String Indexing and Slicing
A string is a sequence of characters representing Unicode characters.
For example the string "Helloworld" can be thought of like this.
Unlike many programming languages, Python does not have a character type and a character is just a string (str) of length 1.
1. String Indexing
You can access elements of the string using square brackets [] and the index of the position you wish to access.
Note indexing starts at 0. So the first character has an index of 0.
For example,
str1 = "String Indexing"
print(str1[0]) # prints the first character "S"
print(str1[4]) # prints the 5th character "n"
1.1 IndexError
If you try the following:
str1 = "String Indexing"
print(str1[15]) # IndexError
You will get an exception as there is no 16th character (index 15 tries to access the 16th character).
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: string index out of range
1.2 Negative Indexing
We can also access the characters of the string from the end.
For example,
str1 = "String Indexing"
print(str1[-1]) # prints the last character "g"
print(str1[-2]) # prints the second to last character "n"
1.3 Length of a String
The len() function will return the length of the string.
str1 = "String Indexing"
print(len("String Indexing")) # prints 15
2. String Slicing
We can easily get substrings from a given string by using string slicing.
For example, we can get "Index" from the string "String Indexing".
The following gets the characters from position 7 to position 11 (12 not included)
str1 = "String Indexing"
print(str1[7:12]) # prints Index
Basically you specify the start index and the end index (not included), separated by a colon, to return a part of the string.
2.1 Start to a Given Position
You can get all characters from the start to a given position by:
str1 = "String Indexing"
print(str1[0:6]) # prints String
## OR
print(str1[:6]) # prints String
The second way is very common and is shorthand for the start index 0.
2.2 A Given Position to the End
You can get all characters from a given position to the end of the string using.
str1 = "String Indexing"
print(str1[4:15]) # prints ng Indexing
## OR
print(str1[4:]) # prints ng Indexing
The second way is quite common and it means we don't need to know the length of the string!
Otherwise, in general you would need to do the following
str1 = "String Indexing"
print(str1[4:len(str1)]) # same as str1[4:15] or str1[4:]
2.3 Negative Slicing
You can also slice using negative indexing.
str1 = "String Indexing"
print(str1[-3:]) # prints out the last 3 characters - "ing"
print(str1[-4:-2]) # prints out character 11 and 12. Same as str1[11:13]
=== TASK===
Copy the following into a new Python file.
# DO NOT EDIT THESE TWO LINES
firstname = input("Please enter a first name: \n") # leave this alone!
surname = input("Please enter a surname: \n") # leave this alone!
# -------------------------------------------------------
# Edit the following line so that it prints out the first character of the first name
print(f"The first character of the first name is {firstname}")
# Edit the following line so that it prints out the last character of the surname (negative indexing)
print(f"The last character of the surname is {surname}")
# Edit the following line so that it prints the initials of the person. e.g. Mary Smith would result in M.S
print(f"The person's initials are {firstname}.{surname}")
# Edit the following line so that it prints the first 3 characters of the first name. For example Mary would print out Mar
print(f"The first 3 characters of the first name are {firstname}")
# Edit the following line so that it prints the last 4 characters of the surname
print(f"The last 4 characters of the surname are {surname}")
- Edit the following line so that it prints out the first character of the first name
print(f"The first character of the first name is {firstname}")
- Edit the following line so that it prints out the last character of the surname (negative indexing)
print(f"The last character of the surname is {surname}")
- Edit the following line so that it prints the initials of the person. e.g. Mary Smith would result in M.S
print(f"The person's initials are {firstname}.{surname}")
- Edit the following line so that it prints the first 3 characters of the first name. For example, Mary would print out Mar
print(f"The first 3 characters of the first name are {firstname}")
- Edit the following line so that it prints the last 4 characters of the surname (note we assume for simplicity that the surname contains at least 4 characters, what happens if it is less than 4?)
print(f"The last 4 characters of the surname are {surname}")
An example of the correct program for the input Mary Smith is given below.
Please enter a first name:
Mary
Please enter a surname:
Smith
The first character of the first name is M
The last character of the surname is h
The person's initials are M.S
The first 3 characters of the first name are Mar
The last 4 characters of the surname are mith