Strings are Objects¶

Strings are objects in Python which means that there is a set of built-in functions that you can use to manipulate strings. You use dot-notation to invoke the functions on a string object such as sentence.lower(). The function lower() returns a new string with all of the characters in the original string set to lowercase. The function capitalize() will return a new string with the first letter of the string capitalized.

Getting Part of a String¶

A string is a sequence of characters. Each character can be referenced by its index, which is a number between 0 and one less than the total number of characters in the string as shown below.

Figure 1: A string with the position (index) shown above each character

The index gives the position of the character in the string.

Note

In many languages characters and strings are different data types, but in Python they are not. A character is just a string of length 1.

Finding a Small String in a Bigger One¶

The string.find(part) function takes a part of a string (often a single character) as input and returns the index of the first place in the string where the part begins. If the part isn’t found, find returns -1.

Note

If a string has more than one occurance of the part, there is an optional argument to find that tells it where to begin searching, so sentence.find("is", 3) will return 4, the first occurance of “is” found starting the search at index 3.

Figure 2: A string with the position (index) shown above each character

A slice is a way to get part of a string. It uses the same square brackets that an index uses, but it has two integer values inside with a : between them that tell where the part begins and ends.

The slice will include all the characters from the first index up to but not including the second index.

Check Your Understanding

csp-4-2-1: What will be printed when the following executes?

str = "This is the end"
str = str[1:4]
print(str)

This is the end
This would be true if we were printing the value of str and we hand't changed it on line 2.
This
This would be true if the first position was 1 and the substring included the character at the end position, but the first character in a string is at position 0 and the substring won't include the character at the last position.
his
This will return a string that starts at position 1 and ends at position 3.

csp-4-2-2: What will be printed when the following executes?

str = "This is the end"
str = str[5]
print(str)

i
This will print the character at position 5 in the string which is i. Remember that the first character is at position 0.
s
This would be true if the first character was at position 1 instead of 0.
is the end
This would be true if it returned from the given position to the end of the string, but that isn't what it does.

Getting the Length of a String¶

The len(string) function takes a string as input and returns the number of characters in the string including spaces.

Check your understanding

csp-4-2-3: Given the following code segment, what will be printed?

street = "125 Main Street"
print(len(street))

13
Don't forget to include the spaces in the count.
15
The len function returns the number of elements in the string including spaces.
10
This would be true if the len function only returned the number of alphabetic characters, but it includes all including spaces.

csp-4-2-4: What will be printed when the following executes?

str = "His shirt is red"
pos = str.find("is")
print(pos)

1
The find function returns the index of the first position that contains the given string.
9
This would be true if it was pos = str.find(" is").
10
This would be true if it was pos = str.find(" is") and the first position was 1, but it is 0.

Write the code to evaluate the length of the string “I like green eggs” and print it. It should print “The length is 17”.

Create variable to hold the string.

Next Section - Strings are Immutable