of 1.
Square brackets [ start : end : step ] can be used to access elements of the string using the parameters “start”, “end” and “step”.
Example:
We want to display the first digit of a credit card we use only [start] position:
creditcard = "5024 1234 5678 2468"
# string pos 0123456789...
# to print the first digit
print(creditcard[0])To display the first 4 digits, display positions 0 – 3
The start index is inclusive, but the end index is NOT included.
creditcard = "5024 1234 5678 2468"
# string pos 0123456789...
# to print the first digit
print(creditcard[0:4]) # <-- Start index is inclusive | end index exclusiveprint(creditcard[0:4]) could also be written print(creditcard[:4]) ⇐ Python assumes we are starting at the beginning of the string.
If we want to print the last half of the card number, the first digit will be in the 10th place. We’d use print(creditcard[10:]) ⇐ Python assumes a blank after the colon means we want everything through to the end.
If we want to print the last digit we can use the ‘negative index’ position where the last digit’s position is deemed to be -1, second last digit is at -2 and so on.
To display the last four digits:
creditcard = "5024 1234 5678 2468"
# string pos 0123456789...
# to print the first digit
print(creditcard[-4:]) # position -4 is 4th from the endThe step parameter specifies the increment between each index in the slice.
If we wish to display every 3rd character of the entire string we’d use:
print(creditcard[0:19:3]) ⇐ where [start:end:step]
print(creditcard[::3]) ⇐ will do the same thing, Python presumes start to end.
If we wanted every second character of the centre 8 numbers (9 characters)
creditcard = "5024 1234 5678 2468"
# string pos 0123456789...
print(creditcard[5:14:2])We expect from the positions indicated an output of 13 68:
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 |
| 5 | 0 | 2 | 4 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 2 | 4 | 6 | 8 | |||
| ^ | ^ | ^ | ^ | ^ |
Exercise:
Write code to display a user’s inputted credit card number securely with the following output: Your credit card number is: XXXX-XXXX-XXXX-2468
To print out a string in reverse order, we simply make the ‘step’ parameter -1
backwardtext = input("Enter some input to be reversed: ")
print(backwardtext[::-1])Exercise:
Write code to input a user’s email address and separate it into a domain name and username. The output should look like:
Your username is bob.down Your email domain is gmail.com
Solution:
email = input("Enter your email address: ")
split = email.find("@") # <-- could also use email.index("@")
username = email[:split] # <-- or email[0:split]
domain = email[split + 1:] # <-- + 1 excludes @ from domain
print(f"Your username is: {username} ")
print(f"Your domain is: {domain}")On line 3 we could use:
split = email.find(“@”) ⇐ if the user inputs an invalid email omitting ‘@’ – split = -1
split = email.index(“@”) ⇐ if the user inputs an invalid email omitting ‘@’ – ERROR
You could also write this code with one less line omitting the “split’ variable – it’s a matter of personal preference and ease of readability:
email = input("Enter your email address: ")
username = email[:email.find("@")]
domain = email[email.find("@") + 1:]
print(f"Your username is: {username} ")
print(f"Your domain is: {domain}")